Learning Module 7:
101 Basic Series
Low Voltage Power Circuit Breakers
Low Voltage Power Circuit Breakers
What You Will Learn
We will step through each of these topics in detail:
Introduction
6
What is a Low Voltage Power Circuit Breaker?
Method Used to Make or Break Circuits
Ratings
Construction/Maintainability
Trip Units
Operating Mechanisms
8
8
8
9
9
9
Principles of Operation and Terminology
Stored Energy
Bus
Control Voltage
Drawout
Behind Door Drawout
Through Door Drawout
Continuous Current Rating
100% Rated
Interrupting Rating
Short Time Rating
Trip Free
Current Sensor
10
11
11
11
11
13
14
14
14
14
14
15
15
Review 1
16
Design and Functional Considerations
Frame or Chassis
Primary Contacts
Arc Extinguishers
Operating Mechanism
Integral Trip Unit
Accessory Items
17
17
18
21
22
23
25
Review 2
28
Mounting Methods
Fixed Circuit Breaker
Drawout Circuit Breaker
29
29
29
Governing Standards
ANSI
UL
IEC
Closing Comments on Standards
30
31
31
31
31
Testing
32
Helping the Customer
Standard Selection Factors
ANSI Example
IEC Example
ANSI and IEC Example
33
33
34
36
36
Page 2
Low Voltage Power Circuit Breakers
Special Selection Factors
37
Review 3
38
Advanced Low Voltage Power Circuit Breakers
39
Standards and Testing
IEEE (Institute of Electrical and Electronic Engineers)
NEMA (National Electrical Manufacturers Association)
UL (Underwriters Laboratories Inc.)
CSA (Canadian Standards Association)
ANSI (American National Standards Institute)
IEC (International Electrotechnical Commission)
Standards Conclusion
Low Voltage Power Circuit Breaker C37.50 Testing
40
41
41
42
42
42
44
44
45
Review 4
48
Construction Method
49
Ratings and Performance
50
Review 5
55
Operational Techniques
Primary Opening and Closing Contacts
Moving Contact Assembly
Stationary Contact Assembly
Operating Arm (Link)
Magnum DS Primary Contact System
Operating Mechanism
Magnum DS Operating Mechanism
Arc Chamber (Chutes)
Magnum DS Arc Extinguishing
Integral Trip Unit
Programmable Protection and Coordination
Circuit Breaker Coordination
Circuit Breaker Coordination Example
56
56
57
57
58
58
62
63
64
65
66
67
67
68
Review 6
70
Protective Systems
Long Delay Protection (L)
Short Delay Protection (S)
Instantaneous Protection (I)
Ground Fault Protection (G)
Overcurrent Protection Combinations
Characteristic Curve Review
Advance Warnings and Alarms
System Diagnostics and Testing
System Monitoring
Power Quality Monitoring
Energy Monitoring and Management
Communications
72
73
73
74
74
74
75
75
75
76
76
76
76
Page 3
Low Voltage Power Circuit Breakers
Applications
Zone Selective Interlocking
76
77
Review 7
80
Meaning of Nameplate Ratings
81
Low Voltage Power Circuit Breaker Summary
Standards and Testing Summary
General Features Summary
Magnum DS Summary
82
82
83
83
Review 8
85
Glossary
86
Review 1 Answers
88
Review 2 Answers
88
Review 3 Answers
88
Review 4 Answers
89
Review 5 Answers
89
Review 6 Answers
89
Review 7 Answers
89
Review 8 Answers
89
Page 4
Low Voltage Power Circuit Breakers
Welcome
Welcome to Module 7, which is about low voltage power circuit breakers.
Figure 1. Molded Case Version of Low Voltage Power Circuit Breakers
You will find the “Advanced Low Voltage Power Circuit Breakers” on page 39 to be
an advanced discussion of the low voltage power circuit breaker. It is intended to
be a continuation of the discussion of low voltage power circuit breakers and Module 5, “Fundamentals of Circuit Breakers.” It also addresses many of the same
topics in a more detailed fashion. Because Eaton’s Cutler-Hammer Magnum DS
Low Voltage Power Circuit Breaker is the latest and most advanced product in the
industry, it will be the low voltage power circuit breaker used for most examples.
Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study
the material carefully then answer the questions without referring back to what
you’ve just read. You are the best judge of how well you grasp the material.
Review the material as often as you think necessary. The most important thing is
establishing a solid foundation to build on as you move from topic to topic and
module to module.
A Note on Font Styles
Key points are in bold.
Glossary items are italicized and underlined the first time they appear.
Viewing the Glossary
Printed versions have the glossary at the end of the module. You may also browse
the Glossary by clicking on the Glossary bookmark in the left-hand margin.
Page 5
Low Voltage Power Circuit Breakers
Introduction
There are both low voltage DC power circuit breakers and low voltage AC
power circuit breakers. The interruption of direct current is distinctly different
from the interruption of alternating current, and generally more difficult at comparable voltages and currents. Large quantities of low voltage AC power circuit
breakers are used throughout industry in comparison to very small numbers of DC
devices. For this reason and the fact that this is an introduction to low voltage
power circuit breakers, only AC designs will be covered. Keep in mind, however,
low voltage DC power circuit breakers do exist and are used in a number of specialty applications, such as rapid transit.
Circuit Breakers are often classified by certain modifying words, such as low voltage power. Low voltage AC power is considered to be for applications at 1000
volts AC and below. For comparison reasons then, medium voltage AC power is
considered to be for application above 1000 volts AC. In general, however, low
voltage power circuit breakers are viewed as 600 volt circuit breakers applied at a
number of different voltage levels, such as 240 or 480 volts.
Sound confusing? Let’s try to clear it up a bit by taking a brief look at why a low
voltage power circuit breaker might be used along with some background information.
Why use a low voltage power circuit breaker over another type of low voltage circuit breaker? Most often the determination is made by the specific application. Let’s consider a number of the more prominent reasons why a low voltage
power circuit breaker is ideally suited for certain applications. Keep these reasons
in mind as you proceed through this module. You will learn about the features and
requirements that support and further explain the following reasons for applying
low voltage power circuit breakers:
•
Continuity of Service - Continuity of service allows for the maximum up time
and minimum down time of equipment. A low voltage power circuit breaker
has a significant Short Time Rating (also: “withstand rating”). This means that
the low voltage power circuit breaker has the strength to withstand the
stresses of a fault for up to 1/2 second or 30 cycles, instead of opening immediately. This ability to delay opening allows for a circuit breaker nearest the
fault to clear the fault. This helps to prevent facility outages or a wide shutdown of facility equipment.
•
Maintainability - A low voltage power circuit breaker is designed to be maintained in the field. This extends the useful service life of the circuit breaker.
Especially for heavy, repetitive duty applications, maintenance of the circuit
breaker is quite an important feature. Low voltage power circuit breakers
allow for the inspection and replacement of parts on site.
•
Safety - Low voltage power circuit breakers are tested as drawout devices in
an enclosure. As such, four distinct circuit breaker positions relative to its
enclosure are provided for maximum operator safety. The four drawout circuit
breaker positions allow for the following uniquely different functions:
Connected Position: The circuit breaker is fully connected and functional.
Test Position: The circuit breaker’s primary connections are disconnected.
Secondary connections are not disconnected and testing can be safely performed because the circuit breaker is not energized. This is not possible with
a circuit breaker that is permanently mounted.
Disconnect Position: Neither the primary nor secondary electrical connections
of the circuit breaker are made. This position is often used as a storage position for the circuit breaker within its enclosure.
Withdrawn Position: In this position, the circuit breaker has no electrical conPage 6
Low Voltage Power Circuit Breakers
nections. It is far enough out of its enclosure, usually on some type of integral
extension rails, to permit inspection and maintenance without turning the
power off to an entire assembly of equipment.
Reliability - Low voltage power circuit breakers are tested for and must be
able to meet high electrical and mechanical endurance ratings. Electrical
endurance is the number of operations at rated continuous current and maximum system voltage. Mechanical endurance is the number of operations with
no voltage applied.
Remote Operation and Reclosing - Low voltage power circuit breakers are
designed for operation remotely. They have two-step Stored Energy mechanisms which permit circuit breakers to rapidly reclose after a fault. The twostep stored energy mechanism makes multiple charge-close operations possible, such as the operating sequence: charge-close-recharge-open-closeopen.
Custom has led to using phrases such as low voltage power circuit breaker,
low voltage metal-frame circuit breaker, low voltage air circuit breaker, and
600 volt power circuit breaker. Although these circuit breakers fall into the classification of 1000 volts and below, real world applications are usually 600 volts and
below, thus the 600 volt reference. In general, such a device must be built and
tested in accordance with a very specific set of standards, such as ANSI Standards. A low voltage power circuit breaker is a device with both an Interrupting
Rating and a short time rating, where the short time rating is composed of two
components:
•
Short Delay Current (expressed in kA)
•
Short Delay Time (expressed in cycles)
This is the primary differentiating feature between a power circuit breaker and a
molded case circuit breaker. The importance of this difference will be discussed a
number of times later in this module.
For many years, low voltage power circuit breakers were essentially an assembly
of parts on a welded metal frame, thus the phrase metal-frame circuit breaker.
Distinguishing one low voltage circuit breaker from another at that point was
rather simple. If it was a metal-frame circuit breaker, it was probably a power circuit breaker. If the circuit breaker parts were enclosed by an insulating material, it
was called a molded case circuit breaker (Figure 2).
Figure 2. Metal-Frame Low Voltage Power Circuit Breaker
Certain hybrid low voltage circuit breakers were later developed and quite successful in certain markets. These circuit breakers had their parts encased in an
insulating material, not unlike a molded case circuit breaker. From a performance
standpoint, however, they performed more like a power circuit breaker. They had
several of the same physical attributes as the power circuit breaker, but were
never able to achieve the short time ratings of a power circuit breaker or pass all
the power circuit breaker test standards.
Page 7
Low Voltage Power Circuit Breakers
This type of circuit breaker, although not tested to all the same standards as a
power circuit breaker, found its application niche to be similar to traditional power
circuit breakers. This design became known as a low voltage insulated case
circuit breaker (Figure 3).
At that point, the line between frame material to identify the type of circuit breaker
became blurred. All this said, the differentiating feature still remains the
device’s ability to meet power circuit breaker test standards, not the frame’s
type of construction.
Figure 3. Low Voltage Insulated Case Circuit Breaker
What is a Low
Voltage Power
Circuit Breaker?
Like much other terminology in the industry, the designation low voltage power circuit breaker can be confusing at times. For now, let’s just say that the set or sets
of standards a circuit breaker complies with determines whether or not the circuit
breaker can be classified as a low voltage power circuit breaker. Applicable standards will be discussed later in this module.
As you might imagine by now, there is a wide variety of low voltage power circuit
breakers available in the market today. We will not concentrate on what circuit
breakers are called. Instead, we will look at characteristics, features and governing standards. Then, no matter who the manufacturer or what a circuit breaker is
called, you will be better prepared to discuss the subject.
Low voltage power circuit breakers are considered rugged, long-lived, flexible
and, to varying degrees, field-maintainable. Let’s briefly look as some of the areas
that might set a low voltage power circuit breaker apart from other types of low
voltage circuit breakers, such as:
•
Method used to make and break circuits
•
Ratings
•
Construction/Maintainability
•
Integral Trip Units
•
Operating Mechanisms
•
Testing
Method Used to Make or
Break Circuits
Because they make and break power circuits in air using Arc Chutes, as opposed
to Vacuum, SF6 or oil, they are considered Air Circuit Breakers.
Ratings
Low voltage power circuit breaker interrupting ratings and frame size designations
can vary to some degree from one manufacturer to another or from one part of the
world to another. One thing that is common to most power circuit breakers is the
fact that they are rated for continuous operation at 100% of their current rating in their enclosure. What you see on the nameplate is what you get. There is
Page 8
Low Voltage Power Circuit Breakers
no derating necessary when enclosed, if they are applied as specified by the manufacturer. This is not the case with all types of low voltage circuit breakers when
applied in an enclosure. Low voltage power circuit breakers also have a short
time rating in addition to an interrupting rating making them naturally suited
for selectivity and coordination with downstream devices. Downstream devices
are devices, such as other circuit breakers, that are farther into the electrical
system.
You will recall from an earlier discussion, and it is worth mentioning again, that the
short time rating is composed of two components - short delay current and
short delay time, which are adjustable (programmable). As far as selectivity is
concerned, let’s say it is the response to a set of circuit or system conditions, usually in terms of current, in a certain time frame. It is really the ability to withstand a
certain level of current (kA) for a given time period (cycles) while a downstream
device selectively takes care of the problem by interrupting. This is also known as
discrimination. The degree of selectivity is usually limited by the sophistication of
the trip unit and the physical ability of the circuit breaker to withstand the potentially large thermal and mechanical stresses created by a fault current.
Construction/
Maintainability
Low voltage power circuit breakers are essentially an assembly of parts on a
metal frame or in an encased housing of insulating material. It is important to
know that no set of standards dictates the type of frame construction for low
voltage power circuit breakers. That decision is left in the hands of the manufacturer. You could look at it like the frame and body of a car holding all the other
parts, like the motor, wheels, bumpers, seats and radio. This type of circuit
breaker, to varying degrees, has the ability to be maintained in the field.
Figure 4. The Frame of a Car is Like the Housing of a Low Voltage Power Circuit Breaker
In addition, it is available in either a Fixed or Drawout configuration, with drawout
being the most commonly used type.
Trip Units
Trip Units today used on low voltage power circuit breakers are almost universally
of the solid state, microprocessor-based design. Years ago this same type circuit breaker used only electromechanical type trip units. Because this type of trip
unit used with a low voltage power circuit breaker is almost non-existent, it is only
mentioned briefly in this module. It is important to note that ANSI Standards
require that the trip units on low voltage power circuit breakers be integrally
mounted.
Operating Mechanisms
Low voltage power circuit breakers operate through two-step stored-energy spring
mechanisms. The springs used to close the circuit breaker contacts, called closing
springs, can be manually or electrically charged. The springs used to open the circuit breaker, called opening springs, are usually charged automatically when the
breaker is closed.
Because of the increased closing forces required and the closing speed, low voltage power circuit breakers use two-step, stored energy mechanisms. That is,
the closing springs are charged and remain charged with the breaker open until a
“close” button or some other type of release is activated to close the breaker. As
mentioned in Module 5, the low voltage power circuit breaker is required by ANSI
Page 9
Low Voltage Power Circuit Breakers
Standards to provide an open-close-open duty cycle. This dictates the need for
a two-step stored energy mechanism.
In the Workplace
Low voltage power circuit breakers are most commonly applied in switchgear
assemblies like the one shown here.
Figure 5. Typical Low Voltage Metal-Enclosed Assembly
Frequently, low voltage power circuit breakers are used to control (and protect
against overloads and short-circuits on) fans, pumps and lighting panels.
An assembly such as this one could be used to serve the HVAC needs of a manufacturing facility.
Because they are built to withstand such intense service conditions, low voltage
power circuit breakers are ideal for industrial applications such as this.
Principles of
Operation and
Terminology
A low voltage power circuit breaker can be applied on any system within the interrupting rating of the circuit breaker. Low voltage power circuit breakers are ideally
suited for applications where there is a requirement for the circuit breakers to
be selective when faced with short-circuit conditions. In addition to our earlier discussion of selectivity, we could also say that “selective” means that the circuit
breaker is capable of remaining closed for a certain period of time with a short-circuit present to allow the problem to be cleared up by a downstream device before
the power circuit breakers open and the larger system is shut down (short time
delay rating capacity). This is the area where short time delay ratings from 0 to 30
cycles play a key role. Obviously, it is assumed that the circuit breaker is applied
properly and will not face short-circuit conditions beyond its capabilities. If it does
see a condition beyond its short time rating, it will open instantaneously.
Time will be taken here to introduce several additional principles and common
terms associated with low voltage power circuit breakers and their application.
This material will be especially helpful from a practical standpoint. These are the
types of terms and topics encountered on the job when working with low voltage
power circuit breakers and their assemblies. Principles and terms presented here
are certainly not all inclusive. Even after this module is completed and you return
to your work location, new terms will surface that should be part of your low voltage power circuit breaker vocabulary. The intent here is to provide a solid background on which to build.
Page 10
Low Voltage Power Circuit Breakers
Stored Energy
Stored energy was briefly touched on earlier in this module and in Module 5.
Because this is a phrase frequently heard with respect to circuit breakers, it
deserves some elaboration. All low voltage power circuit breakers, whether manually or electrically operated, utilize two-step stored energy mechanisms.
Stored energy mechanisms are needed to overcome inherent forces opposed to
the closing process. They also make it possible to close the circuit breaker very
quickly, 5 cycles or less time.
Stored energy is energy held in waiting, ready to open or close the low voltage
power circuit breaker in five cycles or less. Designs are such that the energy
required to open a low voltage power circuit breaker is always available.
On manually operated circuit breakers, closing springs are charged by hand. For
electrically operated circuit breakers, springs are normally charged by a small
electric motor, although they can also be charged manually if power is not available (Figure 6).
Figure 6. Typical Low Voltage Metal Frame Power Circuit Breaker Being Manually Charged
Bus
Bus refers to a conductor or conductors, usually made of copper or aluminum
bars. Bus bars carry current and serve as a common connection for two or more
circuits (Figure 7).
Figure 7. Rear View of Typical Low Voltage Switchgear Assembly
Showing a Maze of Bus Bars Interconnected
Control Voltage
The Control Voltage (or secondary voltage), is usually secondary with respect to
the voltage rating of the circuit in which the circuit breaker is applied. Control voltage is used to operate secondary devices. The voltage used to run the motor that
charges a circuit breaker’s springs automatically is an example.
Drawout
A drawout circuit breaker refers to a circuit breaker that can be moved
within a compartment from one defined position to another without manually disconnecting any connections or turning off the line side power. This is
usually accomplished through the use of a mechanical levering device, some-
Page 11
Low Voltage Power Circuit Breakers
times in combination with the manual assistance of an operator. This is called
racking the circuit breaker into or out of a position. The circuit breaker is first
opened, and then automatic main disconnect devices on a drawout circuit breaker
allow for the circuit breaker to connect or disconnect from the bus. These automatic main disconnect devices are often referred to as Finger Clusters. The
phrase finger cluster comes from the fact that many designs utilize a number of
conductive pieces (fingers) assembled into one cluster. The four typical defined
positions are:
•
Connected
•
Test
•
Disconnect
•
Remove (Withdrawn)
In the Connected position, the circuit breaker is into its compartment as far as it
will go with both primary and secondary electrical connections made. The circuit
breaker is now ready for normal operation (Figure 8).
Figure 8. Connected Position
In the Test position, the circuit breaker is farther out of its compartment with the
primary electrical connections no longer made (Figure 9). Secondary electrical
connections are still made in this position to provide the secondary power required
to test the circuit breaker’s operation, including the trip unit.
