Protective Devices Maintenance and the Potential Impact on Arc Flash Incident Energy
Dennis K. Neitzel, CPE, CESCP
This article provides insight into the electrical safety considerations, as they relate to maintenance of overcurrent protective devices, along with the potential impact on the arc flash energy, for industrial and commercial electrical equipment and systems. It provides valuable information for the electricians, technicians, and engineers who operate and maintain this equipment.
Electrical preventive maintenance and testing are some of the most important tasks to be performed in order to assure the reliability and integrity of electrical distribution equipment and systems, as well as for the protection of personnel. However, preventive maintenance of electrical systems and equipment, specifically with regard to overcurrent protective devices, is often overlooked or performed infrequently. The National Electrical Code (NEC) states that overcurrent protection for conductors and equipment is provided to open the circuit if the current reaches a value that will cause an excessive or dangerous temperature in conductors or conductor insulation. With regard to circuit breakers the only way to accomplish this is through proper maintenance and testing of these devices.
MAINTENANCE AND TESTING
The first step in properly maintaining electrical equipment and overcurrent protective devices is to understand the requirements and recommendations for electrical equipment maintenance from various sources. Examples of sources include, but are not limited to, the Manufacturer’s instructions, ANSI/NETA MTS, NFPA 70B, IEEE Std. 3007.2, NEMA AB-4, and NFPA 70E.
The second step in performing maintenance and testing is to provide adequate training and qualification for employees. NFPA 70E-2015, Standard for Electrical Safety in the Workplace
, Section 205.1 states, “Employees who perform maintenance on electrical equipment and installations shall be qualified persons…and shall be trained in and familiar with, the specific maintenance procedures and tests required.
The Occupational Safety and Health Administration (OSHA), defines a qualified person as “One who has received training in and has demonstrated skills and knowledge in the construction and operation of electric equipment and installations and the hazards involved.” It is important that employees are properly trained and qualified to maintain electrical equipment in order to increase the equipment and system reliability, as well as enhance employee safety.
Electrical Preventive Maintenance Program:
NFPA 70E, Section 205.3 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.
” Section 205.4 further states that “Overcurrent protective devices shall be maintained in accordance with the manufacturers’ instructions or industry consensus standards. Maintenance, tests, and inspections shall be documented.
” It goes on to state in Section 225.3 that “Circuit breakers that interrupt faults approaching their ratings shall be inspected and tested in accordance with the manufacturers’ instructions.
Therefore, the third step is to have a written, effective Electrical Preventive Maintenance (EPM) program. NFPA 70B-2013, Recommended Practice for Electrical Equipment Maintenance,
makes several very clear statements about an effective EPM program as follows:
- “Electrical equipment deterioration is normal, but equipment failure is not inevitable. As soon as new equipment is installed, a process of normal deterioration begins. Unchecked, the deterioration process can cause malfunction or an electrical failure. Deterioration can be accelerated by factors such as a hostile environment, overload, or severe duty cycle. An effective EPM program identifies and recognizes these factors and provides measures for coping with them. “
- “In addition to normal deterioration, there are other potential causes of equipment failure that can be detected and corrected through EPM. Among these are load changes or additions, circuit alterations, improperly set or improperly selected protective devices, and changing voltage conditions.”
- “Without an EPM program, management assumes a greatly increased risk of a serious electrical failure and its consequences.”
- “A well-administered EPM program will reduce accidents, save lives, and minimize costly breakdowns and unplanned shutdowns of production equipment. Impending troubles can be identified — and solutions applied — before they become major problems requiring more expensive, time consuming solutions.”
IEEE Std 3007.2-2010, Recommended Practice for the Maintenance of Industrial and Commercial Power Systems,
states: “In planning an electrical preventive maintenance (EPM) program, consideration must be given to the costs of safety, the costs associated with direct losses due to equipment damage, and the indirect costs associated with downtime or lost or inefficient production.
All maintenance and testing of electrical protective devices addressed here must be accomplished in accordance with the manufacturer’s instructions. In the absense of the manufacturer’s instructions, the latest edition of the ANSI/NETA, Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems, 2015 Edition,
is an excellent source of information for performing the required maintenance and testing of these devices. However, the manufacturer’s time-current curves would valuable information for properly testing each protective device.
