Operate electrical switchgear in the
electricity supply industry
US 12387
Training and Assessment Resource
Level 4
Credits 6
Electricity Supply Industry Training Organisation
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www.esito.org.nz
Contents
Introduction to Training Assessment Resource
Glossary
3
4
1.0 Introduction5
1.1Rating
5
1.2 Switchgear selection
7
2.0 2.1 2.2 2.3 2.4 Switchgear medium 8
Oil filled switchgear
8
Air insulated switchgear
10
Vacuum switchgear 12
SF6 switchgear 13
3.0
Switchgear components15
3.1 Operating mechanisms
15
3.2 Switchgear enclosures 27
3.3 Closing and release mechanisms 30
3.4 Interlocks
30
3.5 Grading capacitors 37
3.6 Anti-pumping mechanisms 37
3.7 Closing and trip coils
37
3.8 Trip circuit supervision
38
3.9 Slow closing mechanism
38
3.10 Communications38
3.11 Auxiliary plant 38
3.12 Direct current (DC) systems38
3.13 Earthing 39
3.14 Operating rods39
3.15 Current and voltage transformer39
4.0 4.1 4.2 4.3 4.4 4.5
4.6
Types of switchgear
Circuit breakers
Reclosers and sectionalisers
Disconnectors
Earth switches
Point of wave switching
Resource management act 1991 (RMA)
40
40
42
45
46
48
48
5.0
Switchgear numbering49
5.1 High voltage switchgear numbering
49
5.2 Location or circuit identifier 49
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5.3
5.4
5.5
5.6
Equipment code
Unit number or unique identifier
Equipment number
11kv indoor switchgear numbering
50
50
51
51
6.0 Operator responsibilities
6.1 Prior to operating switchgear
6.2 Safety tips when operating switchgear 52
52
53
Answers to activities
55
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Introduction to Training
Assessment Resource
This Training Assessment Resource (TAR) contains the information that you require to complete the written assignment
in the assessment pack for this unit standard.
Purpose
People who obtain credit for this unit standard are able to:
Demonstrate knowledge of electrical switchgear in common use in electricity supply systems.
Describe the operating principles of switchgear in common use in electricity supply systems.
Identify and communicate switchgear status.
Operate electrical switchgear under normal service conditions.
Operate electrical switchgear in response to unplanned events.
Report electrical switchgear operation.
Getting started on the ESITO Training & Assessment Resources
Activity: A written or spoken exercise or assignment.
Keypoint: Important information to remember.
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Glossary
When I see this word
It means
Bellows Flexible connection between two compartments, used to ensure that compartments remain sealed.
Cam A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-­versa.
Crude Rough.
Dielectric strength Dielectric strength of an insulating material is the maximum electrical field strength the material can withstand before breaking down.
Fluctuations Unpredictable movements.
Inrush Current The large current that is drawn when a transformer is switched into the circuit.
Instantaneous Occurring, arising, or functioning without any delay; happening within an imperceptibly brief period of time.
Ionization The physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. Phenomenon
A fact or occurrence that appears or is perceived.
Tolerance
The permissible limit or limits.
Velocity
Speed in a given direction.
Pneumatic Energy stored in the form of air pressure.
Sacrificial Something that is allowed to be damaged to prevent damage to
another component.
Satellite Remote/removed from the main unit.
Sprocket A sprocket is a profiled wheel with teeth that meshes with a chain, track or other perforated or indented material. It is distinguished from a gear, in that sprockets are never meshed together directly, and differs from a pulley, in that sprockets have teeth and pulleys are smooth.
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1.0Introduction
The power system in New Zealand uses circuit breakers (CBs), disconnectors, earth switches, reclosers, and
sectionalisers. These are used to control the flow of electricity, to provide safe isolation and to protect equipment
connected to the system. The term used to cover all devices listed above is switchgear. This manual will discuss each of
these components.
Switchgear forms a critical part of the power system. They are used throughout the system to control the flow of
electricity by connecting/isolating the generators, transformers and lines that supply the electricity to the transmission
and distribution networks. They are also used within the transmission and distribution networks to control the flow
of electricity and to connect/isolate different lines within the networks and to connect and isolate the loads to the
transmission and distribution networks.
1.1 Rating
The rating of switchgear is defined by the following:
Nominal voltage
The term nominal voltage, also called voltage class, is used to define the rated system voltage for which the circuit breaker is designed to be used.
Rated maximum voltage
Rated maximum voltage is the maximum continuous voltage for which the circuit breaker is designed to be used.
Rated continuous current
Rated continuous operating current is the maximum operating RMS current that the switchgear is designed to
operate at.
Rated fault current
Rated fault current is also known as rated short-circuit current or maximum interrupt current. This is the maximum current that the circuit breaker can safely interrupt. It is important that this is set above the maximum prospective fault current of the circuit.
Capacitive current rating
Switching capacitive loads is hard on the switchgear and can produce switching surges. Some switchgear designs are better suited than others for switching capacitive loads. Most switchgear will state in the rating the level of capacitive switching it is rated for.
Impulse voltage
This is the rating that gives the switchgear the ability to withstand voltage impulses caused by factors
like lightning.
Duty cycle
Duty cycle is the number of times in a given period that the switchgear can be operated. This may be stated as a time limit (e.g. 3 operations in an hour, or minimum time between operations) or as the number of normal and/or fault operations until maintenance is required.
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Explain the terms below in your own words using an example.
Nominal voltage –
Rated maximum voltage –
Rated continuous current –
Rated fault current –
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1.2 Switchgear selection
Factors that need to be considered when selecting switchgear include:
The power factor of the load. Power factor is used to define the relationship between the voltage and the current. A power factor of 1 indicates that voltage and current waveforms are in phase (resistive load). As capacitance is added to the circuit, the current will start to lead the voltage. A leading power factor of 0 would indicate that the current leads the voltage by 90 degrees or a purely capacitive load.