Figure 9. Test Position
In the Disconnect position, the circuit breaker is even farther out of its compartment with the main Contacts open (Figure 10). Neither the primary nor secondary
electrical connections are made. This is a typical compartment storage position
for a circuit breaker not in use.
Page 12
Low Voltage Power Circuit Breakers
Figure 10. Disconnect Position
In the Remove (or Withdrawn) position, the circuit breaker is out of the compartment on extension rails with the main contacts open and the closing springs discharged (Figure 11). There are neither primary nor secondary electrical
connections. This is the typical last position for a circuit breaker to be in before it is
physically removed from its rails to another location.
Figure 11. Remove (Withdrawn) Position
Behind Door Drawout
This is related to the specific drawout breaker design (Figure 12). Behind the door
drawout means that the breaker compartment door usually must be opened to
Lever (or “rack”) the breaker from one position to another as just discussed under
“Drawout.”
The breaker normally has a Faceplate Shield (or “deadfront shield”) to protect the
operator from dangerous voltages while the door is open. This type of design usually permits the breaker to be in any of three positions (Disconnect, Test, Connected) with the door closed. This design does not permit an individual to know
the status of the circuit breaker or its trip unit without opening the compartment
door.
Page 13
Low Voltage Power Circuit Breakers
Figure 12. Typical Behind the Door Drawout Type Low Voltage Metal-Frame Circuit Breaker
Being Levered From One Position to Another
Through Door Drawout
This is also a drawout related circuit breaker design (Figure 13). Through the
door drawout permits the operator to lever the circuit breaker from the
“Connected” position to the “Test” position to the “Disconnect” position
and vice versa without opening the compartment door. The door has a hole in
it to accommodate protrusion through the door of some small portion of the circuit
breaker as it reaches a position well to the front of the compartment. The operator
is also protected by a deadfront shield, usually a combination of the door and the
faceplate of the circuit breaker. The benefits associated with this design are that a
full view of the circuit breaker front is given along with access to the racking (drawout) device without opening the compartment door.
Figure 13. Three Typical Through-the-Door Drawout Positions of
Low Voltage Power Circuit Breakers in its Compartment
Continuous Current
Rating
The Continuous Current Rating of a circuit breaker is the maximum current rating
the breaker is designed to carry on a continuous basis and remain within the
applicable guidelines for the breaker. It is also referred to as the “Frame Rating” or
the “Frame Size.”
100% Rated
ANSI specifies that low voltage power circuit breakers are to be rated for continuous operation at 100% of their current ratings in their compartment. To meet these
requirements, they are tested for operation within a specific enclosure and, therefore, do not need to be de-rated.
Interrupting Rating
The interrupting rating is the maximum short-circuit current that the circuit breaker
can safely interrupt. ANSI prescribes its minimum preferred ratings for power circuit breakers to meet.
Short Time Rating
The short time rating of a low voltage power circuit breaker is the maximum value
of current the circuit breaker is designed to handle safely for a short period
of time (30 cycles or 0.5 seconds) in the closed position, without damage to the
circuit breaker. This test is repeated twice for a total of one (1) second. The short
time rating is usually equal to the 600 volt interrupting capacity. This attribute is
one of the main features that differentiates a power circuit breaker from
Page 14
Low Voltage Power Circuit Breakers
other types of circuit breakers and allows for system selectivity. The short time
rating was also discussed earlier in this module.
Trip Free
When a circuit breaker is in a Trip Free condition, it cannot, by design, be
closed. Even when intentional efforts are made to close the circuit breaker and it
is in a trip free condition, the main contacts will not touch and the circuit breaker
will automatically return to the tripped position. This is an important safety feature
specific to power circuit breakers.
Current Sensor
Sensor, as used with respect to a circuit breaker, is a common term for a current
transformer which steps current down to useful levels for a specific purpose,
such as providing an input to a trip unit (circuit breaker’s intelligence package).
Page 15
Low Voltage Power Circuit Breakers
Review 1
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. A power circuit breaker has either an interrupting rating or a short time rating.
TRUE FALSE
2. While inside their compartments, most low voltage drawout power breakers
can be in any of the following positions with compartment doors closed:
a. Connected Position
b. _______________ Position
c. _______________ Position
3. Both manually and electrically operated low voltage power breakers use
stored energy mechanisms for opening and closing.
TRUE FALSE
4. The frame rating or the frame size of a low voltage power breaker refers to the
_______________ ________________ rating of the breaker.
5. All low voltage power breakers that meet applicable ANSI Standards are
capable of continuous operation at 100% of their current rating.
TRUE FALSE
6. The _______________ _______________ Rating of a power breaker is one
of the main features that differentiates a power breaker from other types of circuit breakers.
7. ANSI Standards requires low voltage power breakers to have integrally
mounted trip units.
TRUE FALSE
8. One reason a low voltage power breaker utilizes a two-step stored energy
mechanism is so that it is able to provide an ______________________________-_______________ duty cycle.
Page 16
Low Voltage Power Circuit Breakers
Design and
Functional
Considerations
In Module 5, you learned that all circuit breakers have a number of design and
functional characteristics in common:
•
Compliance with Specific Standards
•
Set of Open/Close Contacts
•
Means to Open and Close the Contacts
•
Means to Extinguish an Arc
•
Means to Respond to Overcurrents/Commands
•
Method for Enclosing Circuit Breaker Components
•
Method For Mounting Circuit Breaker
Specific methods used for mounting and using low voltage power circuit breakers
will be covered in the next section. In this section, the concentration will be on how
low voltage power circuit breaker operate to accomplish their tasks and what
accessory items are available to enhance their capabilities.
Basic low voltage power circuit breakers are generally composed of:
•
Frame or Chassis
•
Primary Contacts
•
Arc Extinguishers
•
Operating Mechanism
•
Integral Trip Unit
•
Accessory Items
Let’s take a look at each.
Frame or Chassis
You will recall from Module 5 that all circuit breakers utilize some method to hold
all the parts that make up a circuit breaker, usually called the frame or chassis. A
low voltage power circuit breaker chassis today will be one of two types (Figure 14
and 15):
•
Open Type Metal-Frame (Older Designs)
•
Molded Frame of Engineered Thermoset Composite Resins (Newer Designs)
Figure 14. Molded Frame Construction
Page 17
Low Voltage Power Circuit Breakers
Figure 15. Metal-Frame Construction
The open type metal-frame has a number of pieces welded and/or bolted
together on which the different circuit breaker components are assembled. Components have a tendency to be larger, heavier, and may need to be adjusted.
In the Workplace
The new Magnum DS power circuit breaker utilizes a rigid frame molded from
engineered thermoset composite resins.
Figure 16. Magnum DS Power Circuit Breaker
Molding improves the structural rigidity of the frame, allowing for higher interrupting and short time ratings.
Many individual circuit breaker parts are molded as integral assemblies. This
improves the design by making it smaller and stronger with fewer individual parts,
unlike the metal-frame approach.
Primary Contacts
Primary open/close contacts in a low voltage power circuit breaker provide a
means for isolating or connecting a part of a circuit from or with the rest of the system. The design of the primary contacts is one of the most critical design
considerations relative to the efficiency and overall effectiveness of any low
voltage power circuit breaker. These contacts are used to carry or break the
main continuous load current associated with the system in which the circuit
breaker is applied. Each phase has an associated primary contact. A three-phase
low voltage power circuit breaker, for example, would have three sets of primary
contacts. Keep in mind that primary contacts come in a wide variety of designs
and appearances. All designs do not use the same number of parts nor are all
designs equally efficient. However, all designs act to provide the same general
service.
Low voltage power circuit breaker primary contacts usually have separate arcing
and main current carrying parts. This does not mean that they are necessarily
separate pieces. They could both be part of the same component piece, although
the arcing and main contacts act as individual pieces and perform distinctly different functions.
Page 18
Low Voltage Power Circuit Breakers
In Module 5, the discharge of electric current crossing a gap between two contacts
was discussed (Figure 17). This phenomenon, on a small scale, can occasionally
be observed when pulling a plug from a wall socket.
Figure 17. Electrical Current Crossing a Gap
Arcing also occurs when opening and closing low voltage power circuit breakers
under load, except to a much larger degree. The primary contact design challenge
is to ensure that the arcing is dealt with first to protect the surface of the
main contacts from arc damage. For this reason, primary contacts are mechanically designed such that on closing of the circuit breaker, the arcing contacts
touch (make) before the main contacts. Also on opening of the circuit breaker,
the main contacts part (break) before the arcing contacts. This construction
ensures that arcing takes place on the heat resistant arcing contacts. Usually, primary contacts are replaceable on low voltage power circuit breakers, which can
be needed in time if the operating duty of the breaker is severe enough.
A primary contact assembly is composed of:
•
fixed (stationary) part
•
moving part
A rigid insulating piece through a pushing or pulling motion is used to operate the
moving part of the primary contact assembly.
The fixed and moving main and arcing portions of the assembly can be in any
number of configurations, some more efficient than others (Figures 18 and 19).
Usually the designs for a particular type circuit breaker are the same. The only
variable is the number of parts used to handle the amount of current available.
Larger circuit breaker frames require more and/or larger arcing and main contact
pieces.
Keep in mind that the design goal is to efficiently handle arcing through the heat
resistant arcing contacts so that the main contacts are protected from unnecessary damage. This approach permits the main contacts to be made from low resistance materials, such as silver or silver alloys to minimize the heat developed
during normal operation.
Page 19
Low Voltage Power Circuit Breakers
Figure 18. Contact Assembly Mounted (Front View)
Figure 19. Contact Assembly Mounted (Rear View)
Finally, it was pointed out in Module 5 that some newer low voltage power circuit
breaker designs take full advantage of certain natural facts of physics to assist
with the opening process. You will recall that the concept centers around magnetic
fields established in conductors when current is flowing in the conductors.
The low voltage power circuit breaker design takes full advantage of this electromagnetic force to assist with opening and keeping the circuit breaker closed. In
certain configurations, the force and also the insulator are used to help hold
the contacts closed temporarily during a fault condition, which is where a
power circuit breaker’s short time rating comes from. Circuit breaker designs
taking advantage of this concept can be smaller and lighter and still maintain the
higher withstand (short-time) capabilities associated with low voltage power circuit
breakers. When it is time for the contacts to open, this same force can be used in
the opposite direction to speed the opening action.
Think about the concept of electromagnetic assistance with opening and closing
of contacts in the following fashion (Figure 20). A door could be viewed as the
movable contact. Our super-hero can be considered the rigid insulator used to
push closed or pull open the door (contact). Assistance from the wind (electromagnetic force) in the proper direction would help our super-hero open or keep
the door closed.
Page 20
Low Voltage Power Circuit Breakers
Figure 20. Electromagnetic Force Can Be Used to Help With Opening and Closing
Arc Extinguishers
In Module 5, a number of ways or combinations of ways to extinguish an arc was
discussed. Low voltage power circuit breakers use some type of Arc Extinguishers (arc chutes or arc chambers) mounted above and around the main contacts to
extinguish arcs in air (Figures 21 and 22). This leads to the name low voltage
power air circuit breakers.
Arc chutes, in some form, have been used to extinguish arcs for more than a half
century. The primary purpose of an arc chute is to extinguish an arc each time a
circuit breaker interrupts a current. This is accomplished by confining, dividing and cooling the arc. This accomplished, the arc is not able to sustain itself
through current zero.
Not all arc extinguishers are created equal and, therefore, some are more efficient
than others. Efficiency is very important because the amount of contact damage
caused by arcing is directly related to how fast or efficiently arcs are extinguished.
More efficiency leads to longer contact life.
Figure 21. Top Rear View Magnum DS Circuit Breaker Showing
Arc Chambers and One Arc Chute Removed
Figure 22. One Arc Chute Removed From a Magnum DS Circuit Breaker (Bottom View)
During the arcing process, ionized gases are generated and normally vented, in
some fashion, harmlessly away from the circuit breaker, breaker compartment,
and any operator who might be in close proximity to the equipment. It is also
Page 21
Low Voltage Power Circuit Breakers
known that the high pressure created by these gases, if controlled properly, can
be put to good use during interruption.
To this end, the molded case low voltage power circuit breaker design, for example, utilizes this gas pressure to help with the interruption process while minimizing gas leakage back into the circuit breaker itself. This improvement is
accomplished through the use of seals in the arc chamber and a close fit of pieces
and parts. This can only be accomplished with molded frame designs. Obviously,
the design and process is a bit more involved than just described. For now, the
most important thing to remember is that the original arc extinguisher concept is
still used today, but great strides have been taken to improve upon the original
concept with significant improvements in overall efficiency.
Operating Mechanism
You learned in Module 5 that some type of a mechanism is provided with all circuit
breakers for opening and closing. Low voltage power circuit breakers are no
exception. A low voltage power circuit breaker operating mechanism is composed
of a number of different parts, assemblies and accessories, all dedicated to ensuring that the circuit breaker opens and closes consistently.
The mechanism is a two-step spring charged stored energy type providing
three basic functions:
•
A means to charge the closing springs
•
A means to close/open the circuit breaker using the stored energy of the closing and opening springs
•
A means to perform an Open-Close-Open duty cycle
Two varieties of the mechanism exist:
•
Manual
•
Electrical (Motor Operated)
The manually operated circuit breaker has its closing springs charged manually
through the use of some type of charging handle. The circuit breaker is closed
using a manual close button which is a mechanical process. As the circuit breaker
closes, a set of smaller opening springs are charged. The circuit breaker is
opened using a manual trip (open) button, which is a mechanical process.
Safety interlocks, accessory items and trip units can also cause the circuit breaker
to trip through mechanical means. Most manually operated power circuit breakers
can be equipped with an optional device to electrically release the spring’s stored
energy, thus closing the circuit breaker.
Previously, it was not practical or even possible to convert manually operated low
voltage power circuit breakers to electrically operated circuit breakers in the field.
This is no longer impossible with newer low voltage power circuit breaker designs.
Such designs permit manually operated circuit breakers to be converted to
electrically operated circuit breakers by field installing UL Listed electrical
operators.
Page 22
Low Voltage Power Circuit Breakers
Figure 23. Magnum DS Power Circuit Breaker Being Manually Charged
An electrically operated circuit breaker can be operated every way a manually
operated circuit breaker can be operated. In addition, a small electric motor is normally used to automatically charge the closing springs, and an electrical means to
close or trip (open) the circuit breaker is provided.
Integral Trip Unit
For a circuit breaker to do its job, a means must be provided enabling the circuit
breaker to perform automatically or in response to other commands. In short, the
circuit breaker is a rather dumb device without a brain (intelligence source). This
source of intelligence is the trip unit.
As required by ANSI Standards, low voltage power circuit breakers must be provided with an Integrally Mounted Trip Unit. This means that the trip unit must be
inside of, or part of, the circuit breaker. Prior to the advent of the first solid state
trip unit, electromagnetic type tripping devices, commonly called dual-overcurrent
magnetic trips, were used with all low voltage power circuit breakers. In recent
times, this type of tripping device on low voltage power circuit breakers has disappeared from the scene. For this reason, only microprocessor-based trip units will
be discussed in this module.
A typical microprocessor-based trip unit used with low voltage power circuit breakers offers the following capabilities (Figure 24):
•
Programming
•
Monitoring
•
Diagnostic
•
Communication
•
Testing
Figure 24. Integrally Mounted Trip Units
Page 23
Low Voltage Power Circuit Breakers
The capabilities of a particular trip unit depends on the trip unit design itself and
system requirements. Some trip units can only offer basic features, while others
can offer basic features or, if required by the system, additional sophisticated and
highly advanced features.
The operating response of a trip unit is graphically represented by time-current
characteristic curves. These curves show how and when a particular trip unit will
act for given values of time and current. A characteristic curve is represented by a
band created by a minimum and maximum value of time or current.
The programmable or adjustable features of a trip unit permit movement of its
characteristic curve or parts of the curve (Figure 25). This movement can be done
in both a horizontal and vertical direction. Some trip units even allow the shape of
the curve to be changed.
Figure 25. Typical Trip Curve Horizontal Movement
Most trip units offer protection combinations of:
•
(L) Long delay protection - protection against overloads and short circuits
•
(S) Short delay protection - protection against short circuits
•
(I) Instantaneous protection - protection against short circuits
•
(G) Ground fault protection - protection against ground faults
A trip unit offering all four of these protection at one time is commonly called an
LSIG Trip Unit. Other combinations are also available, such as LI, LS, LSI, LIG
and LSG.
The long, short and ground functions would have programmable values of
current and time. Obviously, instantaneous has no associated time because
the trip is instantaneous (Figure 26). Trip units have these different programmable
features programmed so they coordinate with one another and with the requirements of the system being protected to provide the closest possible system coordination and protection against all eventualities. This coordination discipline is
where you start hearing phrases like curve shaping and close coordination. No
attempt will be made in this module to get into the details of this discipline. It is
quite specialized and best left to individuals with the proper training.
Page 24
Low Voltage Power Circuit Breakers
Figure 26. Advanced Trip Unit Time-Current Curve Adjustments
More advanced trip units are able to monitor and display currents, energy,
power, power quality and power factor. They also may be able to diagnose
problems and provide advance warnings of potential problems, such as harmonics. Two way communications for remote monitoring and control is also
available. This affords the user a cost effective way to monitor and control expansive, multi-location facilities with a wide array of protective equipment and operational machinery.
Trip and no trip tests can usually be performed on the trip unit and circuit
breaker utilizing integral testing capabilities or separate test kits. Normally, the
tests can be performed with the circuit breaker in service and full protection provided during the testing. This type of testing is secondary testing. Primary testing
involves specialty testing equipment and a testing expertise, and is not discussed
in this module.
Accessory Items
Accessories used with low voltage power circuit breakers are usually added to the
circuit breaker to provide additional features, such as status indication and remote
operation. It is possible, however, that some accessories for some circuit breaker
designs might be mounted remotely from the circuit breaker. These devices might
be totally mechanical, totally electrical or a combination. The intent here is to
briefly discuss the function of commonly used accessory items, although all low
voltage power circuit breakers do not necessarily offer all of the devices being discussed, nor is this list all inclusive.
•
Electrical Operator - This is an assembly of devices including a small spring
charging motor that when added to a manually operated circuit breaker converts it to an electrically operated circuit breaker. This allows for remote
operation (open/close) of the circuit breaker. The ability to field install this
device is more common with newer low voltage power circuit breakers. Power
circuit breakers normally use to be either manual or electrical by design, and
could not be easily converted.
•
Operations Counter - An operations counter is a counting device, usually
linked in some fashion to the operating mechanism. It is used to count the
open and close operations of the circuit breaker, and serves as a maintenance aid.
