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Safety maintenance requirements for power circuit breakers
Conference Paper in IEEE Conference Record of Annual Pulp and Paper Industry Technical Conference · June 2012
DOI: 10.1109/PPIC.2012.6293014
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Safety Maintenance Requirements for Power Circuit Breakers
Copyright Material IEEE
Paper No. PPIC –
James R. White
Senior Member IEEE
Shermco Industries, Inc.2425 E. Pioneer Drive
Irving, Texas 75061
jrwhite@shermco.com
(not shown) are pulling the contact assemblies rearward,
exerting pressure on the prop and roller. As long as the prop
and roller remain aligned the circuit breaker will stay in the
contact-closed position. To trip the circuit breaker the trip bar
pushes the prop off the roller, collapsing the linkage and
opening the contacts. Fig. 2 shows the collapsed linkage.
Abstract – This paper looks at the most common
issues causing circuit breaker malfunctions. Power
circuit breakers protect electrical power systems from
damage and people from injury or death if a fault were to
occur during maintenance or operation of the system. If
the circuit breakers are not maintained they cannot be
depended upon to function in accordance with the
manufacturer’s specifications.
This paper reviews
findings of Shermco’s Circuit Breaker Shop while
working on customer’s circuit breakers.
Index Terms – Circuit breaker, maintenance, safety-related
maintenance requirements, circuit breaker failure modes.
I.
INTRODUCTION
National Fire Protection Association (NFPA) 70E® 2012(1)
contains a number of requirements for maintaining
overcurrent protective devices (OCPD). Chapter 1, Section
130.5 Informational Note (IN) No. 1 states, “Improper or
inadequate maintenance can result in increased opening time
of the overcurrent protective device, thus increasing the
incident energy”.130.5 IN No. 4 refers the reader to Chapter 2
for more safety-related maintenance requirements in NFPA
70E®. In Chapter 2, Section 205.3 NFPA 70E® states,
“Electrical equipment shall be maintained in accordance with
manufacturers’ instructions or industry consensus standards
to reduce the risk of failure and the subsequent exposure of
employees to electrical hazards.” 205.4 states, “Overcurrent
protective devices shall be maintained in accordance with the
manufacturers’ instructions or industry consensus standards.
Maintenance, tests, and inspections shall be documented.”
To understand the reason for preventive maintenance (PM)
and the causes for problems, an understanding of the
construction and operation of the typical stored-energy-type
circuit breaker is needed. The issues covered by this paper
are common to all low-voltage power, medium-voltage power
and medium-voltage vacuum circuit breakers.
II.
Fig. 1
Simple Prop and Roller Mechanism
Breaker Closed
CIRCUIT BREAKER OPERATION
Fig. 1 shows a simple prop and roller operating mechanism
using an accordion-style linkage. This is similar to that used
on early circuit breakers, but is not used in modern circuit
breakers due to its inherent weaknesses. The circuit breaker
contacts are held in the closed position by a mechanical prop
and roller mechanism. The centerline of the prop is in line
with the centerline of the main roller. The opening springs
Fig. 2
Simple Prop and Roller Mechanism
Breaker Open
1
This simple operating mechanism is no longer used, due to
the weakness of the linkages. This same principle is used in
modern circuit breakers, although greatly modified. Fig. 3 is
one style of a modern prop and roller mechanism. Without
the extending linkages it is much more rugged and has fewer
issues from misalignment or breakage. Fig. 3 shows the
operating mechanism in the Closed position. The Insulated
Coupling (12) holds the contacts closed due to parts 2, 5, 6,
11 and 14 being placed in an interference fit. The Trip Latch
(11) holds the Secondary Latch (14) from rotating clockwise.
The Secondary Latch is positioned against the Secondary
Latch Roller (6 - Yellow), which in turn extends the Cam
(Main) Roller (5 - Red) against the Prop (2). The Opening
Spring (15) is not shown in this view, but is exerting pressure
on the contacts to open.
Note that the Secondary Latch (14) is held against the
Secondary Latch Roller (6), which is pushing the Main Roller
(5) and its linkage into a vertical position. The Main Roller is
in turn held against the Prop (2), which prevents it from
overextending. The centerline of the Insulating Coupling pin
is in a straight line with the Main Roller through the Camshaft.