If inductance is added to a resistive circuit, the current will start to lag the voltage. A lagging power factor of 0
would indicate that the current lags the voltage by 90 degrees or a purely inductive load. Typically, the power factor of a load will be lagging with a value >0.8. When the voltage waveform on a circuit with a power factor
<1 goes through the zero crossing, there will still be current flowing. The lower the power factor, the larger the
percentage of current that will be flowing when the voltage goes through the zero crossing. This factor makes
it harder to break the current and can affect the rating of the circuit breaker.
Transformer inrush can exceed the rating of switchgear or cause it to operate unnecessarily. Transformer inrush is a phenomenon that is caused by the transformer becoming saturated when it is switched on, due
to residual magnetism of the transformer core. While only a transient effect lasting a relatively short period of time, it can reach values as high as 30 times the rated transformer current. Point on wave switching can be used to lessen the effects of transformer inrush.
Likewise, not accounting for circulating currents from paralleled supplies may exceed the rating of switchgear or cause it to operate unnecessarily. The circulating currents are caused by a voltage mismatch. This can be caused by differing circuit impedance or tapchangers on transformers.
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2.0
Switchgear medium
There are a number of different mediums used in switchgear. They are defined by the medium that is used to insulate
the switchgear and the medium and method used to quench/extinguish/cool the arc.
2.1 Oil filled switchgear
In oil filled switchgear, the oil performs two functions. The first function is to insulate between energised and
de-­energised parts of the switchgear. The second function is to quench/extinguish the arc, as well as dissipate heat
caused by the arc.
When an arc is drawn, it heats the oil in the vicinity of the arc. This oil expands and is forced up through the arc control
system. This draws the arc with it, causing the arc to lengthen and increase the resistances that the arc has to travel
until the arc can no longer be maintained and it is extinguished.
Oil filled switchgear present a health and safety risk due to the risk of fire and explosion and also present an
environmental risk due to the oil contained. They also require significant maintenance and consideration needs to be
given to how to dispose of the waste oil during this maintenance.
There are two types of oil filled switchgear, bulk oil and minimum oil.
Bulk oil
Bulk oil switchgear are also known as dead tank oil filled switchgear. In bulk oil switchgear, the oil is used to insulate
between the phases and from phase to earth. On 110kV and 220kV, the phases are in separate tanks from each other
and can hold up to 20,000 litres of oil. These curcuit breakers are no longer available in the extra high voltage rating as
they require large volumes of oil, and are very large and heavy.
Figure 1 - Bulk oil circuit breaker
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Minimum oil
Minimum oil switchgear are also known as live tank oil filled switchgear. They are similar in construction to bulk oil
switchgear, except that they contain less oil. This is achieved by separating each phase into a separate tank, isolating
the tanks from each other and earth with insulators. In this way, the oil is only required to quench the arc and ensure
isolation across the contacts when in the open position. Therefore, a lot less oil is required.
Figure 2 - Minimum oil circuit breaker
Explain the difference between bulk oil and minimum oil circuit breakers.
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2.2 Air insulated switchgear
Air break
Air break switchgear use air as the insulation medium and there are a number of different configurations. The basic
principle is that the heat formed by the arc is used to extend the arc upwards until the length of the arc is too long to
be maintained.
Figure 3 - Air break switches
Air blast
Like air break, air blast switchgear use air as the insulating medium and to quench the arc. However, with air blast
switchgear compressed air is used to blast the arc causing it to extend and extinguish.
Many air blast circuit breakers have been removed as the release of the air causes a significant amount of noise.
The noise is similar to a rifle shot. When using air blast circuit breakers, consideration needs to be given to the noise and
its effects on neighbours.
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Figure 4 - Air blast circuit breaker
Explain how the blast of air is produced in an air blast circuit breaker.
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2.3 Vacuum switchgear
In a vacuum breaker, there is a set of contacts contained in a sealed cylinder and a vacuum is applied to the cylinder.
The vacuum cylinder is commonly referred to as a vacuum bottle. One of the contacts in the cylinder will be fixed, the
other is able to move and is attached to the operating mechanism via bellows to allow the contact to be opened or
closed. When the circuit breaker is opened, the operating mechanism will draw the moving contact away from the
fixed contact.
Vacuum bottles
Figure 5 - Vacuum circuit breaker
A vacuum has a high dielectric strength as there is no material (no electrons) to conduct. As the contacts are separated,
an arc will be formed by ionization of metal vapours from the contacts. However, the arc is quickly extinguished because
the metallic vapour quickly condenses on the surface of the contacts.
Fixed terminal
End shield
Insulating envelope
Electrodes
Bellows shield
(arc shield)
Vapour
condensation
shield
Bellows
Moveable terminal
Figure 6 - Cross-­sectional diagram of a vacuum bottle
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Figure 6 shows the typical configuration of a vacuum bottle. The outer body of the vacuum bottle is usually made from
glass or ceramic. Contained within the vacuum bottle are both the fixed and moving contacts and the arc shield. The
moving contact is connected to the operating mechanism by bellows. This allows movement of the contact without
breaking the seal of the vacuum bottle. The arc shields and vapour condensing shield prevent metallic vapours forming
on the inside surface of the vacuum bottle.
The contact movement is only a small amount (10 to 30mm) and this type of breaker is typically up to 33kV.
2.4 SF6 switchgear
SF6 curcuit breakers are now the most common curcuit breaker used on transmission circuits. SF6 is the chemical
symbol for Sulphur Hexafluoride. It is a colourless, odourless, heavy gas. SF6 is an electronegative gas. This means
that electrons are attracted to the gas. This gives the gas a very high dielectric strength. At atmospheric pressure, it
has 2-­3 times the dielectric strength of air, and at twice atmospheric pressure, it is comparable to transformer oil.