Molded-Case Circuit Breakers:
The need for inspection of molded-case breakers (MCCBs) will vary depending on operating conditions. Suggested inspection and testing is defined in NEMA AB 4, Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers Used in Commercial and Industrial Applications
. MCCBs receive initial testing and calibration at the manufacturers’ plants. These tests are performed in accordance with UL 489, Standard for Safety, Molded-Case Circuit Breakers, Molded-Case Switches and Circuit Breaker Enclosures
. MCCBs, other than the riveted frame types, are permitted to be reconditioned and returned to the manufacturer’s original condition. In order to conform to the manufacturer’s original design, circuit breakers must be reconditioned according to recognized standards. An example of a recognized standard is the Professional Electrical Apparatus Recyclers League (PEARL) Reconditioning Standards. In order to ensure equipment reliability, it is highly recommended that only authorized and qualified professionals recondition MCCBs.
Circuit breakers installed in a system are often forgotten. Even though the breakers have been sitting in place supplying power to a circuit for years, there are several things that can go wrong. The circuit breaker can fail to open due to a burned out trip coil or because the mechanism is frozen due to dirt, dried lubricant, or corrosion, or it can fail due to inactivity or a burned out electronic component. Many problems can occur when proper maintenance is not performed and the breaker fails to open under fault conditions. This combination of events can result in fires, damage to equipment, or injuries to personnel.
The manufacturers’ literature, or approved industry concensus standards, must be used when maintaining circuit breakers. Most manufacturers, as well as NFPA 70B, recommend that if an MCCB has not been operated, opened or closed, either manually or by automatic means, within as little as six months time, it should be removed from service (the load removed) and manually exercised several times. This manual exercise helps to keep the contacts clean, due to their wiping action when opening and closing, and helps ensure that the operating mechanism moves freely and helps keep the lubrication fluid. This exercise does not necessarily operate the mechanical linkages of the tripping mechanism (see Figure 1). Generally, the only way to properly exercise the entire breaker, including the operating and tripping mechanisms, is to remove the breaker from service and test the overload and short-circuit tripping capabilities. A stiff or sticky mechanism can cause an unintentional time delay in its operation under fault conditions. This could dramatically increase the arc flash incident energy level to a value in excess of the rating of personal protective equipment.
The maintenance procedures and tests, regularly performed on MCCBs, generally includes visual inspection; lubrication where possible; cleaning; insulation resistance tests; contact resistance tests; and overcurrent tests.
Visual inspection and cleaning of an MCCB are some of the simplest tests and therefore are sometimes overlooked. Yet, they are a vital part of breaker maintenance. These basic tests don't take much time to do, yet they can point out and help avoid catastrophic problems. When performing a visual inspection, look for signs of overheating, excessive arcing, bent linkages, cracked insulation, tracking, etc. For a 3-phase breaker, this inspection is simplified because the condition of one phase can be compared with the other two phases.
MCCBs should be kept clean for proper ventilation of the breakers. These types of breakers are usually tripped by a thermal element that senses an increase in temperature due to excessive current draw. However, if dirt accumulates on the surrounding of the breaker, the heat build-up may not be permitted to dissipate properly and result in nuisance tripping. Clean the breaker housing and inspect it for cracks or signs of overheating. Tighten all connections. Exercise the breaker several times to ensure the mechanism has freedom of movement and to allow contact wiping. In addition, larger duty circuit breakers (225 amps or above) should be electrically trip tested to ensure proper operation of the trip elements and trip linkages. If possible, test contact resistance to ensure quality of breaker contacts. All molded-case circuit breaker panels should be cleaned of all dirt, dust, and debris using a vacuum. This should only be performed with the equipment in an electrically safe work condition. If it is required to be performed while energized, only qualified workers, applying all required safe work practices and PPE.