•
Auxiliary Switch - An auxiliary switch consists of “normally open” (NO) and
“normally closed” (NC) contacts (Figure 27). The contacts on some
switches are convertible from NO to NC and vice versa. The contacts are frequently referred to as “a” or “b” contacts. The “a” being open when the circuit breaker is open and the “b” closed when the circuit breaker is open.
In short, these auxiliary contacts change “state” when the circuit breaker main
contacts change “state.” An auxiliary switch is normally mounted on the circuit
Page 25
Low Voltage Power Circuit Breakers
breaker. Contacts from these switches are frequently used for electrical operation of a circuit breaker, remote signaling, and electrical interlocking.
Figure 27. Graphical Representation of a Four Contact Auxiliary Switch (2A and 2B)
•
Undervoltage Release (UVR) - An undervoltage release, normally a circuit
breaker mounted electromechanical device, trips the circuit breaker when the
voltage falls below a predetermined level.
•
Shunt Trip (ST) - A shunt trip is an electromechanical device which is standard on most electrically operated power circuit breakers. When added to a
manually operated circuit breaker, it provides for remote controlled electrical tripping.
•
Spring Release (SR) - The spring release device is standard on most electrically operated power circuit breakers. When added to a manually operated
circuit breaker, it permits the circuit breaker to be closed electrically from
a remote location.
•
Bell Alarm (OTS) - The bell alarm, frequently called an overcurrent trip
switch (OTS) on a power circuit breaker, is normally circuit breaker mounted.
Its function is to provide a signal to indicate that the circuit breaker has
tripped open automatically (trip unit command). It will not operate if the
circuit breaker is tripped open by other means, such as the use of a manual
trip button, an electrical control switch, or the operation of an undervoltage
release device.
•
Locking Devices - Low voltage power circuit breakers normally have a wide
array of mechanical locking devices to prevent unauthorized circuit
breaker operation (Figure 28).
Figure 28. Padlock Shown Mounted on Front of Molded Frame Type Power Breaker
Preventing Unauthorized Use of Open and Close Buttons
•
Mechanical Interlocks - These devices provide a way to mechanically interlock two circuit breakers. A typical use for such a device is to prevent one circuit breaker from closing while another circuit breaker is already closed.
•
Capacitor Trip Device - A capacitor trip device is normally mounted externally from the circuit breaker. It uses a small storage capacitor for AC control
Page 26
Low Voltage Power Circuit Breakers
of the circuit breaker to ensure reliable tripping power during fault conditions.
•
Lifting Device - Because some low voltage power circuit breakers can be sizable and heavy, a variety of devices is usually available to lift and move the
circuit breaker once it is out of its compartment (Figure 29).
Figure 29. Rail Mounted Lifting Device Being Used to Lift a Magnum DS Power Circuit Breaker
From Its Compartment Extension Rails
•
Truck Operated Cell Switch (TOC) - A TOC switch is usually mounted in the
circuit breaker compartment and is activated by movement of a drawout
circuit breaker into and out of the “Connected” position. As the circuit
breaker moves, the contacts are activated providing a means for remote indication of the circuit breaker’s position.
Page 27
Low Voltage Power Circuit Breakers
Review 2
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. For many years low voltage power circuit breakers were open type metalframe circuit breakers. Today, newer low voltage power circuit breaker
designs are _________________ frame type designs.
2. When a low voltage power circuit breaker opens, its arcing contacts part
before the main contacts part to draw any arcs formed away from the main
contacts.
TRUE FALSE
3. Low voltage power circuit breakers are considered air circuit breakers and
use _______________ _______________ to eliminate the arc by confining,
dividing and cooling it.
4. The operating mechanism springs of both manually operated and electrically
operated low voltage power circuit breakers can be charged manually.
TRUE FALSE
5. A low voltage power circuit breaker’s source of intelligence is its
_______________ _______________.
6. Current transformers used in a low voltage power circuit breaker to monitor
and reduce the current to useful levels are also known as _______________.
7. Time-current characteristic curves graphically represent the operating
response of the _______________ ______________.
8. Circle the type or types of protection from the four types listed below that offer
some degree of protection against short circuits.
(a) Long Delay Protection (b) Instantaneous Protection
(c) Short Delay Protection (d) Ground Fault Protection
9. An auxiliary switch is graphically represented below. On the graphic, label
each of the four contacts as either “a” or “b” type contacts.
10. A _______________ _______________ is an electromechanical device used
to provide for remote controlled tripping of a manually operated low voltage
power circuit breaker.
11. A bell alarm on a low voltage power circuit breaker, also referred to as an
overcurrent trip switch, provides a signal to indicate when a circuit breaker
has tripped open for any reason.
TRUE FALSE
12. A TOC switch is activated by movement of a circuit breaker into and out of the
“Connected” position. Indicate next to each letter below what word the letters
represent.
T _______________ O _______________C _______________ Switch
Page 28
Low Voltage Power Circuit Breakers
Mounting Methods
As briefly discussed earlier, low voltage power circuit breakers are usually available in the two following mounting configurations:
•
Fixed
•
Drawout
Total usage of low voltage power circuit breakers today is dominated by the drawout configuration because it provides for easier maintenance and continuity of service. Most circuit breaker manufacturers, however, offer both types.
Fixed Circuit Breaker
Fixed low voltage power circuit breakers usually have fixed primary conductor
stabs protruding from the rear of the circuit breaker. The circuit breaker is bolted in
position within its assembly compartment, and the rear conductor stabs are bolted
to primary bus connections (Figure 30). Secondary connections are also made
manually. Power must be turned off to the assembly to connect a fixed circuit
breaker into the system or to remove it from the system.
Figure 30. Fixed Type DSII Circuit Breaker
Drawout Circuit Breaker
Drawout low voltage power circuit breakers have a levering device to move the
circuit breaker from one compartment position to the next. Usually part of the
levering mechanism is on the circuit breaker with a corresponding part is in its
compartment. Working together, they provide the mechanical means required to
move the circuit breaker. Drawout circuit breakers are designed to be removed
and connected without cutting power to the entire assembly under load conditions
because the circuit breaker, by design, automatically opens before racking can
take place. This means that power to the assembly does not have to be turned off
when a circuit breaker is removed from or inserted into the assembly, thus ensuring continuity of service.
Drawout circuit breaker compartments are provided with extension rails which,
when not in use, are stored inside the compartment (Figure 31). The extension
rails provide a means by which a drawout circuit breaker can be easily removed
from its compartment for inspection, maintenance or movement to another area.
Figure 31. Drawout Molded Case Circuit Breaker Shown on Compartment Extension Rails
Page 29
Low Voltage Power Circuit Breakers
Primary electrical connections between the circuit breaker and the primary bus
are automatically made or broken as the circuit breaker is moved into or out
of the “Connected” position within the circuit breaker compartment. Primary
connectors mounted to the back of the circuit breaker slide onto the primary bus
connectors. These primary connectors, often called finger clusters or disconnect contacts, are frequently composed of a number of spring loaded fingers
(contacts) (Figure 32). The number of fingers (contacts) used is dictated by the
amount of current they will carry. Fingers (contacts) are made of an excellent conducting material or material combination, such as silver plated copper.
Figure 32. Rear View of Magnum DS Power Circuit Breaker Showing Six Primary Finger Clusters
Secondary electrical connections are usually automatically made or broken as the
circuit breaker is moved into and out of its compartment. As the circuit breaker is
moved into the “Test” position from the “Disconnect” position, the secondary connections are made providing the required secondary power for testing or
operating the breaker, but no primary power. The secondaries remain connected
as the breaker moves into the “Connected” position. When the circuit breaker
is moved out of the “Connected” position, the secondaries remain connected and
stay connected until the circuit breaker is moved farther out of its compartment
past the “Test” position. The graphics of the four positions presented earlier in the
module demonstrate the movement and connections.
Governing
Standards
You will recall from Module 5 that circuit breakers are designed, built and tested in
accordance with one or more specific sets of standards. In this module, you will be
introduced to the standards specific to low voltage power circuit breakers. The
intent here is not to present and study the different applicable standards word for
word. That type of undertaking would be a course unto itself. Our goal is to understand a little about low voltage power circuit breaker related standards, where
they were, and where they are today.
Figure 33. Many Standards
Page 30
Low Voltage Power Circuit Breakers
You will hear many people repeat specific standards designations. Many of those
same people do not have an intimate knowledge of what the standards actually
say, nor are we saying they should. The actual product selection based on standards compliance should be left to the experts. It is helpful, however, to know what
specific standards your products comply with and what general topic a specific
standard addresses.
Keep in mind that a standard exists for almost everything. There are specific standards for circuit breakers and others for the structural assembly. Compliance with
these exacting standards ensures customers of the very best possible product
selection with a high degree of comfort. There is no room for compromise
when performance, quality and safety are involved.
A number of years ago, low voltage power circuit breakers and most other types
of equipment were designed and built primarily with only domestic standards in
mind. This approach also was taken by foreign suppliers. A manufacturer would
offer a circuit breaker designed, tested and manufactured in keeping with applicable standards for that part of the world or even particular country. Trying to play a
significant role in other world markets was, at best, extremely difficult. If manufacturers today expect to be global participants, they must offer products that comply
with the standards applicable to a variety of different markets around the world.
This will require that you become familiar with both domestic and international
nomenclature, ratings, procedures and governing standards. The task is greater,
but so is the reward.
Some of the lines separating different types of low voltage circuit breakers were at
times blurred in the past. Low voltage metal-frame power circuit breakers were
built and tested to certain ANSI and UL specifications, while some low voltage
encased circuit breakers were tested to UL specifications specific to molded case
circuit breakers. The newest low voltage power circuit breakers today, like Magnum DS, are tested to specific low voltage power circuit breaker standards, like
ANSI. They are also tested to standards that cover a much broader product
scope, like IEC. The primary testing standards associated with low voltage power
circuit breakers today are:
ANSI
The American National Standards Institute’s ANSI C37.50 is a specific North
American testing standard entitled “Low Voltage AC Power Circuit Breakers Used
In Enclosures.” This standard specifies rigorous tests for product performance.
There are additional C37 standards which govern power circuit breaker and trip
unit construction, such as C37.13 and C37.17 respectively.
UL
Underwriter’s Laboratories Incorporated’s UL1066, for the most part, calls for testing to demonstrate compliance with ANSI C37.50 just mentioned. A UL Label is
affixed to the breaker to indicate successful compliance.
IEC
The International Electrotechnical Commission IEC 947-2 is a more general international testing standard covering a variety of devices, including circuit breakers
of all types, and is entitled “Low Voltage Switchgear and Controlgear.”
Closing Comments on
Standards
Before concluding this section, it might help to minimize confusion if you remember that there is often a great deal of referencing to other standards that takes
place within a specific standard. Successful testing with respect to one standard
often includes automatic compliance with other standards.
Example 1 - ANSI C37.13 details the physical attributes, such as stored energy,
that a low voltage AC power circuit breaker must have, while ANSI C37.50 references C37.13 and details how the described breaker should be tested. The key
Page 31
Low Voltage Power Circuit Breakers
here is that successful testing in keeping with ANSI C37.50 brings with it compliance to C37.13.
Example 2 - In a similar fashion, IEC 947-2 references IEC 947-1 (General
Rules). Compliance with IEC 947-2, therefore, brings with it IEC 947-1 compliance.
Testing
The testing required and the standards that must be met by a low voltage power
circuit breaker depend on the area of the world where the circuit breaker is
applied. To play a major international role, low voltage power circuit breakers
should be able to meet the requirements of ANSI, UL and IEC (Figure 34).
Figure 34. Dominant Worldwide Standards
As you continue through this module, you will become well aware that the
required testing is the key factor in defining the type of circuit breaker. In a very
general and simplistic way, low voltage power circuit breakers undergo a
sequence of four rigorous tests.
1. The first sequence consists of a temperature rise, an overload, and a shortcircuit test.
2. The second sequence is a series of short-circuit tests.
3. The third sequence is an endurance test.
4. The fourth is a momentary rating test.
Molded case circuit breakers, for example, are subjected to tests similar to numbers 1, 2 and 3. The fourth test sequence, momentary rating test, is specific to
power circuit breakers and to some IEC molded case circuit breakers.
Specific testing details will not be covered in this module. It should be pointed out,
however, that the momentary rating test just mentioned (test sequence 4) subjects
the circuit breaker to tremendous physical forces and severe heating effects. Very
simply speaking, the circuit breaker is subjected to its full short time current rating
for two (2) time periods up to 30 cycles each. The short time rating indicates
what magnitude of current the circuit breaker can stand with its contacts
closed for a short period without being damaged. The circuit breaker’s short
time rating is often equal to its 600 volt interrupting capacity. A low voltage power
circuit breaker must be strong enough to survive this test and function properly
after completion.
Page 32
Low Voltage Power Circuit Breakers
Helping the
Customer
Selection of the proper low voltage power circuit breaker for a specific application
is not a difficult process. There are some important questions, however, you must
be able to answer. Fortunately, the most difficult part of the job has already been
done by other qualified individuals when they determined the requirements of the
system.
This includes determining things like:
•
Circuit Breaker type required
•
Application voltage
•
Maximum fault current system could see
•
Continuous current for the system and each branch
•
System frequency
•
Types of trip unit capabilities
•
Programmable functions
•
Accessory needs
•
Mounting configuration
•
Special requirements
Your job is to make sure these types of questions are answered. The more familiar you are with what a particular circuit breaker line has to offer, the easier the
task. Let’s start by looking at what circuit breaker manufacturers do to help.
Manufacturers normally provide a great deal of assistance in the way of printed
material, computer accessible information and direct contact. This does not mean,
however, you should not put forth that extra effort to know personally what is available. Learn to use all the information provided.
Most selection factors fall into one of two categories:
Standard Selection
Factors
•
Standard selection factors
•
Special selection factors
Standard selection factors normally are associated with the circuit breaker’s ratings/standards, operation method, accessory items, and how the breaker will be
mounted. The most common points to consider will be discussed.
1. Standards - Applicable standards were discussed in this module and earlier
modules. You should be told or it will appear in a written specification what
standards the circuit breakers must meet. Newer low voltage power circuit
breakers meet a wide array of standards which will make them acceptable in
most parts of the world. In addition, make sure you are aware of any special
local requirements and/or standards.
2. Ratings - This is a critical part of the selection process. You should already
know what is required. Now you must determine what specific circuit breakers
will meet the rating requirements. Manufacturers normally provide easy to
read tables outlining the ratings of every circuit breaker frame. Keep in mind
there could be more than one table. This is especially true for newer circuit
breaker designs that meet both ANSI and IEC requirements. A manufacturer
might choose to present it as one combined table or two tables. If you know
what is required, you will be able to make a selection from the tables under
normal circumstances.
Page 33
Low Voltage Power Circuit Breakers
ANSI Example
Let’s take a look at a typical type ANSI table for a low voltage power circuit
breaker and see what it has to offer (Figure 35). The table used in this example
will not cover every circuit breaker rating for a particular design.
Enough of the table is presented to give you a good working knowledge on how to
proceed. Each area of the table that is discussed is identified by a circled letter to
simplify the discussion. One last important point should be made before beginning. Always read footnote references carefully. They provide important information and could be critical to the proper selection.
Figure 35. Example ANSI Ratings Table
A: The “Breaker Type” is usually the name given to the circuit breaker by the manufacturer along with some general information about the ratings of that specific circuit breaker type. In the example ratings table shown, XYZ-508 is the first circuit
breaker listed. The XYZ is the circuit breaker’s name. The first number “5” gives
you a general idea what the interrupting rating is of the XYZ-508 circuit breaker at
a voltage of 480 volts. This is a common presentation method because the widest
used application voltage domestically is 480 volts. The last two numbers, “08” in
this case, usually tell you the maximum continuous current rating of the circuit
breaker. XYZ-508 can, therefore, carry 800 amperes continuously, and interrupt
50,000 amperes at 480 volts.
B: This column outlines specifically the maximum continuous current the circuit
breaker will carry. Notice that circuit breaker type XYZ-616 in the example table
will carry a maximum continuous current of 1600 amperes. Notice also that the
last two numbers of the circuit breaker type XYZ-616 (“16”) give you the same
information, with 16 meaning 1600. Take the time to make this same comparison
with circuit breaker type XYZ-632.
C: Notice that the rest of this table is devoted to the interrupting capabilities in
amperes of the different circuit breaker types at different application voltages. Also
notice that the application voltages listed are:
•
208-240 volts
•
480 volts
•
600 volts
The nominal voltage range for the ANSI market is 208 to 600 Volts AC at a frequency of 50 or 60 hertz. Get comfortable with seeing these voltages when talking about ANSI rated low voltage power circuit breakers.
D: You will notice that these two columns are labeled differently. The first column
entitled “With Instantaneous Trip” outlines the interrupting capabilities of each
circuit breaker frame at the different application voltages. These ratings are applicable when the circuit breaker’s trip unit provides instantaneous protection.
Page 34
Low Voltage Power Circuit Breakers
In other words, the circuit breaker can be applied to safely handle faults of the
magnitudes shown.
You will also notice in the column entitled “Without Instantaneous Trip” that
some of the interrupting ratings are somewhat lower than the left column under
208-240 volts. These ratings are the magnitudes that the circuit breaker can tolerate safely for a short delay period of time (30 cycles) before opening at the
short delay current ratings shown. This might sound like a contradiction. It
really is not for a number of reasons. Consider the following points.
1. You will recall from material presented earlier that a low voltage power circuit
breaker’s short time rating is normally the same as its interrupting rating. The
key word here is normally. The partial ratings table being considered here
already indicates that there are some very limited times when a low voltage
power circuit breaker could have a higher interrupting rating if it has instantaneous protection versus just short time protection and no instantaneous. This
was probably the result of a conscious decision to meet some very specific
application requirement for a particular customer or industry, knowing the fact
that a circuit breaker had to have instantaneous to be applied at these somewhat higher ratings.
2. The fact still remains that low voltage power circuit breakers must be and are
only applied in keeping with their nameplate rating. This, in almost all cases,
shows the interrupting rating and the short time rating to be the same. When
electrical systems are being considered, fault calculations are done to determine the maximum fault current a system can experience. Low voltage
power circuit breakers are then selected with ratings that are able to deal
successfully and safely with the worst case fault scenario calculated. In
other words, if a low voltage power circuit breaker with an adequate short
delay current rating is applied, it will stay closed for the appropriate short time
no matter what. This is true because it will not see (experience) more that it
was designed to safely handle. End of that part of the story.
3. On the other hand, a low voltage power circuit breaker, which is already in
the open position, will trip open instantaneously if an attempt is made to
close the breaker on an existing fault. This safety feature prevents damage
that could result from closing on a fault. Today, this feature is normally accomplished through circuitry which is part of the trip unit. This self protecting circuitry is often called a discriminator circuit or may be called a making current
release in newer designs like Magnum DS. Its purpose has nothing to do with
a circuit breaker that is already closed and functioning.