SF6 is however 24,000 times stronger than CO2 as a greenhouse gas. For this reason, many companies have signed a
memorandum of understanding regarding the use of SF6. This requires the companies to monitor and record the use of
SF6 and the amounts of SF6 stored in devices. The usage (or leakage) is limited under this agreement.
SF6 circuit breakers use the SF6 gas as both an insulator and arc control medium. When opening, the SF6 circuit breaker
directs a flow of SF6 past and through the arc and causes the arc to be extinguished. On some SF6 breakers, the heat
from the arc is also used to aid the interruption of the arc. By using the heat generated in the arc to heat the gas, this
creates a pressure rise in the gas (in the interruption area) and then this higher pressure forces the gas through the arc
at a high velocity.
Figure 7 - SF6 circuit breaker
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What environmental effect needs consideration when undertaking maintenance on an SF6 curcuit breaker?
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3.0
Switchgear components
3.1 Operating mechanisms 3.1.1 Operating handles
All switchgear can be controlled manually; many can also be controlled remotely. Where fitted with an automated drive,
the manual handle drives the same mechanism as the automated drive. There are interlocks fitted to prevent the remote
operation being performed when manual operation is required.
Figure 8 - Manual operating handles
3.1.2 Stored energy devices
The drive mechanism on a circuit breaker is usually supplied via a stored energy device. Stored energy devices are
used to ensure that no matter what happens to the control supply, the circuit breaker can always be opened or closed. It
also ensures that the circuit breaker does not stop halfway through its operation and always has the correct amount of
energy to operate the breaker contacts at the correct speed.
Typical storage methods include:
3.1.2.1Springs
Springs are a common method of storing energy. They are used to both open and close the circuit breaker.
A motor or manual handle is used to charge the spring, and the spring is then left charged until required.
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Figure 9 - Close and trip springs on a SF6 circuit breaker
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Closing operation
When the breaker is being closed, the closing latch
(6) is released by the closing coil. The sprocket (7) is
locked to prevent rotation where upon the operating
energy in the closing springs is transferred via section
(8) of the endless chain to the sprocket (11) belonging
to the cam disc (3).
1
2
6 11
3
The cam disc then pushes the operating lever (2)
towards the left where it is locked in its end position by
the tripping latch (1).
7 B
8
9
The last part of the rotation of the cam disc is damped
by the damping device (9) and a locking latch on the
sprocket (11) again takes up the initial position against
the closing latch (6).
A
Charging of the closing springs
The breaker has closed; the motor starts and drives the
1
sprocket (7).
2
1
6 11
6
The sprocket (11) belonging to the cam disc (3), has
its catch locked against the closing
latch (6),
3
whereupon the sections of the chain (8) raise the
spring bridge (10).
11
4
3
7 B
The closing springs (5) are thereby charged
A and
8 its normal operating
the mechanism again takes up
9
position.
8
5
7 B
B
AA
5
10
Table 1 - Typical operating principle of an open and closing spring circuit.
1 2
1
4
3
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B
5
A
B
A
9
5
Closed operation
In the normal service position of the circuit breaker (B),
1
the contacts are in closed
2 position, with closing (5) and
6 11
opening spring (A) charged.
1
The breaker is kept in the closed3 position by the
opening latch (1) which takes up the force from the
charged opening spring. The mechanism is now ready
to open upon an opening command
7 Band can carry out a
A cycle.
complete fast auto re-closing (0 - 0.3 s - CO)
6
11
4
3
7 B
8
9
B
8
A
A
5
5
10
Opening operation
When the breaker is being
opened, the latch (1) is
1
released by the tripping coil.
The opening spring (A) pulls the breaker (B) towards
the open position. The operating lever (2) moves to the
right and finally rests against the cam disc (3).
The motion of the contact system is damped towards
the end of the stroke by an oil-filled BdampingA device
5
(4).
1 2
4
3
B
A
Table 1 (continued) - Typical operating principle of an open and closing spring circuit.
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1 Main shaft
2 Closing spring
3 Cam disc
4 Closing lever
5 Switching shaft
6 Trip spring
7Motor
Figure 10 - Spring charging circuit
3.1.2.2Hydraulics
Energy may be stored in the form of hydraulic pressure. This pressure is then used to operate the breaker
when required.
Interrupters
Top post insulators
Bottom post insulator
Hydraulic operating
mechanism
Oil tank
Figure 11 - Hydraulic operated SF6 circuit breaker
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3.1.2.3Capacitors
Storing energy in capacitors is a relative new method of operating circuit breaker mechanisms. The capacitors are
charged and when the breaker is operated, the capacitors are either discharged through a motor that drives the circuit
breaker mechanism or used to operate a solenoid.
Figure 12 - Control cubicle for a capacitor operated SF6 circuit breaker
3.1.2.4Pneumatic
Energy may be stored in the form of air pressure. This pressure is then used to operate the breaker when required.
3.1.2.5 Battery banks
Battery banks can also be used to store energy. These can then be used to operate a solenoid.
3.1.3 Control cabinets
Switchgear that can be operated remotely or via protection relays will have a control cabinet associated with them.
This will house at least the control and indication circuitry and drive motor. Typically, it would also include the stored
energy device and manual operating devices.
To ensure that the switchgear can be operated remotely and the indication circuits work when the local service power is
interrupted, the control circuit is often DC. The control and indication circuits may also be at different voltages. Therefore,
in the control cabinet, there is often three sets of fuses or MCB, one for the drive motor (typically 400V AC), one for the
control circuitry (DC) and one for the indication circuit (also DC).
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What is the purpose of the stored energy device in circuit breakers?
List 3 stored energy devices.