In this position the contacts cannot open until the linkage
collapses, which cannot happen until the Trip Latch (11)
releases the Secondary Latch (14).
Fig. 4
Operating Mechanism – Tripped
Latch. The Prop and Cam reset to their original position and,
if the breaker Close button is pressed, the Closing Springs
will accelerate the contacts closed. The Cam and prop will
rotate, extending the linkage and forcing the components into
the same positions as Fig. 3.
It is easy to see that if these components begin to gum up
and slow down, the breaker won’t operate properly. But if the
breaker is just sitting there, how does the grease get gummy?
All lubricants dry out over time. Organic lubricants are
composed of emulsifiers and oil. Over time the oil dries out,
leaving the emulsifiers, which thickens. If the breaker is
exercised it helps to keep the lubricants as pliable as
possible. But they still dry out over time. As the breaker
carries load current, heat is produced by the I2R losses. The
higher the load current, the greater the losses will be and the
more heat produced. This heat increases the rate at which
the lubricants dry out, even if the breakers are exercised
regularly.
Fig. 3
Operating Mechanism – Closed
Fig. 4 shows the same mechanism in the Tripped position.
To open the breaker the Trip Latch is rotated clockwise,
allowing the Secondary Latch to rotate counter-clockwise.
When it does, the Main Roller (5) and the Secondary Latch
Roller (6) collapse. This allows the Opening Springs to pull
the contacts open. Part 7 is a bellcrank lever, which is used
to change motion in one direction to motion in another
direction, usually at a 900 angle.
Fig. 5 shows the mechanism in the Reset position. This
position is the condition of the mechanism just before closing.
The Trip Latch (11) and the Secondary Latch (14) are reset to
the same position as when the breaker is in the Closed
position. To put the mechanism in this position the Cam (3)
has to be rotated slightly counter-clockwise until the Prop (2)
is lifted up, allowing the Main Roller (5) and linkage (green) to
slip into the crook of the Prop. By doing this the linkage is
slightly extended and the Secondary Latch (14) engages the
front of the frame. As the linkage collapses, the bellcrank
rotates, allowing the contacts to open, shown (in blue), which
allows a gap between the Trip Latch and the Secondary
Fig. 5
Operating Mechanism - Reset
III.
IMPROPER LUBRICATION
Lack of lubrication is the number one problem with circuit
breakers. Typically, lubrication problems are found on one
out of every four circuit breakers has lubrication issues when
maintenance is performed on them. This lack of lubrication
results in circuit breakers that won’t open, circuit breakers
that won’t close or breakers that open after a delay.
All those little rollers and latch surfaces require lubrication.
The old lubricant has to be cleaned off and new lubricant
applied. Each manufacturer has a specific lubricant they
2
recommend for their circuit breakers; one for the current path
and one for the operating mechanism.
Nearly all
manufacturer’s now recommend (or at least allow the use of)
®
a product equal to or similar to Mobil 28 . This is a red,
synthetic-based lubricant sold in tubes or in cans, which can
be used in both the current path and the operating
mechanism.
It isn’t just the operating mechanism that requires
lubrication. All circuit breakers have lubricant applied to the
contact pivot points at the factory. During the course of time,
this lubricant dries out, gets gummy, dries further, then flakes
off, resulting in metal-to-metal wear. When metal-to-metal
wear takes place, bad things happen to the operation of the
breaker. Wear of the parts is greatly accelerated, parts
become misaligned, and the operation of the breaker slows
down.
A.
The Current Path
Fig. 6
Contact Wear Points
Fig. 6 is a circuit breaker contact assembly. Note the
number of points where wear takes place. The primary points
of wear are at the arcing contact/tensioner contact (2) and the
contact roller/tensioner contact (1). Lack of lubricant at point
1 causes wear across both contact faces, while lack of
lubricant at point 2 causes the breaker to slow down and
grooving to be cut into both the roller and the tensioner
contact. Eventually the breaker will seize if the components
aren’t replaced and new lubricant isn’t applied.
Some technicians will take a spray lubricant and spray it
into the contact pivot point or into the operating mechanism.