For now, how this feature is technically accomplished will not be discussed. Just
be aware that such a feature exists with low voltage power circuit breakers. Future
training material specific to a particular low voltage power circuit breaker design
will discuss just how it is accomplished.
Remember:
•
Low voltage power circuit breakers are applied at their nameplate ratings.
•
Low voltage power circuit breakers are sized and selected for application to
handle the maximum fault that could be encountered where they are applied.
•
Low voltage power circuit breakers are provided with a means to trip (open)
instantaneously if they are closed in on an existing fault.
E: Let’s just briefly take a look at the footnote. It tells you that these ratings are
also the short time ratings of the circuit breaker. The material in D was discussed
Page 35
Low Voltage Power Circuit Breakers
as though we already knew these were short time ratings, and we did. Suppose
we did not know that fact and failed to read the footnote. We would not be as
informed as we should be for the proper circuit breaker selection. It could be like
making the selection blindfolded. Be sure to read the footnotes.
IEC Example
ANSI and IEC Example
IEC Example - An IEC example similar to the one just presented will not be
offered here. Ratings tables and their appearance as to how data is presented can
change from country to country and even manufacturer to manufacturer. The
information presented, however, is usually similar. You should be aware of some
of the noticeable differences in the presented data, and start now to become
familiar with IEC rated breakers. For now, consider the following to get started:
•
The voltage range for the international market is 380 through 690 Volts AC
at a frequency of 50 or 60 hertz.
•
The general continuous current range for low voltage power circuit breakers is
800 through 6300 amperes.
•
The voltage and current abbreviations and names are different, such as:
Ue – application voltage, such as 380 or 690 volts.
In – rated current such as 800 or 6300 amperes.
Ics – rated service short circuit breaking capacity.
Icu – rated ultimate short-circuit breaking capacity.
Icw – rated short time withstand current (similar to the ANSI short time rating
and the circuit breaker is expected to function properly again after having
dealt with the Icw).
Let’s make a quick comparison from a presentation standpoint. Keep in mind, the
important things are:
1. Will the circuit breaker being considered do the job?
2. Will the circuit breaker being considered meet the standards in effect where
the circuit breaker is to be used?
It is not possible to simply take a product designed and tested to one standard
(ANSI or IEC) and certify it to the other standard. A manufacturer must undertake
a concerted design effort to satisfy both standards individually.
Breaker Frame (A)
Rated Voltage (V)
Interrupting (kA)
IEC 800 - 2000
415
690
480
600
40 or 65 or 130
40 or 65 or 85
42 or 100 or 200
42 or 100 or 200
ANSI 800 - 1600
This is by far not an all inclusive example. It is only intended to draw some simple
ANSI and IEC comparisons between some of the most common selection points
that must be considered when selecting low voltage power circuit breakers. You
can see that although not exactly the same, it is primarily a matter of familiarization.
3. Operation Method - As discussed earlier, low voltage power circuit breakers
are either manually or electrically operated. You must always specify the
method of operation. At some point, you will need to know the secondary control voltage being used for an electrically operated circuit breaker. Even if the
circuit breakers are manually operated, it is a good idea to find out the sec-
Page 36
Low Voltage Power Circuit Breakers
ondary control voltage. The control voltage is necessary for the final selection of a number of items, not just electrically operated circuit breakers.
4. Accessory Items - Many of the common accessory items associated with low
voltage power circuit breakers were discussed earlier. You have to be alert for
these items in a specification or ask the customer if any are required. A determination can then be made if a compatible accessory is available to meet the
need. Make a list of the required accessories and the specific requirements that are appropriate for them, such as control voltage, number and
types of contacts and overall function.
5. Mounting Method - You will need to know whether the breakers will be fixed
mounted or drawout. Always check to see if there are any special requirements for either configuration.
Special Selection
Factors
There may not be special conditions to consider, although this should be determined as soon as possible. You may be able to deal with certain special conditions and others might call for assistance from the manufacturer. Do not hesitate
to ask for help. Some conditions or requirements to look for that might not be considered standard are:
•
High or low ambient temperatures
•
Moist or corrosive atmospheres
•
Altitude
•
High shock conditions
•
Unusual circuit breaker mounting conditions
Page 37
Low Voltage Power Circuit Breakers
Review 3
Answer the following questions without referring to the material just presented.
1. Of the two most common low voltage power circuit breaker mounting methods, the fixed configuration is most commonly used because it is less expensive.
TRUE FALSE
2. When a drawout type low voltage power circuit breaker is in the “Connected”
position, both _______________ and _______________ electrical connections are made.
3. If you were called upon to select a low voltage power circuit breaker, name at
least five selection factors, standard or special factors, you might consider
during the selection process.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
4. In a low voltage power circuit breaker ratings table, the “Frame Size” indicates
the circuit breaker’s interrupting rating in amperes.
TRUE FALSE
5. The nominal voltage range for low voltage power circuit breakers in the
domestic market governed by ANSI Standards is _______________ to 600
Volts AC.
6. In terms of the international market governed by IEC Standards, Ue stands for
the _______________ _______________, such as 415 volts.
7. In today’s global market, standards are usually so similar that a low voltage
power circuit breaker designed and tested to IEC Standards can simply be
certified to ANSI Standards and vice versa, without the need for additional
testing.
TRUE FALSE
8. A low voltage power circuit breaker is always provided with instantaneous
protection against closing in on a fault current.
TRUE FALSE
9. Low voltage power circuit breakers are sized, selected, and applied to safely
handle the _______________ fault current it could possibly be exposed to.
10. Circle the letter next to the testing standard that most influences the design
and testing of low voltage power circuit breakers used in the United States.
(a) ANSI C50.51
(b) UL1866
(c) IEC 947-2
(d) ANSI C37.50
Page 38
Low Voltage Power Circuit Breakers
Advanced Low
Voltage Power
Circuit Breakers
The Magnum DS Family of low voltage power circuit breakers is not an extension
of any other low voltage design (Figure 36). It is at the forefront of technology and
development. For this reason, it is an excellent design to discuss when certain
specific examples are required in this module. Keep in mind, however, all low voltage power circuit breakers do not necessarily offer as many features or use the
same advanced technology as Magnum DS. Even though this might be the case,
it does not mean that another design does not qualify as a low voltage power circuit breaker, or that there are not other capable low voltage power circuit breakers.
Figure 36. Magnum DS Family of Low Voltage Power Circuit Breakers 2 Frame Sizes
800 Through 5000 Amperes)
Magnum DS is a low voltage power circuit breaker. It is built and tested to all
applicable ANSI Standards for low voltage AC power circuit breakers and Underwriter’s Laboratories Listed. Because of its flexible design, an International Electrotechnical Commission rated version of Magnum DS is also available to
address international requirements. This IEC version is called Magnum. Everything that is expected of an ANSI rated low voltage power circuit breaker is delivered by Magnum DS, and then some. If you think this sounds a bit biased, it is.
Eaton Electrical is justifiably proud of Magnum DS, and as you learn more and
more detailed information about low voltage power circuit breakers, you will most
certainly agree.
You will recall a discussion of the areas that might set a low voltage power circuit
breaker apart from other types of low voltage circuit breakers. Namely:
•
Method used to make and break circuits
•
Ratings
•
Construction/Maintainability
•
Integrally Mounted Trip Units
•
Operating Mechanisms
•
Testing
You were initially introduced to the primary factors that make the low voltage
power circuit breaker unique. Other factors and methods that were rather common with low voltage power circuit breakers but not necessarily unique were also
discussed. You will revisit a number of the areas just mentioned. Each topic discussed, however, will be presented in more detail with special emphasis placed
on those factors that set the low voltage power circuit breaker apart from other
types of low voltage circuit breakers, such as molded case and insulated case circuit breakers. The general topics to be discussed are:
•
Standards and Testing
•
Construction Methods
Page 39
Low Voltage Power Circuit Breakers
•
Ratings and Performance
•
Operational Techniques
•
Integral Trip Unit
•
Applications
•
Low Voltage Power Circuit Breaker Summary
The last section reiterates many of the facts learned with special attention given to
the unique factors associated with low voltage power circuit breakers. This summary can serve as a review and a future quick reference.
Standards and
Testing
The standards that are applicable to low voltage power circuit breakers and the
testing involved to prove compliance by a specific low voltage power circuit
breaker design are in Module 5 and previously in this module. You learned from
those discussions that standards and testing go to the heart of the matter. This is
true from three very important standpoints:
1. This is the industry’s determination as to whether or not a particular circuit
breaker design is capable of meeting a wide range of published operational
and physical requirements.
2. The proven and stated compliance to specific standards tells potential users
that the equipment from the manufacturers under consideration all meet certain basic standards, which makes the user’s evaluation process much simpler. Once this determination is made, a particular manufacturer can still gain
an evaluated advantage by offering additional unique features and/or an operational design approach preferred by the user.
3. It is a solid way of defining specific types of circuit breakers within a larger
general grouping. For example: The larger general grouping is “Low Voltage
Circuit Breakers.” Specific types within the Low Voltage Circuit Breaker grouping would be “Low Voltage Power, Insulated Case, Molded Case and Miniature.”
As you can see, when a specific type circuit breaker is specified, such as a low
voltage power circuit breaker, the specifier already knows what the base expectations are from each manufacturer.
You will recall, from both Module 5 and previous sections, a map of the world
showing the standards most influential in different parts of the world (Figure 37). It
bears revisiting the map again to emphasize the importance, in today’s global
economy, of having flexible designs capable of complying with all major standards
around the world. In this module the emphasis will be primarily on ANSI and IEC
Standards. You should never lose sight of the fact, however, that there are a number of other standards that can play a critical role in determining what equipment
is acceptable for application in a given area of the world. Even local and/or individual city codes and requirements may have to be considered.
Page 40
Low Voltage Power Circuit Breakers
Figure 37. Most Influential Standards Worldwide
In previous modules, references other than ANSI and IEC were made with respect
to standards and testing, such as UL, IEEE. There is a strong relationship
between ANSI, UL and IEEE. As a matter of fact, you will notice in manufacturer’s
publications for low voltage power circuit breakers and even low voltage metal
enclosed switchgear references made to all. The following two samples are typical statements you might encounter when reading publications for both the power
circuit breaker and the metal enclosed switchgear:
Typical Low Voltage Power Circuit Breaker Statement: “Type XYZ low voltage
power circuit breakers are UL listed, and built and tested to applicable NEMA,
ANSI, IEEE and UL standards (ANSI C37.50, C37.13, UL 1066).”
Typical Low Voltage Metal Enclosed Switchgear Statement: “Type XYZ low
voltage metal enclosed switchgear conforms to NEMA SG3, NEMA SG5, ANSI
C37.20.1, ANSI C37.51 and UL1558.”
It may seem to you like a confusing web at this point. Once the relationship is
understood, it will be clear as to why these references are made. There will be no
detailed discussion of the standards relating to low voltage metal enclosed in this
module, only those relevant to the power circuit breaker. Keep in mind, however, it
works the same way. The standards state different requirements for the different
pieces of equipment, but the intent is the same - an uncompromised piece of
equipment with proven performance capabilities.
For the purpose of this section, let’s identify the key players as a minimum and
elaborate on a couple. This should not be considered as a substitute for the standards themselves. For a full explanation of any standard, consult the standard
itself for details and proper conformance instructions.
IEEE (Institute of
Electrical and Electronic
Engineers)
•
IEEE is an objective technical organization made up of manufacturers, users,
and other general interest parties.
•
IEEE defines technical definitions, technical requirements, temperature limits,
altitude correction, insulation limits, and service conditions. For electrical
equipment, including switchgear, it supplies the test requirements for the low
voltage power circuit breaker construction and test standards, namely ANSI
C37.13 and ANSI C37.50.
NEMA (National
Electrical Manufacturers
Association)
•
NEMA is an electrical equipment manufacturer only organization, such as
Eaton Electrical, General Electric, and Square D. NEMA defines preferred ratings, related requirements, and application recommendations.
•
NEMA Standards normally cover additional information about a product of
specific interest to the manufacturing community, which the American
Page 41
Low Voltage Power Circuit Breakers
National Standards Committee does not include in its scope. NEMA votes on
the suitability of standards for ANSI designation and adopts, by reference, the
appropriate American National Standards.
•
The applicable low voltage power circuit breaker NEMA Standard is SG-3,
and it adopts ANSI C37.16 in its entirety.
•
UL is an independent, non profit, third party testing and certification company
headquartered in Northbrook, Illinois. It functions to develop standards and to
insure that equipment meets relevant published standards.
•
UL also adopts otherwise recognized standards, and, in some instances,
develops their own independent certification tests. In the case of low voltage
power circuit breakers, the UL Standard is UL1066, which was previously
mentioned. UL1066, entitled “Standard for Low Voltage AC and DC Power
Circuit Breakers Used in Enclosures,” calls for testing to demonstrate compliance with ANSI/IEEE C37.13 without change. A UL Label is affixed to the circuit breaker to indicate successful compliance.
CSA (Canadian
Standards Association)
•
The Canadian Standards Association is in the category of a major international standard. Its design and testing requirements are essentially the same
as required by UL. In fact, harmonization programs between UL and CSA are
ongoing to close the gap and/or eliminate differences. The Canadian Standards Association standard most associated with low voltage power circuit
breakers is CSA 22.2-31 for Switchgear Assemblies.
ANSI (American National
Standards Institute)
You were briefly introduced to ANSI. Now let’s take the time to get to know ANSI
much better because ANSI is the key to low voltage power circuit breakers. It is
the recognized North American Authority on equipment standards.
UL (Underwriters
Laboratories Inc.)
ANSI’s Purpose - ANSI is a nonprofit, privately-funded membership organization
that coordinates the development of U.S. voluntary national standards, called
American National Standards. It is also the U.S. member body to the non-treaty
international standards bodies, such as the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). ANSI
serves both the private and public sectors’ need for voluntary standardization.
ANSI’s History - The institute was founded in 1918. It was prompted by the need
for an umbrella organization to coordinate the activities of the U.S. voluntary standards system and eliminate conflict and/or duplication in the development process. The institute serves a diverse membership of over 1300 companies, 250
professional, technical, trade, labor and consumer organizations, and some 30
government agencies.
A simple yet very typical example of why ANSI came into existence can be related
to the low voltage power circuit breaker. In the early days of low voltage power circuit breaker development, manufacturers and users were building and applying
equipment with little thought given to uniform performance or design standardization. The C37 standard was developed and implemented to establish minimum
performance standards for the circuit breaker and its physical design features.
The standard was meant to address even the smallest detail. A close button, for
example, might not say close on it or it varied in color from one manufacturer to
the next. These inconsistencies in design made products confusing for use by
customers. This might seem to be one trivial point, but you can imagine how big
the problem would be when compounded with every aspect of a low voltage
power circuit breaker.
ANSI’s Functions - ANSI functions to:
Page 42
Low Voltage Power Circuit Breakers
•
Coordinate the self-regulating, due process consensus voluntary standards
system
•
Administer the development of standards and approve them as American
National Standards
•
Provide the means for the U.S. to influence development of international and
regional standards
•
Disseminate timely and important information on national, international and
regional standards activities to U.S. industry
These standards are intended to provide guidance, direction, and requirements.
Compliance to these standards does not, nor is it meant to limit manufacturers in
construction, materials, or the technology used.
Specifically relating to power circuit breakers, ANSI standards are written by
either the IEEE Switchgear Committee or NEMA. The electrical standards written
by both of these organizations are reviewed and clarified by the Accredited Standards Committee (ASC) for power switchgear and power circuit breakers. The
ASC standards group is entitled C37.
ANSI Defined Standards for Low Voltage Power Circuit Breakers - Although
there are a multitude of ANSI standards relating to many different types of equipment, only those standards relating to low voltage power circuit breakers are outlined here. The intent is just to make you aware of just how many are applicable to
just one category of electrical equipment. You will notice that each standard is followed by a specific year. As additions or changes are made to a standard, the
year is altered to indicate the latest version. Obviously, staying on top of the latest
version is an ongoing process. You should also note that each standard is given a
broad word definition.
1. ANSI/IEEE C37.13-1990, “Low Voltage AC Power Circuit Breakers Used in
Enclosures”
2. ANSI C37.16-1997, “Preferred Ratings Related Requirements and Application Recommendations for Low Voltage Power Circuit Breakers and AC
Power Circuit Protectors”
3. ANSI C37.17-1997, “Trip Devices for AC and General Purpose DC Low Voltage Power Circuit Breakers”
4. ANSI C37.50-1989, “Test Procedures for Low Voltage AC Power Circuit
Breakers Used in Enclosures”
5. IEEE Standard C37.100-1992, “IEEE Standard Definitions for Power Switchgear”
6. IEEE C37.20.1-1993, “Standard for Metal-Enclosed Low Voltage Power Circuit Breaker Switchgear”
7. ANSI C37.51-1989, “Standard for Switchgear - Metal-Enclosed Low Voltage
AC Power Circuit Breaker Switchgear Assemblies - Conformance Test Procedures”
8. NEMA SG-3-1981, “Low Voltage Power Circuit Breakers”
9. UL1066-1993, “Standard for Low Voltage AC and DC Power Circuit Breakers
Used in Enclosures”
This lengthy list gives you some indication why it is a matter of practicality when a
manufacturer states that a piece of equipment is built and tested to all applicable
Page 43
Low Voltage Power Circuit Breakers
NEMA, ANSI, IEEE and UL standards. It was also mentioned that a great deal of
referencing to other standards takes place within the body of a specific standard.
Successful testing and compliance with respect to one standard often includes
automatic compliance with other standards. It is worth repeating one of the examples given.
Example: ANSI C37.13 details the physical attributes, such as Stored Energy, that
a low voltage AC power circuit breaker must have to comply. At the same time,
ANSI C37.50 references C37.13 and details how the described circuit breaker
should be tested. The key here is that successful testing in keeping with ANSI
C37.50 brings with it compliance to C37.13. There is no need to mention C37.13,
when it is stated that the circuit breaker complies with C37.50.
IEC (International
Electrotechnical
Commission)
IEC presides over the standardization of equipment for a number of other parts of
the world. In view of today’s global markets, there is a significant amount of interaction between the organizations just discussed and IEC.
IEC 947-2 is a multi-part international testing standard covering a variety of
devices, including circuit breakers of all types. It is entitled “Low Voltage Switchgear and Controlgear.”
As far as IEC is concerned, every device tested to IEC 947-2 must be subjected to
several test sequences in order to be approved. Because IEC 947-2 covers both
low voltage power circuit breakers and low voltage molded case circuit breakers,
the exact test sequences performed are not necessarily the same. They depend
on the category of the device.
Category A Device - In general, this is a device without a short time Withstand
Rating, such as a molded case circuit breaker.