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3.1.4Contacts
3.1.4.1 Main contacts
The main contacts are the current carrying contacts. Typically, these are a male/female set of contacts. Usually one
contact is fixed and the other is moved by the control mechanism.
3.1.4.2 Arcing contacts
Often, there is a set of contacts that breaks after and makes before the main current carrying contacts. These contacts
are sacrificial, and are designed to limit the damage to the main contacts. In air break switches, this is commonly
achieved by arcing horns.
Note: Arcing horns make first and break last.
Arcing horns
Figure 13a Air break switch in closed position
Figure 13b Air break switch in open position
What is the function of arcing horns on an air break switch?
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3.1.5 Arc chutes
Arc chutes are physical barriers placed to control the flow of ionised air or gases. This is done for three reasons:
To ensure that the ionised air, or gas does not form a conductive path between phases or from phase to earth. This would result in a short circuit being applied to the switchgear and resultant failure.
To allow the ionised air or gas to cool prior to coming in contact with material that could be damaged.
To draw in clean air or gas that will help extinguish the arc.
WARNING: Care should be taken when handling arc chutes. Many of the older arc chutes will
contain asbestos.
Figure 14 - Arc chutes
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3.1.6 Dash pots or turbulators
Dash pots or turbulators are a device used in oil filled switchgear to help extinguish the arc. They work by forcing oil out
from around the contact and drawing in fresh oil. This has three effects:
The oil flow extends the arc. By extending the arc the resistance of the arc increases and this makes it easier
to break.
It forces out oil that has been carbonised by the arc and draws in clean oil.
It ensures that a conductive path is not formed between phases by the carbonised oil.
Figure 15 - Dash pots on an oil filled circuit breaker
Figure 16 - Diagram of contacts and dash pot on an oil filled circuit breaker
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3.1.7 Gas compression mechanisms
Gas compression mechanisms are used in gas filled switchgear such as SF6 breakers. The purpose of the gas
compression mechanism is to pressurise the gas and when an arc is formed to vent this pressurised gas past the
contact to extend the arc. It will also act to replace ionised gas in the same way as a dash pot would be used in an oil
filled breaker.
Valve closed
Valves open
Figure 17a - Diagram of a gas compression circuit on SF6 circuit breaker; circuit breaker in the closed position
Pressure builds closing this valve
Figure 17b - Diagram of a gas compression circuit on SF6 circuit breaker; circuit breaker beginning to open
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Figure 17c - Diagram of a gas compression circuit on SF6 circuit breaker; arc beginning to form
Pressure continues to build until
this valve open
Valve Closes
Figure 17d - Diagram of a gas compression circuit on SF6 circuit breaker; gas compression valve opening
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Gas vents out past the contacts
extinguishing the arc Figure 17e - Diagram of a gas compression circuit on SF6 circuit breaker;
gas released past the contacts, extinguishing the arc
3.1.8Insulators
Insulators are used to isolate active conductors from each other and from earth material within switchgear. The active
conductors are brought out by insulated bushings. There is often an insulating rod in the drive mechanism.
3.2 Switchgear enclosures
Where switchgear is used indoors, the switchgear is enclosed. There are two common forms of enclosures used.
These are metal clad and gas insulated switchgear (GIS).
3.2.1 Metal clad
As the name suggests, metal clad switchgear is clad with sheet metal. The interrupting devices, buses and control
equipment are completely enclosed by grounded metal barriers which have no intentional openings between
compartments. Automatic grounded metal shutters cover primary circuit elements when the circuit breaker is removed.
If a short circuit or fault occurs, damage should be contained within the metal clad enclosure. However, on some older
metal clad switchgear, not all components are clad. Therefore, a fault may still present a risk.
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voltage-transformer
secondary fuses
voltage-transformer
voltage-transformer
high-voltage fuses
current
transformer
bushbar chamber
circuit earthing
plugs
bushbar earthing
plugs
circuit-breaker
isolating plugs
self aligning secondary
plugs and sockets
circuit spout
safety shutters
bushbar spout
safety shutters
circuit-breaker
isolating socket
withdrawal handle
instrument panel
operating mechanism
circuit-breaker
fixed contact
moving contacts
arc control
chamber
operating handle
truck (removable)
moving contact
crossbar
shutter restoration
device
‘off’ position of
moving contacts
Figure 18 - Cross-­sectional diagram of metal clad switchgear
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Figure 19 - Metal clad switchgear
3.2.2 Racking
Most metal clad switchgear is designed so the switching device can be removed. Removal of the switching device
allows isolations to be applied, or the switching device to be used to earth the output circuit or incoming bus. It also
allows maintenance to be undertaken on the switching device. The removal of the switching device is referred to as
racking. To remove the switching device, it is either withdrawn vertically or horizontally and this is usually achieved by
turning a worm drive.
Manual
racking handle
Circuit breaker to be
racked into position
Automated
racking device
Figure 20a - Using an automated racking device
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Figure 20b - Using a manual racking handle
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3.2.3 Gas insulated switchgear (GIS)
As the name suggests, GIS is insulated with gas. With GIS switchgear, all active components are enclosed in metal enclosures and insulated with a gas, typically SF6. This allows extra high voltage to be switched within a relatively small area.
Figure 21 - GIS installation
3.3 Closing and release mechanisms
The closing and release (trip) mechanism is the device that controls the release of the stored energy to operate the
circuit breaker. An example of closing and release mechanisms is shown in table 1 on page 17.
3.4 Interlocks
Interlocks are used to ensure that unsafe actions are not undertaken. Examples of these include ensuring an earth
switch cannot be closed until the associated disconnector has been opened, or racking down a circuit breaker
before opening the circuit breaker. There are many more examples. Interlocks can be achieved either mechanically
or electrically.