To the user, it appears that he has solved the problem. The
breaker functions like it should, opening and closing on
repeated cycling. However, what really happens is that any
remaining lubricant has now been flushed out of the pivot
points and replaced with a 30-day lubricant. More than likely
when the circuit breaker heats up due to current flow through
it the lubricant will quickly dissipate and significant wear will
begin. Other people will use white lithium grease or some
non-approved lubricant. None of these are acceptable, even
if they seem to work at the time.
One of the reasons manufacturers have been so specific
about the lubricants used in their circuit breakers is that the
breakers have to perform under a variety of conditions, many
times in very adverse conditions. Using lubricants not
designed or proven to function well in these conditions is
short-sighted and can very possibly ruin an expensive
breaker or worse, cause it to seize during a fault condition.
Fig. 7 is a picture of the arcing contact and roller from a
circuit breaker showing the wear often seen. It just isn’t this
particular manufacturer or model of circuit breaker, by the
way. Lack of lubrication is a big problem in all breakers.
Fig. 7
Contact Assembly Wear
Besides the issue of the damage caused to breakers, there
are also issues involved with damage to the equipment the
circuit breaker is protecting as well as possible injury to
personnel that may be working on the breaker when it fails to
clear the fault. A delayed trip (or no trip) could push the
incident energy well above the ratings of the personal
protective equipment the worker may be wearing.
B.
Contact Resistance Testing
The fix for increased contact resistance sometimes is not
very straightforward. There’s no way to see inside the
contact pivot points and it’s too expensive to take the breaker
apart just to see what’s going on inside. For determining the
condition of the contact pivot point area a micro-ohmmeter
test (often referred to as a contact resistance test) can be
performed, (see Fig. 8). By measuring the resistance of the
entire current path, any increase in resistance can be
determined.
In low and medium-voltage power circuit
breakers a good part of that increased resistance is going to
be due to the lubricant drying out. By measuring the
resistance each year a trend can be established and when
the resistance becomes too great, the breaker is due for a
tear-down and relubrication.
3
In general, the contact resistance should be as low as
possible and as consistent as possible between the contacts
of all three phases. In general, that’s good advice, but really
doesn’t help to diagnose problems.
A 4,000 amp 480-volt breaker should have a reading of
something less than 30μΩ if it’s in good condition. Smaller
breakers may have readings of 60 to 80μΩ. Molded-case
480V circuit breakers can have contact resistance readings in
the hundreds of micro-ohms.
This information is not
presented to confuse the issue, but this issue is not as simple
as one may think.
Occasionally a manufacturer will
recommend values, but not often.
reset properly. Clean any accessible latch and roller surface
you can reach and relubricate. If a roller is sticky or hard to
spin, it may need to be removed and lubricated. This can be
more than a ten-minute job, especially if it is hard to get to.
Main Roller and Prop
Fig. 9
Prop and Roller Mechanism
Also be aware that some rollers may be under tension from
latches or springs, and always be aware that these are
spring-charged mechanisms that move very fast and very
hard. Before starting any maintenance procedure on a circuit
breaker always cycle it to be certain the springs are
discharged and the contacts are open. The Trip Paddle also
has bearings that, when they get sticky, will cause the
breaker to seem as though it is in a “trip-free” condition.
Fig. 8
Contact Resistance Test Using a Micro-Ohmmeter
One Rule-of-Thumb for low-voltage power circuit breakers
that can be used is that readings up to 100μΩ indicate a
circuit breaker in good operating condition, while readings
that exceed 300μΩ are an indication of trouble. For mediumvoltage breakers the values need to be somewhat lower;
readings above 200μΩ are reason to investigate for
problems, and a circuit breaker in good operating condition
should have readings less than 60μΩ. Any value in between
may indicate that the lubricant is beginning to dry out, but
hasn’t reached the problem plateau yet. The important part
of this process is to perform the test annually and trend the
changes.
IV.
IMPROPER ADJUSTMENT
The second most common problem is improper adjustment
of the breaker. The mechanism is usually pretty close,
because if it wasn’t, you wouldn’t be able to operate the
breaker. However, misadjustment of the contact assemblies
causes the majority of the problems. There are three
primary, common adjustments on low and medium-voltage air
circuit breakers:
A. Contact pressure or contact wipe
B. Contact sequence or contact make
C. Contact gap
C. The Operating Mechanism
Operating the breaker is good for the mechanism. It
loosens up the grease that’s still viable, spreads it around.