Loose connections result in higher resistances and high resistance connections create heat, which is one of the biggest causes of electrical fires. If the connection is very loose, you may see charred or melted thermoplastic insulation on the conductors with the naked eye. But not always. Another area of concern in circuit breakers and switches is high-contact resistance caused by wear of the contact surfaces. As contacts open and close, especially during fault conditions, the contact faces are eroded. This material is sprayed into the arc chutes, along with carbon and other arc by-products. Insufficient contact pressure is often the result of wear and erosion. IR scanning can identify these conditions.
For MCCBs, the visual inspection is sometimes difficult if not impossible. Some MCCBs come from the factory sealed – breaking the seal jeopardizes manufacturer’s warranty. This means the only inspection that can be carried out is on the enclosure and cable terminations. If the MCCB is not sealed, the cover can be removed. This will allow inspection of current-carrying conductors and the mechanism. Again, there are some MCCBs that have arc chutes sealed in place. This makes the inspection of the moving and stationary contacts difficult.
Another visual inspection that should be performed regularly is infrared thermography (IR). IR scanning is recommended as a regular maintenance procedure. The IR scanning must be accomplished with the MCCB energized, closed, and loaded with normal full-load current. IR scanning and analysis have become an essential diagnostic and predictive maintenance tool throughout all industries and have been used to detect many serious conditions requiring immediate corrective action. Many forced outages have been avoided by early detection and correction of problems before the equipment fails. IR scanning is nonintrusive and is accomplished while equipment is in service. Since the equipment is in service and the equipment doors are opened or covers removed, potential electrical hazards exist and all required personal protective equipment (PPE) must be used, where applicable, for shock and/or arc flash.
Loose connections result in higher resistances and high resistance connections create heat, which is one of the biggest causes of electrical fires. If the connection is very loose, you may see charred or melted thermoplastic insulation on the conductors with the naked eye. But not always. IR scanning will identify this condition.
Another area of concern in circuit breakers and switches is high-contact resistance caused by wear of the contact surfaces. As contacts open and close, especially during fault conditions, the contact faces are eroded. This material is sprayed into the arc chutes, along with carbon and other arc by-products. Insufficient contact pressure is often the result of wear and erosion. Insulation resistance, contact resistance, and overcurrent tests require specialized testing equipment for proper testing.
Low-Voltage Power Circuit Breakers:
Low-voltage power circuit breakers are manufactured under a high degree of quality control, of the best materials available, and with a high degree of tooling for operational accuracy. Manufacturer’s tests, per UL 1066 Low-Voltage AC and DC Power Circuit Breakers Used in Enclosures
, show these circuit breakers to have durability beyond the minimum standards requirements. All of these factors give these circuit breakers a very high reliability rating when proper maintenance is performed, per the manufacturer’s instrctions. However, because of the varying application conditions and the dependence placed upon them for protection of electrical systems and equipment, as well as the assurance of service continuity, inspections and maintenance must be made on a regular basis, as recommended by the manufacture.
Maintenance of these breakers will generally consist of keeping them clean, adjusted, and properly lubricated. In addition, it is also necessary to periodically check the circuit breaker contacts for wear and alignment and inspect the arc chutes, especially after the breaker has opened under a fault condition. The frequency of maintenance will depend to some extent on the cleanliness and environmental conditions of the surrounding area. If there is very much dust, lint, moisture, or other foreign matter present then more frequent inspections and maintenance may be required.
Industry standards and manufacturer’s instructions recommend a general inspection and lubrication after a specified number of operations or at least once per year, whichever comes first. If the breaker remains open or closed for a long period of time, it is recommended that arrangements be made to remove the breaker from service and to open and close it several times in succession. Mechanical failure would include an unintentional time delay in the circuit breakers tripping operation due to dry, dirty, or corroded pivot points or by hardened or sticky lubricant in the moving parts of the operating mechanism.
Figure 2 provides an illustration of the numerous points where lubrication would be required and where dirt, moisture, corrosion or other foreign matter could accumulate causing a time delay in, or complete failure of, the circuit breaker operation.
Several studies on electrical equipment failures have been completed over the years by IEEE. These studies have generated failure statisics on electrical distribution system equipment and components. IEEE Std. 493 (the Gold Book) "IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems
" contains the information and statistics from these studies and can be used to provide faliure data of electirical equipment and components, such as circuit breakers. The results of these studies were based upon low- and medium-voltage power circuit breakers (drawout and fixed) as well as fixed mounted molded-case circuit breakers. The results of the study indicated:
- 32% of all circuit breakers failed while in service.