3.4.1 Mechanical interlocks
Mechanical interlocks can be achieved by placing a mechanical device that blocks another operation. A mechanical
interlock can be directly linked between the devices, or controlled by an interconnecting control.
An example of a directly connected mechanical interlock is an arm attached to a disconnector that locks the drive mechanism of the earth switch until the disconnector is in the fully open position.
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In this example, there is a locking arm that needs to be moved to unlock the drive mechanism of the earth switch. However, the locking arm would only be able to be moved once the disconnector is in the fully open position or moved
as part of the disconnector operation.
Disconnector
open
Earth switch
closed
Interlock
mechanism
Disconnector
drive
mechanism
Earth switch
drive mechanism
Figure 22a - Disconnector and earth switch with interlock
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Interlock
Hole in the earth switch drive arm.
Note: The pin from the interlock
will not fit in the current position.
Therefore, the disconnect cannot be
closed until the earth switch is open.
Note: When the earth switch is
opened, the hole will align with the
pin in the interlock arm. Therefore, the
disconnector could be closed.
Figure 22b - Disconnector and earth switch interlock
Another way of achieving a mechanical interlock is to use a keyed interlock system. The earth switch drive mechanism
would be locked, and the lock would only be able to be freed by a key that is housed on the disconnector. The key
would only be released from the disconnector once the disconnector is in the fully open position. Once removed from
the disconnector, the disconnector would not be able to be closed until the key is removed from the earth switch and
returned to the disconnector. The key would only be able to be removed from the earth switch when the earth switch is
in the fully open position. In this way, the earth switch and the disconnector cannot be closed at the same time.
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Disconnector
closed
Earth switch
open
Disconnector
drive mechanism
Earth switch
drive mechanism
Figure 23a - Disconnector and earth switch with keyed interlock
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Disconnector drive
mechanism lock
Earth switch drive
mechanism lock
Note: Key in lock.
This key cannot be
removed until the
disconnector is in
the open position.
Figure 23b - Disconnector and earth switch keyed interlock
Note: Once the key is removed, the disconnector is in the open position.
Key
Note: This key can only
be removed from the
disconnector when the
disconnector is in the
open position.
Figure 23c - Disconnector keyed interlock
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Figure 23d - Earth switch keyed interlock
Earth switch drive
mechanism lock
Note: This pin locks the drive
mechanism in the open
position.
Note: The pin can only be
released with the key from
the disconnector.
Figure 23e - Earth switch keyed interlock
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3.4.2 Electrical interlocks
Electrical interlocks are achieved by blocking the electrical signal of one function by another. Using the same example,
the operating circuit of an earth switch would be blocked by a limit switch on the associated disconnector. In this way,
the earth switch could not be closed until the disconnector is in the fully open position and the limit switch is made.
A similar interlock would be placed that ensures that the disconnector cannot be closed until the earth switch is in the
fully open position.
WARNING: Interlocks are there to ensure unsafe acts are not undertaken. Interlocks should
not be defeated or forced. Switchgear is designed so that force is generally not required to
operate it. If something does not move freely, it should not be forced. Should something not
move freely, advice should be sought from an experienced operator or maintainer.
What is the purpose of an interlock?
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3.5 Grading capacitors
Some circuit breakers have two or more interrupting units. To ensure that the dielectric stress is evenly shared between
all interrupting units during the opening and when open, capacitors can be fitted across each interrupting unit. As the
voltage across capacitors cannot change instantaneously, no one interrupting unit takes the full operating voltage during
the opening. When all are open, the capacitors work as a voltage divider ensuring all the dielectric stress across each
interrupting unit is shared equally.
Interrupting units
Grading capacitors
Figure 24 - Circuit diagram of a double break circuit breaker fitted with grading capacitors
3.6 Anti-pumping mechanisms
Anti-pumping mechanisms are designed to ensure that for each close signal, only one close will occur. Should an
operator close a circuit breaker with a fault on the circuit, the breaker will open quickly after closing. However, without
an anti-pumping mechanism once the breaker opens, should the close signal still be present, the breaker would try to
close again. Therefore, without the anti-pumping mechanism, the breaker would bounce between open and closed until
the operator removed his finger from the close button.
3.7 Closing and trip coils
To release the stored energy device and either open or close the circuit breaker, a trip or closing coil is energised. This
operates a solenoid or latch and releases the energy to operate the circuit breaker. Often the latch systems are complex
and have up to 3 actions to release the energy from just a small coil. Sometimes there are 2 trip coils on switchgear and
either one can operate the breaker.
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3.8 Trip circuit supervision
To trip a circuit breaker, a protection relay sends a signal to the trip coil of the circuit breaker. The trip coil then operates
the circuit breaker mechanism. Should a fault occur on the circuit between the protection relay and the trip coil (e.g. the
trip coil burns out or a wire is dislodged from its terminal), then the circuit breaker would not trip when there is a fault on
the system. This would be a “hidden fault”; this means that the fault on the tripping circuit (e.g. a burnt out coil) would
go unnoticed until it was required to operate.
To ensure that this does not occur, many circuit breakers are fitted with trip circuit supervision. This is achieved by
applying a small voltage to the trip circuit. This is not enough to operate the tripping coil or cause any damage. However,
it is large enough to drive a small current around the tripping circuit. This is then linked to an alarm or indication circuit
and should the tripping circuit failure occur, then an alarm will be raised.
3.9 Slow closing mechanism
Some circuit breakers are or can be fitted with a slow closing mechanism. This is a maintenance tool or action that
allows the circuit breaker to be closed slowly by hand. The purpose of closing the breaker slowly is to ensure that there
is no damage and that the circuit breaker closes smoothly. It can also be used as a crude means of determining the
circuit breaker timing (to ensure that all poles of the circuit breaker close within the allowable tolerance). However, it is
recommended that circuit breaker timing is undertaken with specialised timing equipment.