Listening to the breaker and watching it when it operates may
indicate when the breaker’s in trouble. Listen to the sound
the breaker makes when it operates. It should have a clear,
solid sound. If you operate five breakers, you can tell if one
of them is operating slower than the others. Look at the
lubricant on latch faces and around rollers. If it’s dirty, dusty
or thick, it needs to be replaced. If a circuit breaker is
operated and it won’t reset until it’s been cycled a few times,
there’s a very good chance the mechanism is gummed up. In
general, expect the mechanism to need cleaning at about the
same time as the current path does.
Fig. 9 shows a circuit breaker’s operating mechanism. The
arrow in Fig. 9 is pointing to the main roller in the prop-androller mechanism. If this roller gums up, the breaker won’t
A.
Contact Pressure
Too much contact pressure leads to a number of problems,
including cracked contact faces, excessive wear and bending
of components. A properly adjusted breaker will have
enough contact pressure to reduce contact resistance to
specifications, but not so much that it damages the breaker.
To measure contact pressure, most manufacturers will
require a measurement from a stationary part of the contact
assembly to the contact face with the breaker open. The
same measurement is repeated after the breaker has been
“Slow Closed”. Slow or Maintenance Closing allows the
breaker contacts to be closed slowly by bypassing the springcharging mechanism. Fig. 10 shows a circuit breaker having
the contact pressure being measured.
4
Fig. 10
Using the Slow Close Function
Fig. 12
Contact Vaporized During Arc Interruption
Be aware that the opening springs are fully charged at this
point and the breaker is very dangerous.
When taking
measurements, it is necessary to put your hands around the
contacts where they could be injured if the breaker were to
open, so caution is needed.
Each manufacturer has a
specific method of checking contact pressure and for Slow
Closing the circuit breaker.
Fig. 11 shows the result of too much contact pressure. The
result of Fig.11 is Fig. 12, where the contact face cracked,
then during a fault pieces of the face blew off the contact and
the entire contact melted. This is because the arcing contact
face is made of an alloy of zinc, copper and silver, whereas
the portion under the face is just silver-plated copper. The
damage to the contact face in Fig. 11 was not immediately
apparent when an inspection was first made. Only after
cleaning the contact surfaces did this damage show up.
B.
Contact Sequence
When a breaker closes, all three contacts should make at
about the same time. If they are too far off, the arcing
contacts will suffer accelerated wear, le
ading to failure
of the main contacts. Most manufacturers of low-voltage
power circuit breakers recommend that all contacts touch
within 0.125” (1/8”) of each other.
The adjustment point for contact sequence is the same as
for contact pressure; the stud between the insulating link and
the contact assembly. The circuit breaker shown in Fig. 13 is
representative of most circuit breakers.
Fig. 13
Multi-Purpose Adjustment Point
C. Contact Gap
Fig. 11
Contact Cracking Due to Overpressure
The contact gap is not quite as critical as the other two
adjustments. Many times no specific gap is given in the
manufacturer’s instruction books. However, if the contacts
don’t open far enough, a breaker will suffer a restrike
condition, where the breaker cannot interrupt the fault and is
5
destroyed. Air is a good dielectric when dry, but, it is not a
great dielectric and when contaminated with moisture, carbon
or other conductive particles, or if there is an insufficient air
gap between the contacts, a restrike can occur.
Fig. 15
Hardened Stop Pin Bent by Brute Force
Abuse and neglect are not limited to careless technicians.
It also is dished out by management that think of their
electrical protective devices as overhead; another cost they
don’t need. This attitude is short-sighted. Fig. 16 is a
medium-voltage circuit breaker; one of many we received
from a customer. The cobwebs are not uncommon, as
insects see electrical equipment as good nesting places.
There is no doubt that this breaker has been neglected for
many, many years. The chances of it actually opening during
a fault are pretty slim, which means that a lot of very
expensive switchgear is going to be damaged and there is a
possibility of someone being seriously injured or even killed.
How much is a person’s life worth? How much does
maintaining your switchgear cost? Managers don’t often look
at these issues in that perspective. Often it is a matter of
spending the money ear-marked for maintenance on
something more pressing. H. Landis Floyd, in a presentation
th
to the 13 Annual IEEE Electrical Safety Workshop,
estimated the costs of a serious electrical injury at $23 million
dollars. Mr. Floyd used the Consumer Price Index to adjust
his number for inflation based on the referenced paper (2).
