See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/261507121 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 CITATIONS READS 2 3,282 1 author: James White Shermco Industries, Inc. 14 PUBLICATIONS 13 CITATIONS SEE PROFILE All content following this page was uploaded by James White on 25 August 2015. The user has requested enhancement of the downloaded file. 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 7 View publication stats VITA