3.10Communications
Depending on system criticality and location of the switchgear, communication circuits may be used. The
communication circuits can be used to operate the switchgear from a remote location or to provide information on
its status, such as whether the switchgear is open or closed, current and voltage values, or the condition of the
insulating medium.
3.11Auxiliary plant
Auxiliary plant is a term that is used to describe equipment that is required for the switchgear to operate, but does not
form part of the switching mechanism. Examples of auxiliary plant include an air compressor that is used to compress
the air on an air blast circuit breaker, or a motor that is used to operate a disconnector.
3.12Direct current (DC) systems
Direct current (DC) systems are often used for the protection, operation and indication circuits of switchgear. DC is used
as it can be supplied from batteries, which means supply can be maintained when the AC supply voltage is interrupted.
Therefore, even in fault or black out conditions, the switchgear can still be operated and the indication circuits will still
function correctly.
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3.13Earthing
All switchgear must be effectively connected to earth. This ensures that, if a fault occurs, current will flow to operate
protection circuits and more importantly, the voltage rise of the equipment is limited to a safe level.
3.14Operating rods
Operating rods is a term used to define the mechanical linkages in switchgear. These can be between the
operating handle and/or drive motor and the switching mechanism, or between the switching mechanisms of
multi-pole switchgear.
3.15Current and voltage transformer
Current transformers (CT) and voltage transformers (VT) are often used with switchgear for both protection and
indication circuits.
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4.0
Types of switchgear
4.1 Circuit Breakers
As the name suggests, circuit breakers are used to connect or interrupt circuits under both load and fault conditions.
Circuit breakers are designed to be used with protection relays to automatically open the circuit on the detection of
a fault. However, circuit breakers can also be used to open and close circuits manually or via an automated process.
Circuit breakers use all types of mediums discussed in the earlier section. The choice of which type of insulating and
interrupting medium to use, depends on the rating required and the environment the circuit breaker will be operating in.
Interrupting units
Isolating posts
Figure 25a - 220kV SF6 circuit breaker
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Interrupting units
Isolating posts
Figure 25b - 220kV SF6 circuit breaker
Figure 25c - 11kV indoor metal clad oil filled circuit breakers
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4.2 Reclosers and sectionalisers
Reclosers and sectionalisers are forms of circuit breakers. Nowadays, reclosers and sectionalisers typically use
vacuum technology. However oil filled reclosers are still common. The difference between standard circuit breakers
and reclosers or sectionalisers is that once operated, a circuit breaker is designed not to close again until the circuit
has been inspected and an operator applies a close signal. A recloser, on the other hand, is designed to open under
a fault condition, but then automatically recloses a selected number of times. The time between recloses is usually
adjustable. This means that on circuits, such as a country distribution circuit, where temporary short circuits can occur,
the recloser will open and then reclose. If the fault has been removed it will remain closed, and if the fault is still present,
the recloser will open. An example of a temporary fault, is a bird whose wings touch two conductors or a possum that
shorts a conductor to the crossarm. In both of these examples, the fault usually blows the animal clear of the conductor.
Therefore, when the recloser recloses, the fault has gone and power is maintained. However, if a circuit breaker was
used, a linesman would need to travel the length of the line and then reset the breaker, which could take several hours.
Figure 26a - Vacuum recloser
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Figure 26b - Oil filled recloser
As the name suggests, a sectionaliser is designed to sectionalise a circuit to aid in identifying the location of the fault.
By setting different reclosers to have a set number and duration of recloses, it is possible to break a long distribution
line into smaller sections. This is illustrated in Figure 27. Where the last sectionaliser in a long distribution circuit
recloses once (sectionaliser 3), the sectionaliser in the middle recloses twice (sectionaliser 2) and the sectionliser at
the start of the line recloses three times (sectionaliser 1). Thus the line has effectively been divided into three sections.
Should a fault occur in the middle section of the line (line section 2), then sectionaliser 2 and 3 would be locked out.
When sectionaliser 1 recloses for the third time, the fault would have been cleared and sectionaliser 1 would remain in
the circuit. Therefore line section 1 would remain energised and the linesman would know that the fault must be in line
section 2. By using the sectionaliser discussed above, more people have retained supply and the amount of line required
to be searched has been reduced. Reducing the amount of line to patrol will also significantly reduce the time required
to return the affected sections to service.
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Sectionaliser 1
Sectionaliser 2
Sectionaliser 3
Line section 3
Line section 1
Fault
Line section 2
Figure 27 - Single line diagram of a sectionaliser circuit
What is the difference between a circuit breaker and a recloser?
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4.3Disconnectors
As the name suggests, disconnectors are designed to disconnect a circuit. The difference between a disconnector and a
circuit breaker is that disconnectors are not designed to open a circuit under a fault condition. Disconnectors on 220kV
and 110kV transmission circuits and many on lower voltage circuits are designed so that they can only be opened once
the circuit has been de-­energised. However, there are some disconnectors (e.g. an air break switch on a rural 11kV
distribution circuit) that are designed to switch load current.
The purpose of a disconnector is to provide a physical break in the circuit to allow maintenance to be undertaken on the
circuit, or to isolate the line for other purposes.
Figure 28a - Disconnector in the closed position
Figure 28b - Disconnector in the open position
What is the difference between a circuit breaker and a disconnector?
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4.4 Earth switches
As the name suggests, earth switches are designed to connect an earth to a circuit. The purpose of an earth switch
is to connect an earthed conductor to a circuit to provide a safe condition for maintenance of the circuit or to a load
connected to the circuit.
Earth switches are typically an air break switch, however other types are possible. It is also common to interlock earth
switches with disconnectors to ensure that they cannot be closed onto an energised circuit.