Category B Device - This is a device with a short time withstand rating, such as a
power circuit breaker and certain molded case circuit breakers. Typically, these
devices are referred to as Air Circuit Breakers or just ACBs.
IEC 947-2 was developed with assistance from members of the U.S. National
Committee. Still, a number of significant differences exist between IEC 947-2 and
applicable ANSI standards. In particular, the various ratings of a circuit breaker
can differ when tested to each standard. Therefore, any product comparisons
made between products tested to these different standards (domestic versus
international) should only be made with a thorough understanding of the differences.
Standards Conclusion
This might seem to be a monumental amount of information about standards. It is
only the tip of the iceberg. This is not to imply that you must be an expert on standards to deal with power circuit breakers. You can, however, begin to appreciate
just how much effort, investment, and plain hard work goes into being able to legitimately print in a document a statement such as:
“Magnum DS Low Voltage Power Circuit Breakers are UL Listed and built and
tested to all applicable ANSI Standards.” Keep in mind that all these standards
establish minimum requirements. There is nothing prohibiting a manufacturer
from exceeding standards by offering additional features and/or using newer and
improved operational techniques for more efficient and higher levels of performance. Magnum DS does just that in a number of areas.
The majority of the remaining discussions in this module will be presented as they
relate to applicable ANSI Standards. Remember, however, that other standards
do exist in other parts of the world. They must be complied with to participate in
the international segment (Figure 38).
Page 44
Low Voltage Power Circuit Breakers
Figure 38. Circuit Breaker Identification
Low Voltage Power
Circuit Breaker C37.50
Testing
Testing of a low voltage power circuit breaker in keeping with required ANSI Standards provides the first glimpse at what makes a low voltage power circuit breaker
unique. Remember, low voltage power circuit breakers are applied at or below
their nameplate ratings. That nameplate rating is a result of having successfully
completed a series of rigorous tests. This is referred to as a 100% rating.
Although every detail of the testing will not be covered here, you will have an
appreciation for just how demanding these ANSI defined tests are for low voltage
power circuit breakers. The tests will be described as four test sequences. It
should be pointed out here that all tests are performed using a Drawout circuit
breaker in its enclosure for each frame size.
The first three test sequences are similar in many ways to the tests performed on
other types of low voltage circuit breakers, such as a molded case circuit breaker.
It is the fourth test sequence performed on a low voltage power circuit breaker
that differentiates the power circuit breaker from other types of circuit breakers.
Note that all the following test sequences, except for Test Sequence 4 in which the
circuit breaker has no Trip Unit, are preceded and followed by a calibration test
and dielectric check.
Test Sequence 1 - This test sequence consists of a temperature rise test, an
overload switching test, and then a short circuit test. The circuit breaker is
equipped with an instantaneous trip.
1. The circuit breaker is loaded to 100% of the maximum rating of the frame size
(in normal enclosure) until the temperature is constant. The standard lists the
maximum permissible temperature rises at various parts of the circuit breaker.
2. The circuit breaker is then subjected to number of opening operations on
over-load switching.
3. The circuit breaker is then given a 3-phase short circuit test at its maximum
voltage rating, which is 635 volts for this 600 volt rated circuit breaker. The
short circuit current in this case can be no less than the 600 volt interrupting
capacity of the circuit breaker being tested. The three maximum voltages
used during testing and typically listed on the nameplate are listed below
along with their corresponding application voltages:
Maximum Voltage
Application Voltage
635 volts
508 volts
254 volts
600 volts
480 volts
240 volts
4. The short circuit test consists of initiating current through the closed circuit
breaker, causing it to trip. After 15 seconds, the circuit breaker is re-closed on
the fault, and then allowed to trip open to clear the fault. This is known as an
O-CO (open-close open) test.
5. The short circuit test is followed by a calibration check and a dielectric test.
Test Sequence 2 - This test sequence consists of a series of short circuit tests on
a circuit breaker equipped with selective tripping (no instantaneous).
Page 45
Low Voltage Power Circuit Breakers
1. Once again, all short circuit tests are preceded by a dielectric test and a calibration test. After the interruptions, the circuit breaker is given another dielectric test and the calibration is again checked.
2. Each short circuit test is an O-CO test meaning that the circuit breaker interrupts the full fault current twice.
3. After the 3-phase short circuit tests are completed at different prescribed voltages, single phase tests are performed. A new circuit breaker may be used
for each test, or each test may be done on different poles of the same circuit
breaker.
4. One single-phase test is done at each of the same three maximum voltage
ratings used for the three-phase tests (635, 508 and 254 volts) at the appropriate Interrupting Rating for that voltage.
Test Sequence 3 - This test sequence includes tests of mechanical and electrical
endurance.
1. A circuit breaker is calibrated, given a dielectric test, and subjected to a large
number of operations. Some of the operations are at no load and some at full
load.
2. The required number of operations varies by circuit breaker frame size.
3. After the endurance test, the same circuit breaker is given a full 3-phase OCO short circuit test at 635 volts and a dielectric withstand test.
The required number of operations for the endurance test just described is about
the same for larger frame power circuit breakers and somewhat higher for smaller
frame power circuit breakers compared to molded case circuit breaker endurance
tests. For the sake of comparison, refer to the two endurance ratings tables, one
for power circuit breakers and one for molded case circuit breakers (Figures 39
and 40).
Figure 39. Low Voltage Power Circuit Breaker Endurance Ratings (Per C37.50)
Figure 40. Low Voltage Molded Case and Insulated Case Circuit Breaker Endurance Ratings (Per UL
489)
Test Sequence 4 - This test sequence includes a short time current (withstand)
test. Molded case and insulated case circuit breakers are not usually subjected
to this type of test and, therefore, have no full 30 cycle Short Time Rating. This
is one of the key differences.
1. For this test, the circuit breaker does not have a trip unit or the trip unit is disconnected. During the short-circuit testing, the circuit breaker can be tripped
instantaneously by a shunt trip.
Page 46
Low Voltage Power Circuit Breakers
2. The circuit breaker is closed and then energized at its full short time current
rating. The short time rating is usually equal to the 600 volt short circuit current rating.
3. The current is left on for 30 cycles (1/2 second), then off for 15 seconds, then
back on for another 30 cycles. The circuit breaker remains closed during this
test sequence.
4. After the short time current (withstand) tests, the same circuit breaker is given
a 3-phase short circuit test sequence at full short circuit current rating and 635
volts. The circuit breaker is opened as quickly as possible by the shunt trip,
which is energized at the same instant the power is applied. The intent is to
force the circuit breaker to open during the worst case conditions (full current
asymmetry). This is where the short time rating for the circuit breaker comes
from.
5. After the final test, the circuit breaker calibration is checked and given a
dielectric withstand test.
During Test Sequence 4, the circuit breaker is subjected to tremendous physical
forces from the magnetic fields and to severe heating effects from the current.
Think about it. Both the magnetic forces and the heating vary with the square
of the current (Figure 41). For example, a 4000 ampere frame low voltage power
circuit breaker at 85,000 amperes is subjected to forces and heating more than
450 times normal for each 30 cycle test. It is quite awesome.
Figure 41. Magnetic Force Versus Current
Page 47
Low Voltage Power Circuit Breakers
Review 4
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. ANSI is the recognized North American Authority on equipment standards.
TRUE FALSE
2. Cutler-Hammer’s Magnum DS low voltage power circuit breakers are built
and tested to all applicable ANSI Standards. The IEC Standards version of
this Cutler-Hammer low voltage power circuit breaker is called
___________________.
3. Magnum DS low voltage power circuit breakers carry a UL Label which indicates the circuit breaker’s successful compliance with UL Standard Number
___________________.
4. The primary North American testing procedures for low voltage power circuit
breakers used in an enclosure are stated in ANSI Standard C37._____
_____.
5. All low voltage circuit breakers have full 30 cycle short time rating because all
of them must pass a momentary rating test.
TRUE FALSE
Page 48
Low Voltage Power Circuit Breakers
Construction
Method
You learned that low voltage power circuit breakers are essentially an assembly of
parts on a metal frame or in an encased housing of insulating material. Because
the makeup of both approaches was adequately discussed, the details will not be
repeated. One point, however, that should be repeated centers around what applicable standards are required relative to the construction method. The frame construction used must hold all the circuit breaker parts in place and be capable of
withstanding the tremendous physical forces and severe heating effects a power
circuit breaker could be subjected to while performing its function. Standards do
not specify the exact nature of the construction or the construction material.
Those decisions are left to the circuit breaker manufacturer.
Obviously, the construction method and materials used must result in a strong,
rigid design. For many years, the preferred approach was the open type metalframe which had a number of pieces welded and/or bolted together. With the significant technological strides made in the areas of insulating material and molding
processes, versatile rigid frame housings of high strength engineered thermoset
composite resins have become available. Not only does the rigid frame housing
type low voltage power circuit breaker meet the stringent requirements of ANSI, it
exceeds them in a number of instances.
The Magnum DS circuit breaker uses a 3-piece construction (Figure 42):
A 2-piece engineered thermoset composite resin case completely encloses
the current paths and arc chambers.
The operating mechanism sits on the front of the case and is electrically isolated from the current contact structures. It is in turn covered by an insulating
front cover.
Figure 42. Magnum DS Low Voltage Power Circuit Breaker
Rigid Frame Construction (Right Side View)
The rigid frame construction results in a more compact, lighter weight low voltage
power circuit breaker. Previous circuit breaker designs with many parts that were
once produced and attached individually to the frame can now be molded as an
integral part of the rigid frame. The overall strength and rigidity of the engineered
thermoset composite resin design can even contribute to higher performance
capabilities by the power circuit breaker. This was the result with Magnum DS
which has higher short time ratings (withstand) than previously available power
circuit breakers, along with higher interrupting capabilities. In the way of
review, remember that the short time rating consists of the following two components:
•
Short delay current component (kA)
Page 49
Low Voltage Power Circuit Breakers
•
Short delay time component (cycles)
In addition to the improved performance characteristics just mentioned, the rigid
frame housing type low voltage power circuit breaker has individual arc chambers
that (Figure 43):
•
Insulate and isolate Arcing from other poles, the rest of the circuit breaker,
and personnel
•
Provide support for the current path pole assembly
Figure 43. Top Rear View Magnum DS Power Circuit Breaker
Showing Arc Chambers with One Arc Chute Removed
Ratings and
Performance
The rigid frame housing also provides for the rigid mounting of circuit breaker
components which:
•
Improves the mechanism’s life by eliminating frame deflection
•
Provides for the consistent operation of mechanism
•
Eliminates the need for mechanism adjustments
Some of the most useful information about a low voltage power circuit breaker can
be found in its ratings table. Ratings tables were briefly covered during the Circuit
Breaker Selection discussion. The ratings published in these tables are backed up
by testing as outlined in applicable standards. These ratings tell quite a story.
They indicate how a particular circuit breaker will perform during a given set of
application circumstances. In a similar regard, the ratings indicate what circuit
breaker should be selected for a specific application.
Although ratings tables from different manufacturers often look similar and even
reflect many of the same ratings, it is not a good idea to assume that all low
voltage power circuit breakers have the same ratings. Get into the habit of
reviewing different tables and comparing the published ratings. There is no room
for unexpected surprises in circuit breaker selection.
Figure 44. Example ANSI Ratings Table
Page 50
Low Voltage Power Circuit Breakers
To demonstrate the value of making these comparisons, let’s once again look at a
partial ANSI ratings table, contrasting the ratings for one type low voltage power
circuit breaker (Type XYZ) with those of the Magnum DS low voltage power circuit breaker (Figure 44). Once you have taken the time to look the table over a bit,
several comparison examples will be discussed. Keep in mind that this table only
presents excerpts and is not intended to show the ratings of all available circuit
breakers for an entire line. It can be assumed that both circuit breakers used in
the examples meet all applicable ANSI standards for low voltage power circuit
breakers.
The comparisons to be discussed are identified in the table by a circled letter, for
example.
A - Notice first that the ratings given for the Magnum DS power circuit breaker
apply for all system voltages. Ratings for other power circuit breakers can vary by
voltage. Uniform ratings across all application voltages for the 3200 ampere frame
circuit breakers and below simplifies the selection process. Interrupting ratings
higher than 100,000 amperes are available with Magnum DS frame sizes above
3200 amperes for certain application voltages. Therefore, the uniformity is not
totally maintained on the larger frame circuit breakers.
B - The short time rating (withstand) for Magnum DS once again applies across
the entire range of application voltages, while the other is only applicable to 480
volts. In addition, notice that Magnum DS has higher short time current ratings
than does the other power circuit breaker. A broader range and higher short time
ratings allow Magnum DS to be applied in systems with higher available short circuit currents while maintaining full selectivity.
C - An interrupting rating of 100,000 amperes is available with 3200 ampere frame
and below Magnum DS circuit breakers at all application voltages. This rating is
not at all available with the other power circuit breaker. Notice further that the
short time rating is 85,000 amperes (3200 amperes and below). What does this
accomplish?
1. These ratings mean that this particular Magnum DS rating has a hefty short
time (withstand) capability and can withstand a short circuit of 85,000
amperes for the full 30 cycles required by ANSI.
2. It also means that this Magnum DS rating can be applied on a system that
could experience a short circuit as high as 100,000 amperes. The circuit
breaker would be selective up to 85,000 amperes, and would open instantaneously from 85000 to 100,000 amperes.
D - One more area on a ratings table to be alert to is with and without instantaneous trip. It is possible for some circuit breakers to have higher interrupting ratings if they can trip instantaneously. In those instances, the interrupting rating
might be lower if the circuit breaker is required to provide short time protection.
This is not the case, however, with Magnum DS at interrupting ratings of 85,000
amperes and below.
As a general rule, the ratings for most ANSI rated low voltage power circuit breakers are similar where system requirements are less stringent. When the application requirements become more stringent and/or special, the capabilities of
different power circuit breakers begin to differ. Look at ratings tables closely. They
have a story to tell.
Extended Interrupting Ratings: Interrupting ratings can be extended up to
200,000 amperes by providing a combination of a low voltage power circuit
Page 51
Low Voltage Power Circuit Breakers
breaker connected in series with current limiting fuses (current limiters). This combination is provided on systems where the overload and switching functions of the
circuit breaker are required, and available fault currents could exceed the interrupting rating of the circuit breaker by itself. For smaller frame circuit breakers, the
current limiters usually are integrally mounted in the circuit breaker (Figure 45).
For larger frame circuit breakers, the current limiters are usually mounted in a
separate drawout truck and positioned adjacent to the circuit breaker.
Figure 45. DSL II Fused Type Circuit Breaker (Side View)
Altitude and Ratings: Low voltage power circuit breakers are applicable at their
full voltage and current ratings up to a maximum altitude of 6600 feet (2000
meters) above sea level. When a circuit breaker is installed at higher altitudes,
the ratings are subject to correction factors in accordance with ANSI Standards.
Fortunately, you are not often faced with this situation. Even if you are, it probably
has already been taken into account by the specifier. It is good, however, to be
aware of such application exceptions.
Current Waveforms and Ratings: The subject of current waveforms and their
effect on circuit breaker ratings will not be discussed in great detail in this module.
Once again this consideration is usually made by the specifier and taken into
account when a particular circuit breaker is specified. You should be somewhat
familiar with the concepts. If for no other reason, it could help to explain why a certain circuit breaker rating selection was made for a particular application. It will
also be good background information later in this module when applications are
discussed. You noticed in the ratings table just discussed that interrupting ratings
and short time ratings were expressed in symmetrical amperes. For most discussions and selections involving low voltage power circuit breakers, this will be the
case. Without getting into too much detail, it is important to know that there are
two common ways to rate protective devices in amperes. These ratings are symmetrical and asymmetrical. The symmetrical and asymmetrical ampere ratings
can be quite different for the same device. To clarify the significance of the two
terms, let’s briefly discuss each individually.
The graphic shown of a Symmetrical AC Current represents a fault current flowing in a circuit (Figure 46). The fault current has a sine wave shape and is symmetrical with respect to the horizontal axis. That is, the current rises and falls
equally above and below the horizontal axis, so the shapes are symmetrical.
The graphic shown of an Asymmetrical Current becoming Symmetrical represents an offset fault current wave (Figure 47). It rises above the horizontal axis
considerably more than it goes below for the first few cycles. This wave is said to
be offset from or asymmetrical with respect to the horizontal axis. This condition
occurs in circuits containing reactance which are short-circuited at some time
Page 52
Low Voltage Power Circuit Breakers
other than when the current is passing through the zero point on the cycle. It
occurs in all 3-phase circuits in one or more phases. When this happens, a DC
current is superimposed on top of the AC current causing asymmetry. The DC
component actually decays to zero within a short time after the fault occurs. The
final decay of the DC component signifies a change from asymmetrical to symmetrical. How fast this actually happens depends upon the quantity relationship of
reactance and resistance in the circuit, the X/R ratio. The more resistance in the
circuit, the faster the DC component decays, or the larger the X/R ratio, the
longer the decay time.
Figure 46. Symmetrical AC Current
Figure 47. Asymmetrical Current Becoming Symmetrical as DC Components Decay
Continuing this discussion of current waveforms, let’s take a look at some background information. Low voltage power circuit breakers typically part their Contacts after several cycles of fault current, assuming there is no intentional time
delay. In short, contact parting only begins once time delays are expired. The circuit breaker can be called upon to interrupt more than the symmetrical value of
fault current, as calculated from the impedance of the circuit. This is because of
the presence of the DC component just discussed.
We will not discuss here why the degree of asymmetry can be different, just know
that the degree can vary. What is of real importance is the rate at which the DC
component decays and the change to symmetrical takes place. This rate, and
hence the current value, can be related directly back to the ratio of circuit reactance to circuit resistance or the X/R ratio.
For this reason, the X/R ratio is a significant and specified factor for standards testing. It is also an important ratio to know when selecting circuit breakers
for application on a system. As long as the X/R ratio for the system does not
exceed the tested X/R for the circuit breakers, you are home free.
If the system X/R ratio exceeds the tested X/R for the circuit breakers, the circuit
breakers would have to have their published interrupting capacity and short
delay current capability de-rated. A circuit breaker with a higher interrupting
capacity and higher short delay current rating might be needed to accommodated
the de-rating factor.
Page 53
Low Voltage Power Circuit Breakers
As previously mentioned, these types of system determinations are normally
made well ahead of time by the specifier or consultant, which is when the equivalent system short circuit rating is normally specified.
For your information, the ANSI tested X/R ratio for power circuit breakers is
6.6. This 6.6 ratio was not always the case. As a matter of fact, power circuit
breakers were not always rated in symmetrical terms. That is history and we will
not discuss why a change was made from asymmetrical to symmetrical.