Fig. 14
Contact Gap (Arrow)
The contact gap is measured with the breaker open from
the closest points between the stationary and moving contact
assemblies. Fig. 14 shows the measurement points. If the
contact gap is too small, it can be adjusted using the same
stud that is used for the other adjustments on most circuit
breakers. Again, a properly adjusted breaker is going to be a
compromise between all three specifications. A breaker with
good contacts should fall within the manufacturer’s tolerances
for all three adjustments
V.
ABUSE AND NEGLECT
The third problem is abuse and neglect, which are not too
far removed from each other. People seem to take out their
frustrations or anger on breakers and other inanimate objects,
causing damage that can be dangerous to the power system
and themselves. Of course, some of the abuse is due to lack
of training, not maliciousness. An example is the racking
gear in Fig. 15. This came out of a low-voltage circuit breaker
that had been racked in so hard, it bent the hardened stop pin
(arrow). If this pin shears, the breaker will rack in, then, when
it reaches the end of its travel, racks right on back out. The
cause; over- torquing the racking handle. As the lubricant
gets gummy it will take more effort to rack in the breaker. If
the stabs haven’t been lubricated, it increases the force
required to make the primary stabs connect.
Fig. 16
Severely-Neglected Operating Mechanism
6
VI.
®
maintenance is required by NFPA 70E to ensure worker
®
safety. The 2012 edition of NFPA 70E contains specific
maintenance requirements in Article 130 and 205.
ELECTRICAL PROBLEMS
Electrical problems represent about 25% of the issues on
power circuit breakers. These are usually pinched wiring,
bad connectors or faulty components, such as motor cut-off
switches, spring charging motors or closing coils.
Plastic subcomponents represent about 10% of circuit
breaker failures. Fig. 17 shows a common electrical failure,
where a non-standard repair has led to the wire fraying.
VIII.
ACKNOWLEDGEMENTS
The author wishes to acknowledge the manager of
Shermco Industries’ Circuit Breaker Shop, Jim Miller, for his
assistance in preparing this paper.
The author appreciates the time and effort given by the
following people in reviewing this paper. Their comments and
suggestions greatly enhanced my poor writing.
1. Daleep C. Mohla
2. Daryld Ray Crow
3. Warren Hopper
4. Dennis Netizel
5. Al Havens
IX.
Fig. 17
Common Electrical Problem
REFERENCES
1.
®
NFPA 70E , “Standard for Electrical Safety in the
Workplace”, 2012 edition
2.
Wyzga, R., Lindroos, W., Health Consequences
of Global Electrification, Annals of the New
York Academy of Sciences, Chen C-T, Lee RC,
Shih J-X, Zhong M-H, eds. New York Academy
of Sciences Press, New York, 1999, Vol. 888:
pp1-7.
X.
VII.
CONCLUSIONS
James White is the Training Director for Shermco Industries,
Inc. located in Irving, Texas. He is a Senior Member of the
IEEE, the recipient of the 2011 IEEE/PCIC Electrical Safety
Excellence Award and was the 2008 IEEE Electrical Safety
Workshop Chairman. James is the Alternate interNational
Electrical Testing Association (NETA) representative on
NFPA 70E®, Primary representative on NFPA 70B®, and is
®
®
the NETA representative to NEC CMP-13 and ASTM F18 .
James received NETA’s Outstanding Achievement Award in
2012 and is also a certified Level IV Senior Substation
Technician with NETA, an inspector member of IAEI and
serves on the NETA Safety and Training Committees.
Power circuit breakers are critical components for electrical
system protection. These devices are often neglected, as
they can be in service for several years and perform well
under normal operating conditions. When a short circuit
occurs, however, they may fail to operate exposing the power
system and maintenance personnel to the deadly effects of
an arc flash. This paper illustrates examples of several types
of common failure and the importance of addressing each.
Lubrication issues can only be resolved by disassembly of the
circuit breaker, cleaning the old lubricant and relubricating the
required parts. Use of the proper lubricant is just as
important. These steps will restore a malfunctioning circuit
breaker to its proper working condition. Proper, documented
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