If the earth switch is not interlocked with a disconnector, it is important that another method is utilised to ensure that the
circuit is de-­energised and cannot be re-­energised when the earth switch is closed.
Earth switch
contact arm
Figure 29a - Earth switch in the closed position
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Earth switch
contact arm
Figure 29b - Earth switch in the open position
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4.5 Point of wave switching
Some breakers are fitted with point on wave switching control. This requires that each pole of a breaker can be
switched independently, or in the case of one mechanism for a complete circuit breaker one phase is controlled and the
other two are mechanically staggered through linkages. The controller analyses the waveform and switches each pole at
a preset point on the wave. The purpose of point on wave switching is to limit the inrush current of a transformer. Inrush
current is the current that flows when a transformer is energised. The amount of inrush current that flows depends on
where on the voltage waveform the transformer is energised. While the inrush current quickly decays, it can be larger
than the short circuit current. Therefore, by controlling the point on the voltage waveform that each pole of the breaker
closes, the amount of inrush current can be minimised. This in turn minimises the stress on the transformer and the
electricity grid.
Point on wave switching is also used for switching large capacitor banks. This limits the voltage fluctuations by
switching in the bank at 0 voltage point on the sine wave.
4.6 Resource Management Act 1991 (RMA)
The overall theme of the Resource Management Act 1991 (RMA) is ‘the sustainable management of natural and physical
resources for current and future generations’. The RMA requires that every person has a duty to avoid, remedy or
mitigate adverse effects arising from an activity carried out by or on behalf of that person, whether or not the activity is
in accordance with a resource consent.
Regional and local authorities (councils) are required to produce district or regional plans which dictate what activities
are permitted and what activities require resource consent. Resource consent allows an activity otherwise unauthorised
by a district or regional plan to take place.
Generally, if an activity results in an adverse effect then the permitted activity or resource consent conditions will seek to
control these effects. The discharge of contaminants and nuisance noise effects are restricted in this way.
This may affect the operation of switchgear, for example the installation of an air blast circuit breaker in a residential
area. Due to the noise produced by the operation of the breaker, the activity may not be permitted under a plan and a
resource consent may not be granted authorising it. If resource consent were granted for the activity, then conditions
will be imposed restricting the operation in order to control the effects. This may result in the operational procedure
having to be modified.
The plans produced by the councils and/or resource consents may also affect the maintenance of switchgear.
For example during maintenance of an oil filled circuit breaker, the disposing of used oil will be controlled. The district
or regional plan or resource consent will place restrictions on how the oil is to be disposed. This may result in the
maintenance procedure having to be modified.
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5.0 Switchgear numbering
5.1 High voltage switchgear numbering
All high voltage switchgear has a unique number to distinguish it from other switchgear and thereby ensure that the
correct piece of equipment is operated. Although each piece of equipment has a unique number, there is a logic to the
numbering that helps determine what it is and what it is associated with.
220kV
A Bus
G1
T1
C1
DS16 DS265
220kV
B Bus
DS264
DS224
DS223
DS226
G
CB12
CB262
DS243
DS245
CB222
CB242
Figure 30 - Example of switchgear single line diagram
An example of switchgear numbering is HLY CB 12.
5.2 Location or circuit identifier To identify equipment on one site or circuit from another, a three digit code is used. Examples of this include:
HLY
Huntly Power Station
TKU
Tokaanu Power Station
If all equipment is associated with the one site or circuit, this describer is often dropped.
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5.3 Equipment code
The equipment code is a series of letters that represent equipment. These include:
CB Circuit breaker
DS Disconnector (Note: Only recently changed to DS. DIS was the old descriptor and will still be shown on many drawings and equipment labels.)
ES Earth switch
While the equipment code always appears on the operating order, it may not be on the equipment.
5.4 Unit number or unique identifier The first numbers after the equipment code are the unique identifier numbers. These are chosen to fit into a pattern
based on the physical layout of the station or switchyard, and for ease of reading should be numbered so that equipment numbers increase from left to right on the switchyard (as viewed from the feeder end). This method will produce a
logical pattern of numbers in an outdoor structure.
For stations with more than one outdoor voltage, distinct groups of suitable size should be allocated for each voltage:
e.g. 66kV:
40
-­
139
110kV:
200
-
300
11kV, 22kV, and 33kV switchgear should always be numbered above 1000. Satellite stations controlled from the same
master station should each have different blocks of numbers.
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5.5 Equipment number
The last number in the sequence is used to identify what the equipment is. These include:
0 Bus earth switch
1 Bypass air-­break switch only
2 Circuit breaker
3 Air-­break switch connecting a line or transformer to a circuit breaker capable of connecting it to
bus ­bar “A”
4
Air-­break switch connecting a circuit breaker to a bus bar
5 Air-­break switch connecting a line or transformer to a circuit breaker capable of connecting it to
bus bar “B”
6 Air-­break switch connecting a line or transformer to a circuit breaker or other air-­break switches.
(In a complex layout, this number is always to be used for the switch nearest the line).
7 Bus section air-­break switch. Air break switch connecting a bus section or bus coupler circuit breaker to
a busbar
8 Bus coupler circuit breaker
9 Earthing switch.
Explain what equipment is associated with the following numbers:
234
289
52
5.6 11kv indoor switchgear numbering
11kV switchgear is numbered in the 1000s, usually 2000 to 2999. Usually, the number will increase from left to right.
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6.0 Operator responsibilities
Incorrect operation of switchgear could result in serious health and safety consequences and damage to equipment.
It is important that the person operating the piece of equipment understands how the equipment operates and the
consequences of the operation. For this reason, it is important that only personnel trained and authorised to operate
switchgear undertake the operation, or the operation is undertaken under the direct supervision of an experienced
operator. It is also critical that all company procedures are understood and followed.