At some point, the industry made the change and determined that an X/R ratio of
6.6 for power circuit breakers was typical, and would be a good base to work with
for testing and application. At least all the manufacturers were working with the
same standardized starting point. Now, when the calculated short circuit current X/
R ratio for a particular system is higher than the standard 6.6 X/R ratio for low voltage power circuit breakers, a table can be consulted to determine what de-rating
factor should be applied to the circuit breaker’s interrupting rating to insure proper
circuit breaker sizing and selection.
To give you an idea of what these de-rating factors look like, refer to the partial
table of low voltage power circuit breaker de-rating factors (Figure 48). Keep in
mind that these considerations and decisions must be made for all types of circuit
breakers, not just low voltage power circuit breakers. Before this discussion is
concluded, let’s take a look a one simple example.
Figure 48. Interrupting Rating De-Rating Factors for All Low Voltage Power Circuit Breakers Table
Example: The total available fault current from all sources that this low voltage
power circuit breaker could see was calculated to be 48,000 amperes symmetrical. For this example, we are only considering what circuit breaker could be used
to deal instantaneously with a potential fault of this magnitude. From some selection chart you might select a circuit breaker with a 50,000 ampere interrupting
capability.
In this example that would not be a good selection because it has also been
determined that the system X/R ratio is 9.94. You can see from the de-rating table
that a 9.94 ratio equates to a de-rating factor of 0.937. This factor used on the circuit breaker’s interrupting capability of 50,000 amperes reduces it to 46,850
amperes, not sufficient to deal with the potential fault current of 48,000 amperes
[50,000 x 0.937 = 46,850].
Obviously, a circuit breaker with a higher interrupting capability must be selected
for this application. If a circuit breaker with an interrupting capability of 65,000
amperes is selected, you can see from the calculation that it would do the job
[65,000 x 0.937 = 60,905].
Page 54
Low Voltage Power Circuit Breakers
Review 5
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. The frame or housing of a low voltage power circuit breaker is not required by
applicable standards to be manufactured from any specific type of material.
The manufacturer of the circuit breaker makes that decision.
TRUE FALSE
2. The short time rating of a low voltage power circuit breaker is also known as
the ___________________ rating.
3. Two different components combine to make a low voltage power circuit
breaker’s short time rating. One is the short delay time component and the
other is the __________________ __________________
__________________ component.
4. All low voltage power circuit breakers meeting the same applicable ANSI
Standards must offer all the same ratings.
TRUE FALSE
5. Magnum DS low voltage power circuit breaker interrupting ratings are the
same for all application voltages on 3200 ampere frame circuit breakers and
below.
TRUE FALSE
6. The highest interrupting rating available from any unfused 3200 ampere
frame and below low voltage power circuit breaker is offered by Magnum DS.
That interrupting rating is ___________________ amperes.
7. Two parts of the graphic below are labeled. One component has an arrow
pointing to it but is not labeled. Label the unlabeled component next to the
arrow pointer.
8. Use the information provided here to determine what would be the minimum
interrupting capacity a low voltage power circuit breaker should have to safely
deal with a maximum available fault current of 63 kA on an instantaneous
basis. The system in which this circuit breaker will be applied has an X/R ratio
of 6.59. Circle the correct multiple choice answer, (a), (b), (c) or (d).
(a) 42,000 amperes interrupting capacity
(b) 65,000 amperes interrupting capacity
(c) 85,000 amperes interrupting capacity
(d) Enough information is not provided to determine the answer
Page 55
Low Voltage Power Circuit Breakers
Operational
Techniques
Generally, all low voltage power circuit breakers use the same functional parts to
accomplish their task. You learned earlier that all of them have:
•
a set of main and arcing contacts to isolate or connect circuits
•
a two-step stored energy mechanism to open or close the contacts
•
a mechanical and/or electrical mechanism to charge the springs
•
arc chambers (chutes) to extinguish any arcs formed during opening
•
the intelligence to operate automatically or respond to commands
Does this mean that all low voltage power circuit breakers are virtually the same?
No.
You will recall that ANSI Standards provide for the minimum requirements to be
classified as a low voltage power circuit breaker. In addition, the standards do not
state how something must be accomplished, only what must be accomplished. It
was already pointed out earlier in this module that ANSI Standards do not dictate
the construction method. If there is a better way to accomplish the same results,
do it. What differentiates one low voltage power circuit breaker from another are
the design techniques used to accomplish the task. Techniques that can result in:
•
improved operational efficiency
•
better performance characteristics
•
higher ratings
•
additional features
•
smaller and lighter devices
In short, the customer ends up benefiting by being able to do more and/or do it
better by comparing and then using one circuit breaker over another. Let’s take a
look at some of the design techniques in a little more detail.
Primary Opening and
Closing Contacts
Low voltage power circuit breakers generally have separate arcing and main current carrying contacts. This does not necessarily mean, however, the arcing
and main contacts are physically two separate pieces. On some designs, they
are two distinct parts of the same assembly. On other designs, the arcing and
main contacts are just different contact areas on the same contact.
The arcing and main contacts are designed mechanically so that on closing, the
arcing contacts make before the main contacts. On opening, the main contacts part before the arcing contacts. This type of construction ensures that arcing takes place on the more heat resistant arcing contacts. Because the damaging
arcing takes place on the arcing contacts, the main contacts are protected and
can be made of pure silver or a silver alloy. This main contact composition is much
lower in resistance and minimizes the heat developed during operation. Contacts
are usually maintainable and/or replaceable, a key feature for a low voltage
power circuit breaker.
A complete primary contact assembly generally consists of three primary parts
(Figures 49 and 50):
•
Moving Contact Assembly
•
Stationary Contact Assembly
•
Operating Arm
Page 56
Low Voltage Power Circuit Breakers
Figure 49. DSII Type Primary Contact Assembly Mounted (Front View)
Figure 50. DSII Type Primary Contact Assembly Mounted (Rear View)
Moving Contact
Assembly
The moving contact assembly is hinged or pivoted in some manner within the circuit breaker permitting it to open and close as required. It is made up of a main
contact and an arcing contact.
1. The moving main contact is connected to the load conductor through
either a hinged or flexible connection. It also varies in size depending upon
the amount of current. When it closes, it butts up against the stationary portion
of the contact completing the circuit. It opens the circuit when it pivots in the
opposite direction and breaks contact with the fixed portion.
2. The moving arcing contact makes first with its fixed counterpart upon closing,
and separates last upon opening. It can be a Butt type design or a Finger
(wedge) type design.
Stationary Contact
Assembly
The stationary contact assembly is in a fixed (stationary) position within the circuit
breaker. It is made up of a main contact and a fixed arcing contact or some part
that acts as a fixed arcing contact.
1. The stationary main contact is rigidly connected to the line conductor.
The contact itself is often a butt type contact, and varies in size depending
upon the amount of current it is designed to carry. By butt contact, it is meant
that a connection is made when the moving part of the contact butts solidly up against the stationary part of the contact.
Page 57
Low Voltage Power Circuit Breakers
2. The stationary arcing contact can be a wedge type contact or a butt type contact. By wedge type contact, it is meant that the moving part wedges in
between two fixed parallel arcing fingers (contacts).
Operating Arm (Link)
A rigid operating arm (link), usually made of an insulating material, is the connection between the moving contact assembly and the circuit breaker’s operating
mechanism. It operates the moving contact assembly during the opening
and closing process in conjunction with the operating mechanism. The operating link is sometimes considered to be part of the operating mechanism.
Now that primary opening and closing contacts have been reviewed in a general
way, let’s take a closer look at how all of this works together. We will review the
innovative Magnum DS approach. Keep in mind, however, that all designs are not
the same and some are more effective than others.
Magnum DS Primary
Contact System
All low voltage power circuit breaker designs try, with varying degrees of success,
to maximize the combination of two important design aspects:
•
Contact Finger Design and Movement
•
Natural Electromagnetic Laws
The Magnum DS primary contact system features a very effective and efficient
combination of these two design aspects.
1. The Magnum DS movable contact design centers around a single contact
finger that performs both the main contact and arcing contact functions.
These two functions are performed on different parts of the same contact
finger. A highly conductive alloy pad is part of the copper contact finger and
functions as the moving main contact. The upper portion of the same copper
contact finger functions as the moving arcing contact. A complete movable
contact assembly is merely a combination of a number of single contact fingers. The exact number of contact fingers required depends upon the frame
size and interrupting rating of the circuit breaker (Figures 51 and 52).
Figure 51. Graphic of Single Magnum DS Contact Finger
Page 58
Low Voltage Power Circuit Breakers
Figure 52. Graphic of Magnum DS Multi-Finger Configuration
You learned that design efficiencies often separate one circuit breaker from
another. The contact design used from one circuit breaker to another is a great
example of just such a difference in efficiency. If a visual comparison is made
between the Magnum DS moving contact assembly and its counterpart on a number of other low voltage power circuit breakers, one thing will be obvious - the difference in the total mass (Figure 53). This is especially true in the area of arcing
contacts. The finger and wedge type arcing contact design, used on DS II for
example, is not nearly as efficient at controlling an arc as the butt type arcing contact used by Magnum DS. Consequently, the finger and wedge design makes up
for this shortcoming with additional mass. On the other hand, Magnum DS can
reduce mass and weight because of its more efficient arc control.
Figure 53. Magnum DS (Left) and DS II (Right) Moving Contact Assembly Comparison
To simplify this discussion, let’s talk in terms of a single contact finger. Once you
grasp the concept in terms of one contact finger, it will not matter how many fingers are needed to carry the current. The design concept remains constant.
Improved overall reliability is the result of this simple design.
The opening and closing action of the single contact finger can best be described
as a “Heel-Toe” motion (Figure 54). The main contact portion can be viewed
as the “Heel” and the arcing contact portion the “Toe.” If you rock your foot
forward and backward from heel to toe and toe to heel, you demonstrate the
movement of the contact design. Notice as your foot rocks, there are times when
only the toes touch, times when only the heel touches, and a brief time when they
touch simultaneously.
Page 59
Low Voltage Power Circuit Breakers
Figure 54. “Heel-Toe” Movement
When the circuit breaker is closed, the “Heel” (main contact portion) is in contact with the stationary contact and carrying current (Figure 55). At the same
time, the “Toe” (arcing contact portion) is separated from the stationary arcing contact (arc runner). At this point, it is similar to your foot resting on its heel
with your toes off the floor. Under short circuit conditions, the force on the “Heel”
(main contact portion) to keep the main contact closed is enhanced by the positioning of the fulcrum point, which is located in close proximity to the contact, and
electromagnetic force on the moving conductor.
As the circuit breaker is called upon to open, the “Toe” touches down before
the “Heel” lifts off. During the brief time that both the “Heel” and “Toe” are in contact with their fixed counterparts, the current that is present commutates
(changes its path) from the main contact portion of the contact finger to the arcing portion. When the “Heel” (main contact portion) lifts off, any remaining current
is driven to the ‘Toe” (arcing contact portion) by the arc voltage produced.
Remember, the voltage drives the current. The path the current travels is very
short and the transfer is made quickly. The result is very little arcing damage to
the ‘Heel” (main contact portion). This protection of the main contacts is a primary reason why the main contacts have a longer life, and can be made of higher
conductive materials. This use of purer conductive materials results in better thermal performance. Once the main separation is made, it is followed by separation
of the “Toe” (arcing contact portion) where the main arc is created. How the circuit
breaker deals with the arcing that takes place will be discussed when Arc Chutes
are discussed.
Page 60
Low Voltage Power Circuit Breakers
Figure 55. Magnum DS Heel-Toe Contact Action
Cross Sectional View of Part of Circuit Breaker Only)
Fully Closed Position (Gap at Toe)
All Toe (Arcing)
Contacts Touch
Only Center Toe (Arcing) Contacts Touch
Simultaneously Touching of
All Contacts (Heel and Toe)
Full Open Position
(No Contacts Touching)
When the circuit breaker is called upon to close, the “Heel-Toe” action functions to
first touch down the “Toe” followed by the “Heel.” Most of the impact on the
contacts during closing is absorbed by the “Toe” portion. Once again the main
contacts are protected resulting in a longer life. As was the case during the opening operation, the “Heel” and “Toe” are simultaneously closed for a short period of
time to ensure that any arcing is handled by the “Toe” (arcing contact portion).
Closing is completed as the contact finger rocks back on its “Heel” and the “Toe”
lifts off the fixed arcing contact area (arc runner).
In general, the movable contact can be attached to the load conductor through a
hinged connection or attached with a flexible connector. Because of its complex
opening and closing action, Magnum DS utilizes two braided connectors (flexible shunts) to attach each contact finger. Not only do the flexible braided connectors eliminate extra bolted joints which are natural hot spots, they help to
reduce the mounting space required for the primary contacts. The flexible connectors (shunts) also allow the finger to have the motion necessary for the “Heel-Toe”
(rocking) motion.
2. Naturally generated electromagnetic forces are cleverly used by Magnum
DS to assist with the withstand and opening process (Figure 56). A compact
C Loop current path design permits electromagnetic forces, primarily produced as a natural result of a fault current, to supplement the contact forces
provided by the operating mechanism. The contact fingers are pivoted in such
a way that the magnetically induced forces create a force which Levers the
main contact onto the stationary contact (blow-on force), temporarily increas-
Page 61
Low Voltage Power Circuit Breakers
ing the closing contact pressure during a fault current. This accomplishes two
things significant to Magnum DS:
•
The operating mechanism does not have to be as large or heavy.
•
Higher short time ratings (withstand) are achievable.
Figure 56. Compact C Loop Design (Physics of Blow-On Forces)
Furthermore, when the circuit breaker is tripped, the fulcrum (pivot point) is
released and the electromagnetic forces on the contact fingers are converted to
an opening force to speed up the opening action. You might consider these
uses of electromagnetics as design magic, actually they are design ingenuity.
These natural laws of physics were around before the circuit breaker. It is now that
designs are making good use of electromagnetic forces to help keep the contacts
closed when necessary, and to help to open them quickly.
Operating Mechanism
Low voltage power circuit breakers operate through spring stored energy mechanisms to perform the opening and closing functions. Both manually operated
and electrically operated mechanisms are available. The closing springs can
be manually or electrically charged with an electrically operated mechanism, and
manually charged only with the manually operated mechanism (Figure 57). Manual charging is accomplished through the use of some type of manual charging
handle on the front of the circuit breaker. The mechanism is such that when the
circuit breaker is closed, the opening springs are simultaneously charged. The
spring mechanisms are two-step stored energy mechanisms. That is, the closing springs are charged and remain charged with the circuit breaker open until a
close button or electrical solenoid releases them, closing the circuit breaker.
Figure 57. Magnum DS Closing Springs Being Manually Charged
Circuit breaker closing, once the spring is charged, is rapid and occurs within
five cycles. The spring charging time is typically several seconds.
Page 62
Low Voltage Power Circuit Breakers
A number of different devices associated with the mechanism are accessible from
the front of the circuit breaker (Figure 58). As a minimum, these devices are:
•
Manual On (Close) Button
•
Manual OFF (Trip) Button
•
Spring Charged Indicator
•
Main Contacts Position (Open/Close) Indicator
Figure 58. Magnum DS Circuit Breaker Front Cover
The operating mechanism consists of two major parts:
•
Stored energy or spring charging mechanism
•
Mechanism for closing and opening the circuit breaker
These two basic parts are normally combined into one overall operating mechanism. There are two varieties as previously mentioned: manually operated and
electrically operated. In the past, low voltage power circuit breakers were either
manually operated or electrically operated, and could not be converted from one
to the other in the field. Today, a number of manufacturers offer circuit breakers
that can be converted from manual to electrical in the field by the addition of an
electrical operator. If a circuit breaker is manually operated, it can be:
•
Electrically tripped with an electrical shunt trip device
•
Electrically closed with an electrical spring release device
Once again, all operating mechanisms are not designed or constructed the same,
although they perform the same general function. Some are larger and heavier
than others, while some are more efficient than others. Let’s take a brief look at
some features of the Magnum DS operating mechanism.
Magnum DS Operating
Mechanism
The Magnum DS operating mechanism is based on the well proven DSII power
circuit breaker design, which has a recognized reputation for reliability. It is compact and light in weight for a number of reasons. Two of the most important are:
1. The clever use of naturally generated electromagnetic forces to create a
blow-on force to assist the mechanism with keeping the primary contacts
closed. This design technique led to a lighter more compact mechanism.
2. The rigid frame (housing) provides for very rigid mounting conditions for
the operating mechanism. This leads to a longer mechanism life and
improved operations with lighter and fewer mechanism parts.
Page 63
Low Voltage Power Circuit Breakers
The Magnum DS operating mechanism is the type that can be converted in the
field from manually operated to electrically operated with the addition of an electrical operator (Figure 59). The electrical operator mounts inside the front cover of
the circuit breaker. All Magnum DS circuit breakers are pre-wired to accept the
addition of electrical operators simply and quickly.
Figure 59. Field Installable Electrical Operator Shown Mounted with Front Cover Removed
Arc Chamber (Chutes)
Whether you call the arc extinguishing part of the circuit breaker an arc chamber
or arc chutes, all low voltage power circuit breakers utilize somewhat similar
means to extinguish arcs that are generated when contacts part. Once again,
however, some methods are more effective than others. The purpose is to extinguish the arc as soon as possible. This is accomplished by combining a number of
well proven techniques, such as:
•
De-ionizing the arc gases
•
Stretching the arc
•
Breaking the arc into pieces
•
Cooling the arc
For a long time, low voltage power circuit breakers have had an individual arc
chute mounted on the top of each circuit breaker pole. The arc chute is designed
such that it fits well down over the circuit breaker’s arcing contacts. This arc chute
positioning confines the arc inside the arc chute at all times.
The arc chute housing is made of a strong insulating material. This housing holds
a number of pieces called splitters, usually in a vertically stacked arrangement.
The splitters are often made of steel in a special shape and/or configuration to aid
arc interruption. When a fault occurs and the contacts separate, the splitters
attract the arc further up into the arc chute where the arc is stretched, cooled, deionized and extinguished.
In open metal-frame low voltage power circuit breakers, individual insulating barriers are mounted as required between the different circuit breaker poles, which
also separates individual arc chutes (Figure 60). This is often necessary to help
maintain the proper electrical clearances.
Figure 60. DSII Metal-Frame Circuit Breaker with One Arc Chute Shown Removed
Page 64
Low Voltage Power Circuit Breakers
Magnum DS Arc
Extinguishing
The Magnum DS low voltage power circuit breaker takes full advantage of the
fact that its rigid frame housing is molded from high strength thermoset engineered composite resins. The arc chamber is actually an integral part of the circuit breaker frame. The arc chamber insulates and isolates each pole from one
another, from the rest of the circuit breaker, and from operating personnel. It also
provides the space required to mount and enclose individual arc chutes over each
set of contacts.