6.1 Prior to operating switchgear
Some of the points of consideration required include:
The use of the correct personal protective equipment (PPE). Each company will have differing requirements based on the equipment and the associated risks. It is important that the company’s recommended PPE is checked and worn prior to undertaking the operation of any switchgear.
The use of: arc rated overalls, arc rated jacket, arc rated pants, arc rated switching hood, hard hat, safety glasses, insulated gloves and insulated mats should be considered.
Ensuring the correct sequence is followed. For example, when isolating a circuit for maintenance, it is important that the circuit breaker/s are opened first, then the disconnector/s should be opened and lastly the earth switch/es are closed. The reason for following this sequence is the disconnector may not be rated to open under load. Doing so may cause damage to the disconnector or may even result in personal injury. Likewise, it is important to ensure that the disconnectors are open prior to closing the earth switches.
Again, failure to open the disconnector prior to closing the earth switch could result in damage to personnel
and equipment.
Recording the status of equipment after a fault. This should include the position of the switchgear and the
status of all protection flags and alarms.
The condition of the switchgear. This is particularly important following a fault. If there are any concerns about the condition of the switchgear, it should be checked by a suitable experienced operator or maintainer prior to operation.
Ensuring that large currents are not driven by voltage mismatches. This is especially important when paralleling circuits.
Ensuring that the load will be shared, and that large circulating currents are not formed. Circulating currents are caused by a mismatch in the terminal voltage of the transformer. This is a particular issue where the transformers are fitted with onload tapchangers. If paralleling transformers with onload tapchangers, it is advisable to ensure that they are identical transformers and the tapchangers are set to “master-­slave”. This will ensure that the tapchangers do not get out of synchronisation.
Ensuring that conditions are not set up so they would exceed the normal or fault ratings of the switchgear.
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6.2 Safety tips when operating switchgear
Where possible, operate switchgear remotely. Although very reliable, switchgear does fail. It is a lower risk situation if
the operator is not in the area. Where it is not practical to operate the switchgear remotely, the following are some basic
safety measures.
Safety tips when operating ground mounted switchgear locally:
Wear suitably rated arc flash clothing and hood.
Wear insulated gloves.
Ensure no one is in the vicinity. If it is necessary for others to be in the area, ensure they too have suitably rated arc clothing.
Do not stand directly in front of the switchgear. Stand to the side and operate at arms length. Should a fault occur, the arc is likely to come directly out from the switchgear.
Check that you have your hand in the correct position and then turn your head just prior to pushing the operating mechanism. Should there be a fault, the arc is less likely to damage your eyes and face.
Just prior to operating the device close your eyes. Again, this will help protect your eyes.
Just prior to operating the device take a large breath. Should something go wrong, your natural reaction is to take a large breath. If there has been an arc there may be superheated gases that could cause damage to your lungs. Having taken a large breath just prior, you are unable to take another breath and this will avoid damage to
your lungs.
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When operating overhead switchgear from ground level:
Wear suitably rated arc flash clothing and hood.
Wear insulated gloves.
Wear a hard hat.
Ensure that you have stable footing.
When switching, look down. Do not look at the switching device. Should an arc occur and you are looking at the switching device, this could cause arc blindness or particles to enter your eyes.
What should be recorded after a fault and why?
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Answers to Activities
Page 6
Explain the terms:
Nominal voltage
Nominal voltage, also called voltage class, is used to define the rated system voltage for which the circuit breaker is designed to be used.
Rated maximum voltage
Rated maximum voltage is the maximum continuous voltage for which the circuit breaker is designed to be used.
Rated continuous current
Rated continuous operating current is the maximum operating RMS current that the curcuit breaker is designed to operate at.
Rated fault current
Also known as rated short-­circuit current, or maximum interrupt current. This is the maximum
current that the circuit breaker can safely interrupt. It is important that this is set above the maximum perspective fault current of the circuit.
Page 9
Explain the difference between bulk oil and minimum oil circuit breakers.
Bulk oil as all three phases in the one tank and uses the oil to insulate between phases and earth, whereas minimum oil has a separate tank for each phase and the isolation between the phases
is achieved by insulators.
Page 11
Explain how the blast of air is produced in an air blast circuit breaker.
Air is compressed and stored in a tank. This is then released when required.
Page 14
What environmental effect needs consideration when undertaking maintenance on an SF6
curcuit breaker?
SF6 is a strong greenhouse gas. Therefore, any source of release needs to be controlled and capture
the gas.
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Page 21
What is the purpose of the stored energy device in circuit breakers?
Page 21
List 3 different stored energy devices.
Springs
Hydraulics
Capacitors
Pneumatic
Battery Banks
Page 22
What is the function of arcing horns on an air break switch?
They make first and break last, thereby saving the main contacts from any arc damage.
Page 36
What is the purpose of an interlock?
To help ensure unsafe acts are not undertaken.
Page 44
What is the difference between a circuit breaker and a recloser?
A recloser is designed to automatically close a selected number of times after a fault, whereas a circuit breaker is designed to operate and then lock out until reset.
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What is the difference between a circuit breaker and a disconnector?
Disconnectors are not designed to switch fault currents, many are not designed to switch load current,
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The stored energy device is to ensure that if the control power supply is lost the curcuit breaker will still operate. Furthermore, it ensures that the operation will not be stored halfway through the operation.
The stored energy device also ensures that the correct amount of energy is used to operate the
curcuit breaker.
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Explain what equipment is associated with the following numbers:
234 – Disconnector (air break switch) connecting a circuit breaker to a busbar.
289 – Earth Switch
52 – Circuit breaker
Page 54
What should be recorded after a fault and why?
The position of the switchgear and the status of all protection flags and alarms. This is to aid in the
investigation of why the circuit breaker operated and what the likely cause of the fault was.
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