It is a well known fact that a high pressure in the arc chamber contributes to
improved interruption. The top of the arc chutes (arc chute exits) are baffled with a
cover to create the back pressures. Magnum DS also uses an arc chamber seal
to:
•
Prevent undesirable back flow of gas generated during arcing into the rest of
the circuit breaker
•
Confine gas to maintain pressure for improved interruption.
By designing the contact carrier assembly to be a close fit with the molded enclosure, and by including the arc chamber seal, the reverse flow of gas is minimized.
The arc chamber seal design not only permits Magnum DS to increase the interruption performance, it is also a safety feature to protect the mechanism and operators from potentially harmful gases. This precise fit type design is able to be
accomplished because of Magnum DS’s engineered frame design. Metal-frame
construction, by nature, cannot be constructed to such close tolerances.
You will recall during the discussion of Magnum DS contact operation that a compact C Loop current path is used during normal operation. This design permits
the circuit breaker to take advantage of electromagnetic forces to keep the contacts closed - the blow-on effect. When the circuit breaker is called upon to open
and deal with the interruption of a fault current and the generated arc, the same
blow-on force is converted to an opening force to help speed the opening
action.
After the main contacts part, any remaining current is transferred to the arcing
contacts by the arcing voltage (Figure 61). Magnetic action is enhanced and the
arc is drawn to the arc chute. As the arcing contacts separate, the moving arcing
contact discharges into the arc chute plates while the integral arc runner helps to
draw the arc up into the arc chute. Remember, the integral arc runner serves a
dual purpose. It functions as the fixed arcing contact and is also a critical part of
the arc chute itself.
Figure 61. Magnum DS Arc Running System
Page 65
Low Voltage Power Circuit Breakers
Finally, the alternating V shape of the arc chute plates (splitters) is very effective at dividing and cooling the arc (Figure 62). The result is a fast and efficient arc
extinguishing process.
Figure 62. Bottom View Magnum DS Arc Chute Showing Alternating V Shaped Plates
Integral Trip Unit
All low voltage power circuit breakers must have the intelligence to operate automatically or to respond to specific commands. This intelligence package is the trip
unit. You will recall that ANSI Standards require that trip units used with low voltage power circuit breakers be part of (integral to) the circuit breaker (Figure 63).
We discussed a low voltage power circuit breaker’s tripping system. The system
included a trip actuator, a current sensor (transformer) for each phase, and
the trip unit (brains of the system). You also know that trip units come in a wide
variety of shapes and sizes with an even wider array of capabilities. Most low voltage power circuit breaker trip units used today are RMS sensing microprocessor-based designs. Usually they are selected based on the system requirements
for protection and other needs, such as communications capabilities.
Figure 63. Magnum DS Microprocessor-Based Trip Unit
Shown Mounted on Left and Unmounted on Right
The discussion here will focus on what types of features, protective and otherwise, are available today with digital microprocessor-based trip units. Keep in
mind, some of the most sophisticated electrical systems might utilize every feature a family of trip units has to offer. Other system applications might just need
the most basic protection package. In short, these tripping systems can be tailored to meet very precise system requirements. Generally, sophisticated microprocessor-based trip units are capable of providing combinations of a number of
capabilities, such as:
•
Programmable protection and coordination
•
Advanced warnings and alarms
•
System diagnostics and testing
Page 66
Low Voltage Power Circuit Breakers
Programmable
Protection and
Coordination
•
System monitoring
•
Power quality monitoring
•
Energy monitoring and management
•
Communications
Precise system coordination can be achieved when an expansive number of timecurrent curve shaping adjustments are part of the trip unit’s capabilities. This
means that a large number of incremental set points must be available for both
current pickup and time settings. To offer the most flexibility, the slope of a timecurrent curve should be adjustable along with the current and time settings. To
optimize the coordination effort, programmable adjustments should include the
following:
•
Long delay pickup (current)
•
Long delay time and slope adjustments
•
Short delay pickup (current)
•
Short delay time and slope adjustments
•
Instantaneous pickup (current)
•
Ground fault pickup (current)
•
Ground fault time and slope adjustments
Current (pickup) settings, delay time settings and slope selections determine the
resultant characteristic curve. Let’s take the time here to review several of these
issues. We will briefly look at the different types of protection available with special
attention given to short delay protection (withstand capabilities), a premier feature
of low voltage power circuit breakers.
Circuit Breaker
Coordination
The word Coordination is used quite often in these training modules and out in
industry. All of us have an idea what the process involves. It is a big responsibility
to consider all the variables in a distribution system, do the analysis and make the
ultimate equipment decisions. Equipment decisions that must place safety first
followed by a number of other considerations, like equipment protection,
power outages, cost, space limitations, and unusual application conditions.
Coordination is the process of selecting and applying circuit breakers in an
electrical distribution system to localize a fault condition and restrict power
outages to the equipment affected. The goal of coordination is improved selectivity. Proper coordination requires a comparison of the operating characteristics
of the circuit breakers in the system. Time-current characteristic curves are used
to determine if coordination will exist when choosing the size of the circuit breakers or just analyzing an existing system.
By overlaying the time-current curves of two circuit breakers or comparing them in
some other manner, you can determine whether selectivity exists. If the curves of
the two circuit breakers intersect, for example, the intersection area indicates conditions under which both circuit breakers may trip. If such a pair of circuit breakers
were used in an electrical distribution system, those conditions could result in both
circuit breakers tripping. This would be a needless interruption of power to some
portions of the system. On the other hand, if the curves of the circuit breakers are
distinctly separate and do not intersect, the circuit breakers are said to be coordinated.
Page 67
Low Voltage Power Circuit Breakers
Preparing for and doing a coordination study can be quite an effort, especially with
a large, diverse system. Accurate system one-line diagrams must be prepared. All
pertinent data including types and ratings for all circuit breakers, available short
circuit currents and any special requirements must be determined and shown on
the diagrams.
This brief discussion will give you some idea as to what went on behind the
scenes before you were asked for a particular circuit breaker with specific ratings
and operating characteristics. Obviously, this subject is far more involved than
this, but at least you will have a better appreciation for the process.
Circuit Breaker
Coordination Example
Refer to Figure 64 for this example. A system one-line diagram is prepared first
showing pertinent data, like types and ratings of circuit breakers and available
short circuit currents. For convenience, the one-line diagram for this example is
shown superimposed on the coordination curve.
A coordination study begins with the circuit breaker nearest the load and works
toward the source. For this example, a small motor is the load. The time-current
characteristics for the motor during starting and normal running might look like the
curve superimposed on the coordination curve. The downstream 50 ampere
molded case circuit breaker one (MCCB1) is selected first with its corresponding
curve put in place. This is followed in a similar manner by the curve for the 200
ampere molded case circuit breaker two (MCCB2), and the upstream 800 ampere
power circuit breaker 1 (PCB1) curve. Notice that all three circuit breaker curves
are distinctly separate, which means coordination exists between the circuit
breakers.
Because the curve representing the motor’s startup and normal running does not
intersect the curve for MCCB1, MCCB1 should not trip under normal conditions.
Should the motor’s rotor become locked and continuously draw some potentially
harmful level of current, MCCB1 would be the first in line to deal with the problem.
MCCB2 and PCB1 are positioned to deal with higher, more potentially destructive
current levels.
Although this is a very minor example, it demonstrates the procedure. The more
involved the system, the more involved and time consuming the coordination process.
Page 68
Low Voltage Power Circuit Breakers
Figure 64. Selectively Coordinated System Example
Page 69
Low Voltage Power Circuit Breakers
Review 6
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. When a low voltage power circuit breaker closes, its arcing contacts make
before its main contacts. The arcing contacts also break before the main contacts when the same circuit breaker opens.
TRUE FALSE
2. The Magnum DS movable contact design centers around a single contact finger as shown graphically below. Two parts of a contact finger are being
pointed to by arrows. Identify those two parts by filling in the blank spaces
next to the arrows.
A—_____________ ____________ contact
B—_____________ ____________ contact
3. A “Heel-Toe” contact structure is utilized by Magnum DS power circuit breakers. When all the “Heels” of all the contact fingers are touching and all the
“Toes” are not, the circuit breaker is in what position? Circle the best answer
from the choices below.
(a) the circuit breaker is ready to close
(b) the circuit breaker is open
(c) the circuit breaker is closed
(d) None of the above
4. The naturally generated force used by Magnum DS power circuit breakers to
temporarily increase the closing contact pressure is called the
________________-________________ force.
5. Magnum DS power circuit breakers also use a similar naturally generated
force to speed up the circuit breaker’s opening action.
TRUE FALSE
6. The stored energy spring mechanism of both manually operated and electrically operated low voltage power circuit breakers can be charges manually.
TRUE FALSE
7. The arc chamber of a Magnum DS power circuit breaker _____________ and
______________ each circuit breaker pole from one another, from the rest of
the circuit breaker, and from operating personnel.
8. Because Magnum DS power circuit breakers use arc chambers, they do not
use arc chutes like older power circuit breaker designs.
TRUE FALSE
9. Most modern low voltage power circuit breaker trip units are true
_________________ sensing microprocessor-based designs.
Page 70
Low Voltage Power Circuit Breakers
10. ANSI Standards require low voltage power circuit breakers to have an integrally mounted trip unit.
TRUE FALSE
Page 71
Low Voltage Power Circuit Breakers
Protective Systems
All electrical power distribution systems are intended to provide power to equipment with the utmost in safety. System protection, with the primary goal of safety,
is designed to balance the remaining goals of conductor/equipment protection and
service continuity at the most reasonable cost. To meet these goals, protective
equipment, under fault or overload conditions, must isolate the affected section of
the power system to maintain service to other sections, minimize equipment damage, and limit the duration of any outage. Depending on the design, these functions are performed by protective systems with varying degrees of effectiveness.
There are three basic approaches for designing a power system protection package. The system can be:
•
Series-Combination Rated
•
Fully Rated
•
Selectively Coordinated
Each approach safely protects the system. The degree, however, to which service
continuity is preserved on unfaulted portions of the system and the initial cost of
the system differs by the system design selected.
Low voltage power circuit breakers are capable of functioning on any system
selected. Because of their short time ratings, however, low voltage power circuit
breakers are ideally suited for selectively coordinated systems. Only the
selectively coordinated system will be discussed.
The Selectively Coordinated Protective System maximizes service continuity. All circuit breakers are fully rated to interrupt the maximum fault current
available at their point of application on the system. With selective coordination,
only the circuit breaker nearest the fault operates to isolate the faulted circuit from
the power source.
A selectively coordinated system requires that each upstream circuit breaker
have short time protection. The upstream circuit breakers must be capable of
withstanding the thermal and magnetic stresses delivered by the fault current for the period of time it takes the circuit breaker nearest the fault to
interrupt the fault. This type of duty requires the withstand capabilities only available with a low voltage power circuit breaker.
In most instances, the initial cost of a selectively coordinated system is
higher than the other systems mentioned. The continuity of service, however,
cannot be matched.
The selectivity of a system can be based on any one of the following alternatives:
•
Magnitude of the fault current (current selectivity)
•
Duration of time during which system experiences fault current (time selectivity)
•
Combination of current and time (complete selectivity)
Let’s take a look at a simple system example to illustrate each of the just mentioned alternatives.
Selectively Coordinated System Example:
Take a look at the graphic of a simple power system using all low voltage power
circuit breakers (Figure 65). The large X marked “C” represents where a fault
occurs. Circuit breaker “B” is the downstream circuit breaker and “A” the
upstream circuit breaker.
Page 72
Low Voltage Power Circuit Breakers
For a completely selective system, upstream circuit breaker “A” requires
greater trip time than does downstream circuit breaker “B” for all values of current; hence complete selectivity. Complete selectivity requires that the time-current characteristic curves of the two circuit breakers should not overlap at any
point. The curves would maintain a sufficient time interval (spacing on the coordination plot shown) to allow for normal operating time of the downstream circuit
breaker (B) before a “trip” operation would be initiated for the upstream circuit
breaker (A).
Current selectivity occurs between circuit breakers “A” and “B” when downstream circuit breaker “B” has a lower Continuous Current Rating and a lower
instantaneous trip setting than upstream circuit breaker “A”. Current selectivity
increases with increased differences in the continuous and instantaneous settings
of the upstream and downstream circuit breakers.
Time selectivity occurs between upstream circuit breaker “A” and downstream
circuit breaker “B” when, for the same value of current, the upstream circuit
breaker utilizes a longer time delay than the downstream circuit breaker. Time
delay selectivity is best achieved with circuit breakers having trip units
equipped with significant short time delay capabilities. The upstream circuit
breaker must be sufficiently delayed in initiating a trip operation to allow the downstream circuit breaker enough time to clear the fault completely. This means that
the upstream circuit breaker must be capable of withstanding the thermal and
magnetic stresses introduced by the fault current during the time delay. Historically, this type of selectivity has been and continues to be achieved with power circuit breakers having short time capabilities along with significant withstand
ratings.
Figure 65. Selectively Coordinated System Example
Power system maximum
fault current through
breakers A and B is 65 kA
to fault at C.
Breaker A is completely
selectively coordinated
for all values of fault current; breaker B isolates
all faults at C.
Breaker A is current
selectively coordinated
for all fault conditions of
less than 50kA; both A
and B trip for faults above
50kA at C.
Breakers A and B relatively close in continuous
current rating; breaker A
is time selectively coordinated for Fault Currents
to C above 30 kA.
Long Delay Protection
(L)
Long Delay Protection reacts to overload conditions and certain short circuit
conditions. The protection consists of a long delay current setting, a long delay
time setting, and quite often a curve slope adjustment for the time setting.
Short Delay Protection
(S)
Short Delay Protection reacts to short circuit conditions (Figure 66). It is similar
to long delay protection in that it has a current setting, a time setting, and quite
often a curve slope adjustment for the time setting. Good system coordination
depends heavily on the flexibility of the trip unit’s short delay protection and the
withstand capabilities of the circuit breaker. Low voltage power circuit breaker time
adjustments are adjustable up to 0.5 seconds or 30 cycles. Low voltage power
circuit breakers are designed, built and tested to withstand faults in the system in
which they are applied for this extensive amount of time.
Page 73
Low Voltage Power Circuit Breakers
Figure 66. Typical Short Delay Protection Adjustments
Typical Short Delay Current (Pickup) Adjustment
Typical Short Delay Time Adjustment Typical Short Delay Time Adjustment
With Flat Slope
With (I2T) Slope
Instantaneous
Protection (I)
Instantaneous Protection reacts to high level fault conditions. The instantaneous setting establishes the current level at which the trip unit’s instantaneous
tripping function will trip the circuit breaker with no intentional time delay.
Ground Fault Protection
(G)
Ground Fault Protection reacts to ground fault conditions. The protection also
consists of a ground fault current setting, a ground fault time setting, and quite
often a curve slope adjustment for time.
Overcurrent Protection
Combinations
The overcurrent protection provided to a system can be provided in a number of
combinations of the long (L), short (S), instantaneous (I) and ground fault (G) protection functions just covered. It is common to hear trip units described in terms of
the protective functions offered using only the first letters (L,S,I,G) as the description. The following descriptions are just two examples of how trip units or circuit
breakers are often described:
1. LS Trip Unit - This tells you that this particular trip unit offers long (L) and
short (S) protection. It does not provide any other of the specifics or capabilities associated with that protection.
2. LI Circuit Breaker - This means that the circuit breaker is equipped with a trip
unit that offers long (L) and instantaneous (I) protection. Once again, specifics
are not provided.
The common protective function combinations you will encounter are:
•
LI (long and instantaneous)
•
LS (long and short)
•
LSI (long, short and instantaneous)
•
LIG (long, instantaneous and ground)
•
LSG (long, short and ground)
Page 74
Low Voltage Power Circuit Breakers
•
Characteristic Curve
Review
LSIG (long, short, instantaneous and ground)
The operating response of a trip unit is graphically represented by time-current characteristic curves. These curves show how and when a particular trip
unit will act for given values of current. The more versatile the trip unit, the easier
it is to accomplish close coordination and achieve optimum protection.
The programmable or adjustable parts of a trip unit permit movement of its characteristic curve or parts of the curve. The movement can be done both horizontally
and vertically on the time-current grid (Figure 67).
Figure 67. Advanced Trip Unit Time-Current Curve Adjustments
Advance Warnings and
Alarms
The more capable microprocessor-based trip units using various methods offer
early warnings of a specific condition or alarms of detected conditions. Many of
these features can be indicated directly by the trip unit, indicated by an accessory
device, or indicated on the screen of a remote computer. What is available and
how it is accomplished is a function of the specific trip unit. Some typical examples
of these advance warnings and alarms are:
1. High Load Alarm - The high load alarm is activated when the load current
exceeds a selected current level to give an advanced warning of the condition.
2. Long Delay Trip Alarm - The long delay trip alarm operates after the circuit
breaker trips due to an overload condition.
3. Short Circuit Trip Alarm - The short circuit trip alarm operates after the circuit breaker trips due to conditions, such as an instantaneous trip or a short
delay trip.
4. Ground Fault Trip Alarm - The ground fault trip alarm operates after the circuit breaker trips due to conditions that exceed the ground fault settings.
System Diagnostics and
Testing
Depending upon the sophistication of the trip unit, microprocessor-based trip units
can store information in memory for use during diagnostic activities. In a number
of instances, the reason for certain situations or operations can be directly determined from the trip unit.
Integral testing capabilities are also part of most microprocessor-based trip units
(Figure 68). The circuit breaker can usually be tested so the circuit breaker will
actually trip (Trip Test Mode) or tested without the actual operation of the circuit
breaker (No Trip Test Mode). Most circuit breaker designs usually continue to provide protection during the testing process
Page 75
Low Voltage Power Circuit Breakers
Figure 68. Close-Up of One Type of Magnum DS Trip Unit’s Front Accessible Testing Port
System Monitoring
Power Quality
Monitoring
Energy Monitoring and
Management
Communications
Advanced microprocessor-based trip units are capable of monitoring much or all
of the following data:
•
Steady-state value of phase, neutral and ground currents
•
Minimum and maximum current values
•
Average demand current
•
Cause of trip
•
Magnitude of fault current causing trip operation
The most sophisticated trip units are capable of calculating and displaying in
some manner power quality information. Typical displayed information would be:
•
Percentage of harmonic content
•
Total harmonic distortion (THD)
Once again, the more sophisticated trip units can monitor and display in some
manner power and energy values, such as:
•
Power in kilowatts
•
Peak demand in kilowatts
•
Total energy in kilowatt-hours
•
Forward and reverse energy in kilowatt-hours
Most microprocessor-based trip units are capable of communicating in some
manner. Two way communications is possible via a network twisted pair for
remote monitoring and control. The circuit breaker, through the trip unit, is able to
respond to open and close commands via the communications network.
Many Eaton Electrical devices, including trip units, are IMPACC compatible
devices. Eaton’s Cutler-Hammer IMPACC network is a unique system that centralizes multiple monitoring, protection, and control devices in a building’s electrical distribution system. Compatible devices can be remotely monitored, controlled
and even programmed. The IMPACC Communications Network called INCOM is
a noise immune communications network. INCOM interconnects microprocessor
based electrical distribution and control products with remote personal computers
into an information and control communications network.
Applications
You learned that low voltage power circuit breakers have a number of unique features and/or capabilities. Some of the features make them the ideal or, in some
instances, the only choice in circuit breakers. Most of this centers around short
time capabilities. The more and/or closer the required system coordination, the
more valuable a low voltage power circuit breaker becomes. Because a number of
topics have been discussed, let’s take a look at several application examples.
This will help to pull a number of these capabilities together in a real way.
Page 76
Low Voltage Power Circuit Breakers
Zone Selective
Interlocking
Today’s modern microprocessor-based trip units are usually available with zone
selective interlocking. Zone selective interlocking is provided for the short delay
and ground fault delay tripping functions for improved system protection.
The zone selective interlocking feature is a means of communications over a
pair of wires between two or more compatible trip units. Zone selective interlocking makes it possible for programmed trip unit settings to be altered automatically
to respond to different fault conditions and locations. This results in the effects of
the interruption being localized and provides positive coordination between circuit
breakers provided.
A typical ground fault protection scheme using zone selective interlocking is
graphically illustrated (Figure 69). For circuit breakers outside the zone of protection, the trip unit on the circuit breaker nearest the fault sends an interlocking signal to the trip unit of the upstream circuit breakers. This interlocking signal
restrains immediate tripping of the upstream circuit breakers until their programmed coordination times are reached. Thus correctly applied zone selective
interlocking can reduce damage due to short circuit or ground fault conditions.
Figure 69. Ground Fault Protection Scheme Using Zone Selective Interlocking
Magnum DS Application Example:
You learned that Magnum DS low voltage power circuit breakers have:
•
Higher short time ratings (withstand capabilities)
•
Even higher interrupting ratings
These facts, when applied to a real world application, result in interesting results.
Let’s take a look at an industrial user with a large transformer feeding a facility.
This user has multiple service entrances utilizing an outdoor liquid filled 3750 kVA
transformer. Each of the connections from the transformer enter the facility and
each feed separate 3000 ampere main circuit breakers (M1 and M2). Each main
circuit breaker in turn feeds a number of 800 ampere feeder circuit breakers (F1
through F6). This might be a good time to review the one-line diagram for this
example (Figure 70). The diagram represents an example using insulated case
circuit breakers first and then Magnum DS power circuit breaker.
Page 77
Low Voltage Power Circuit Breakers
Figure 70. Insulated Case and Magnum DS Application Example One-Line Diagram
Prior to Magnum DS low voltage power circuit breakers, the user utilized insulated case circuit breakers in order to obtain a high enough interrupting capacity
for the application. It was calculated that this system had a potential fault capability of 93,500 amperes when everything that could contribute was considered.
From the calculation, it was obvious that the system required circuit breakers with
an interrupting rating of 100,000 amperes. The user would not consider the
expense of fused power circuit breakers and did not have enough floor space anyway. The only acceptable solution at that time was using insulated case circuit
breakers with an interrupting capacity of 100,000 amperes.
There was a drawback to this solution. The insulated case circuit breakers only
had a short delay current rating of 25,000 amperes for the feeder circuit breakers
and 35,000 amperes for the main circuit breakers. Thus, for a fault greater than
35,000 amperes on the load side of the starters within any of the motor control
centers, total coordination would be lost.
A 50,000 ampere fault, for example, at point “A” on a branch circuit of motor control center 1 (MCC1) would take the entire motor control center 1 (MCC1) out of
service because of circuit breaker (F1) opening. In addition, all the other motor
control centers connected to Bus #1 would be lost because of main circuit
breaker (M1) opening.
With the availability of Magnum DS low voltage power circuit breakers, the picture
and the results would be quite different. If Magnum DS circuit breakers with a
short delay rating of 85,000 amperes and an interrupting rating of 100,000
amperes were used, the coordination achieved would be much better with far less
equipment out of service. In addition, Magnum DS circuit breakers would require
less floor space.
Let’s conclude this application example by tabulating a comparison between the
insulated case circuit breaker, the Magnum DS power circuit breaker and a DSLII
(integrally fused DSII) power circuit breaker. For the sake of comparison and this
tabulation, let’s assume that floor space is not limited and that fused power circuit
breakers could be applied.
By comparing the data presented in Figure 71, it is clear that the DSLII power circuit breaker could handle the 50,000 ampere fault condition just described. There
are, however, three drawbacks associated with using the DSLII power circuit
breaker in lieu of the Magnum DS power circuit breaker. They are:
1. The initial installation cost for DSLII would be the highest of the three circuit
breakers.
Page 78
Low Voltage Power Circuit Breakers
2. DSLII circuit breakers would require significantly more floor space than the
other two circuit breakers.
3. The DSLII does not have a short time rating. Therefore, it cannot selectively
coordinate with downstream devices.
Figure 71. Application Tabulation Table
Page 79
Low Voltage Power Circuit Breakers
Review 7
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. Of the three approaches to system protection listed below, circle the one that
offers the best service continuity.
(a) Series-Combination Rated
(b) Selectively Coordinated
(c) Fully Rated
2. The short delay time adjustment of a low voltage power circuit breaker is
adjustable up to ___________________ seconds or ___________________
cycles.
3. The area represented by dotted lines on the time-current graphic below represents a typical short delay ___________________ adjustment.
4. What types of protection would an LSG type trip unit offer?
______________________________
______________________________
______________________________
5. If a trip unit offers a slope adjustment on ground fault protection, it means that
the portion of the curve representing the ground fault current adjustment can
have its slope changed.
TRUE FALSE
6. Zone selective interlocking permits two or more compatible devices, such as
trip units, to communicate over a ___________________ of
__________________.
Page 80
Low Voltage Power Circuit Breakers
Meaning of
Nameplate Ratings
Low voltage power circuit breakers are applied at their nameplate ratings. This
is a good time to elaborate a bit on the subject, especially because we just covered a real application situation with Magnum DS circuit breakers. Let’s take a
closer look at a Magnum DS nameplate and see what can be learned.
Nameplates are prominently displayed on the front of the circuit breaker. Some of
the information presented is general in nature, but most of it is specific to that circuit breaker. Refer to the Magnum DS nameplate graphic, and let’s briefly cover
the referenced areas (Figure 72).
Figure 72. Typical Magnum DS Nameplate
A—Magnum DS, this is the name of the family of low voltage power circuit breakers.
B—This is the identification number for this particular low voltage power circuit
breaker. Refer to the Magnum DS Identification System Graphic to interpret this
number (Figure 38). Identification numbers present a significant amount of information. In addition, identification numbers are frequently used in industry when
discussing and/or ordering circuit breakers. This is a good time to start getting
used to the concept of a product numbering system, whether it is called a catalog
number or an identification number.
The Magnum DS Identification System Table is rather self explanatory. Let’s just
consider and interpret the identification number on the nameplate, MDS-C32.
Figure 73. Magnum DS Identification System
1. The “M” stands for Magnum.
2. The “DS” identifies that this circuit breaker is tested to and for application in
keeping with ANSI Standards.
Page 81
Low Voltage Power Circuit Breakers
3. The “C” indicates that this circuit breaker has an interrupting capacity of
100,000 amperes.
4. The “32” indicates the frame size in amperes, which is 3200 amperes for this
circuit breaker.
C—Additional ratings specific information is presented here, some of which you
already know by understanding the identification number.
D—This is a very key area of the nameplate. The majority of information on how
this particular circuit breaker can be safely applied is presented in this area. Each
line starts off by indicating the maximum voltage at which the ratings that follow
apply. You will notice that the ratings are the same for every voltage on the Magnum DS nameplate. This is an area where assumptions cannot be made. The
same ratings apply for a specific Magnum DS frame size for all three maximum
voltages. This is not the case with every type of low voltage power circuit
breaker. Some have reduced ratings at certain voltages. Be observant!
The 100,000 indicates that this circuit breaker will safely interrupt a fault current
as high as high as 100,000 amperes instantaneously.
The 85,000 indicates that this circuit breaker has this withstand capabilities to
safely deal with a fault current as high as 85,000 amperes on a short time basis
up to 0.5 seconds or 30 cycles.
E—This is another area of the nameplate where information specific to this particular circuit breaker is indicated. The voltage rating of the circuit breaker’s factory
installed accessories are shown here. In addition, specific manufacturing information pertaining to this circuit breaker can be found in this area, like the General
Order Number (G.O. #).
F—This area of the nameplate outlines a reference which specifies enclosure
construction details to maintain published ratings.
Low Voltage Power
Circuit Breaker
Summary
In this training module, many points were made about Low Voltage Power Circuit Breakers. Issues were presented that applied to all low voltage power circuit
breakers, and to specific low voltage power circuit breakers. In this summary, we
will briefly review many of the points made. Some of the issues are unique to low
voltage power circuit breakers while others are common, but not necessarily
unique. A general summary is presented first followed by a Magnum DS summary. This summary is not intended to be all inclusive, especially as it relates to
Magnum DS. It will, however, be an excellent review and act as a good future reference.
Standards and Testing
Summary
1. ANSI and IEC are the two most dominant standards relating to low voltage
power circuit breakers worldwide.
2. ANSI Standards are the recognized North American Authority on low voltage power circuit breaker design.
3. ANSI C37.50 is the primary standard for testing low voltage power circuit
breakers.
4. ANSI C37.50 testing is composed of four test sequences. The fourth test
sequence is unique to low voltage power circuit breakers. Other types of low
voltage circuit breakers are not subjected to this test sequence. This fourth
test sequence determines whether or not a low voltage power circuit breaker
has withstand capabilities to receive a 30 cycle short time rating. During
the actual testing, the circuit breaker is subjected to two 30 cycle tests.
Page 82
Low Voltage Power Circuit Breakers
5. ANSI Low voltage power circuit breakers are always rated for continuous
operation at 100% of their current ratings in their enclosure.
6. ANSI Standards do not specify the nature of construction or construction
material for low voltage power circuit breakers.
7. ANSI Standards require low voltage power circuit breakers to provide an
open-close-open duty cycle. This is accomplished with a two-step spring
stored energy mechanism.
8. ANSI Standards require low voltage power circuit breakers to have integral
trip units.
General Features
Summary
1. Precise system coordination with unmatched short time ratings and withstand capabilities
2. Applied in a system at nameplate ratings
3. 5 cycle closing maximum
4. Strong, rigid frames to deal with the physical stresses associated with
short time ratings (withstand capabilities)
5. Two-step stored energy spring operating mechanism
6. Both manual and electrical operation
7. Integral trip units
8. Typically supplied as four position drawout circuit breakers
9. Through the door or behind the door drawout
10. Fixed mount configurations
11. Fused configuration for 200,000 ampere interrupting
12. Maintainable design
Magnum DS Summary
1. Meets or exceeds ANSI/UL Standards for low voltage power circuit breakers
2. 3 cycle closing
3. Rigid frame housing of engineered composite resins
4. Rugged, maintainable construction
5. Compact and lightweight
6. Through the door drawout or Fixed
7. Higher short time ratings (withstand) than DSII
8. Higher interrupting ratings than DSII
9. Field convertible manual to electrical operation
10. Field installable UL listed accessories
11. Front viewable and mounted electrical accessories
12. Only two frame sizes, 800 through 5000 amperes (smallest 3200 ampere
frame in industry)
13. Heel-toe contact structure
14. Improved contact material
15. Compact “C” loop current path
Page 83
Low Voltage Power Circuit Breakers
16. Braided current path connections
17. Sealed arc chamber for isolation and insulation
18. Programmable, RMS sensing microprocessor-based trip units
19. Moving contact wear indicator
20. Zone selective interlocking
21. IMPACC compatible communications
Page 84
Low Voltage Power Circuit Breakers
Review 8
Answer the following questions without referring to the material just presented.
1. A Magnum DS power circuit breaker identified as MDS-616 is a
___________________ ampere frame circuit breaker.
2. What is the interrupting capacity of a Magnum DS power circuit breaker with
an identification number of MDS-C32?
___________________ kA interrupting
3. The Magnum DS family of power circuit breakers uses only two different
frame sizes to cover _________________ through _________________
amperes.
4. All low voltage circuit breakers are rated for continuous operation at 100% of
their current ratings in their enclosure.
TRUE FALSE
5. The two-step spring store energy mechanism used in a low voltage power circuit breaker makes it possible for the circuit breaker to perform an openclose-open duty cycle, which is required by ANSI Standards.
TRUE FALSE
6. A drawout Magnum DS power circuit breaker can be levered into or out of
three different positions within its enclosure without ever opening the enclosure’s front door. Name these three positions.
(a) ______________________________ position
(b) ______________________________ position
(c) ______________________________ position
7. Magnum DS power circuit breakers have field installable electrical accessories which can be viewed from the front of the circuit breaker. They must, however, be installed at the factory to be UL listed.
TRUE FALSE
8. Magnum DS power circuit breakers offer higher short time ratings as well as
higher _____________________ ratings.
Page 85
Low Voltage Power Circuit Breakers
Glossary
Air Circuit Breaker
A circuit breaker that makes and breaks power circuits in
air using arc chutes.
Arcing
The effect generated when electrical current bridges the
air gap between two conductors that are not touching.
Arc Chute
A component of the arc extinguisher in a low voltage
circuit breaker. It elongates and cools an arc.
Arc Extinguisher
A common method used to extinguish an arc. In
general, it confines, divides and cools the arc.
ANSI
American National Standards Institute.
Butt
A design type for making contacts in a circuit breaker.
Circuit Breaker
A reusable overcurrent protection device. After tripping
to break the circuit, it can be reset to protect the circuit
again.
Connected Position
The position in which the circuit breaker is fully
connected and functional.
Contacts
Method to open and close the circuit as the contacts
come together or separate.
Continuous Current
Rating
The amount of current the breaker can carry
continuously at 60 cycles without exceeding the
temperature rise limit, according to ANSI charts.
Control Voltage
Voltage used to operate secondary devices.
Coordination
The process of selecting and applying circuit breakers in
an electrical distribution system to localize a fault
condition and restrict power outages to the equipment
affected
CSA
Canadian Standards Association
Disconnect Position
The position in which neither the primary nor secondary
electrical connections of the circuit breaker are made.
This position is often used as a storage position for the
circuit breaker within its enclosure.
Drawout
A type of circuit breaker that can be moved into or out of
its structure without unbolting, often on a racking
mechanism.
Faceplate Shield
A device to protect the operator from dangerous
voltages while the breaker door is open.
Finger
A design type for making contacts in a circuit breaker.
Finger Clusters
Automatic main disconnect devices for a circuit breaker.
Fixed
A type of circuit breaker that is bolted into a fixed
position with bus or cable mechanically bolted to
breaker terminations.
Ground Fault
Protection
A form of protection which reacts to ground fault
conditions.
IEEE
Institute of Electrical and Electronic Engineers. An
objective technical organization made up of
manufacturers, users, and other general interest parties
Page 86
Low Voltage Power Circuit Breakers
Integrally Mounted
Trip Unit
The trip unit is inside of, or part of, the circuit breaker.
International
Electrotechnical
Commission
Abbreviated IEC. This organization is associated with
equipment used internationally.
Interrupting Rating
Also “Ampere Interrupting Capacity (AIC).” A rating of
the amount of current that a protective device, such as a
fuse or circuit breaker, can safely interrupt.
Instantaneous
Protection
A form of protection which reacts to high level fault
conditions.
Lever
The act of moving the breaker from one position to
another.
Long Delay
Protection
A form of protection which reacts to overload conditions
and certain short circuit conditions.
NEMA
National Electrical Manufacturers Association.
SF6
An arc extinguishing technology involving the use of
sulfur hexafluoride gas.
Short Delay
Protection
A form of protection which reacts to short circuit
conditions.
Short Time Rating
A rating for how fast a circuit breaker will open,
expressed in cycles.
Stored Energy
A mechanism used to overcome inherent forces
opposed to the breaker closing process, which stores
energy until it is needed to help open the breaker.
Test Position
The position in which the circuit breaker’s primary
connections are disconnected. Secondary connections
are not disconnected and testing can be safely
performed because the circuit breaker is not energized.
Trip Free
Breaker cannot be prevented from tripping, even when
holding the handle in the ON position.
Trip Unit
Device that trips the operating mechanism in case of a
short circuit or overload condition.
Underwriter’s
Laboratories
UL. An independent laboratory that tests equipment to
determine whether it meets certain safety standards
when properly used.
Vacuum
An arc extinguishing technology. Features a pair of
separable contacts enclosed in a vacuum-tight
envelope. Because the environment inside the
interrupter envelope is a vacuum, an arc cannot be
sustained easily.
Withdrawn Position
In this position, the circuit breaker has no electrical
connections. It is far enough out of its enclosure to
permit inspection and maintenance.
Withstand Rating
See “Short Time Rating.”
Page 87
Low Voltage Power Circuit Breakers
Review 1 Answers
1. False
2. b. Test, c. Disconnect
3. True
4. Continuous current
5. True
6. Short time
7. True
8. Open Close Open
Review 2 Answers
1. Molded
2. False
3. Arc chutes, arc extinguishers
4. True
5. Trip unit
6. Sensors
7. Trip unit
8. Circles around a, b, and c
9. Left to right: b, b, a
10. Shunt trip
11. False
12. Truck, Operated, Cell
Review 3 Answers
1. False
2. Primary, secondary
3. Any 5 of the following:
Standards
Ratings
Operation Method
Accessory Items
Mounting Method
High or Low Ambient Temperatures
Moist or Corrosive Atmospheres
Altitude
High Shock Conditions
Unusual Circuit Breaker Mounting Conditions
4. False
5. 208
6. Application Voltage
7. False
8. True
9. Maximum
10. D
Page 88
Low Voltage Power Circuit Breakers
Review 4 Answers
1. True
2. Magnum
3. UL1066
4. C37.50
5. False
Review 5 Answers
1. True
2. Withstand
3. Short delay current
4. False
5. True
6. 100,000
7. DC Component
8. B
Review 6 Answers
1. False
2. upper right blank: Moving Arcing
lower left blank: Moving Main
3. C
4. Blow-on
5. True
6. True
7. Insulates, Isolates
8. False
9. RMS
10. True
Review 7 Answers
1. B
2. 0.5, 30
3. Short Delay Current
4. Long Delay Protection, Short Delay Protection, Ground Fault Protection
5. False
6. Pair, Wires
Review 8 Answers
1. 1600
2. 100 kA
3. 800, 5000
4. False
5. True
6. a: connected b: test c: disconnect
7. False
8. Interrupting
Page 89