- 9% of all circuit breakers failed while opening.
- 7% of all circuit breakers failed due to damage while successfully opening.
- 42% of all circuit breakers failed by opening when it should not have opened.
- 77% of fixed mounted circuit breakers (0-600V including molded case) failed while in service.
- 18% of all circuit breakers had a mechanical failure
- 28% of all circuit breakers had an electric-protective device failure.
- 23% of all circuit breakers failures were suspected to be caused by manufacturer defective component.
- 23% of all circuit breaker failures were suspected to be caused by inadequate maintenance.
- 73% of all circuit breaker failures required round-the-clock all-out efforts.
Another survey was conducted specifically on low-voltage power circuit breakers and the results concluded:
RELIABILITY AND INTEGRITY
- 19.4% of low-voltage power circuit breakers with electromechanical trip units had unacceptable operation.
- 10.7% of low-voltage power circuit breakers with solid-state trip units had unacceptable operation.
In reviewing the data from these studies, it can be seen that nearly 1/3 of all circuit breakers failed while in service and thus would not have been identified unless proper maintenance was performed. In addition, 16% of all circuit breakers failed or were damaged while opening. The fact that 42% of all circuit breakers failed, by opening when they should not have opened, suggests improper circuit breaker settings or a lack of selective coordination. This type of circuit breaker failure can significantly affect plant processes and could result in a total plant shutdown.
Also of significance is that a very large percentage of fixed mounted circuit breakers, including molded-case had a very high failure rate of 77.8%. This is most likely due to the fact that maintenance of this style of device is often overlooked, but certainly is just as important.
The fact that 18% of all circuit breakers had a mechanical failure and 28% had an electrical protective device failure suggests that both the mechanical linkages, as well as the trip units, need to be maintained. Furthermore, although mechanical maintenance is important, proper testing of the trip unit is much more critical.
Also of importance, is the realization that maintenance and testing is needed because nearly ¼ of all circuit breaker failures were caused by a manufacturer’s defective component and nearly another ¼ of all circuit breaker failures were due to inadequate maintenance. Thus, if proper maintenance and testing is performed, potentially 50% of the failures could be eliminated or identified before a problem occurs. But perhaps the most important issue for an end user is downtime. With regard to this concern, the study indicated 73% of all circuit breaker failures required round-the-clock all-out efforts. This could most likely be greatly reduced if preventive maintenance was performed on a regular basis.
As can be seen by the statistics above, failures can and do occur, therefore maintenance and testing is crucial to the reliability and safety of all circuit breakers.
RESETTING CIRCUIT BREAKERS AFTER AUTOMATIC OPERATION
Another issue associated with the failure of circuit breakers is resetting a circuit breaker that has tripped by automatic means due to an overcurrent condition (overload, short-circuit, or ground-fault). OSHA 29 CFR 1910.334(b)(2) addresses this situation very clearly:
“Reclosing circuits after protective device operation. After a circuit is deenergized by a circuit protective device, the circuit may NOT be manually reenergized until it has been determined that the equipment and circuit can be safely reenergized. The repetitive manual reclosing of circuit breakers or reenergizing circuits through replaced fuses is prohibited.
NOTE: When it can be determined from the design of the circuit and the overcurrent devices involved that the automatic operation of a device was caused by an overload rather than a fault condition, no examination of the circuit or connected equipment is needed before the circuit is reenergized.”
The safety of the employee manually operating the circuit breaker is at risk if the short-circuit condition still exists when reclosing the breaker. OSHA no longer allows the past practice of resetting a circuit breaker one, two, or three times before investigating the cause of the trip. This previous practice has caused numerous burn injuries that resulted from the explosion of electrical equipment. Before resetting a circuit breaker, it, along with the circuit and equipment, must be tested and inspected by a qualified person to ensure a short-circuit condition does not exist and that it is safe to reset the breaker. Any time a circuit breaker has operated and the reason is unknown, the breaker, circuit, and equipment must be inspected and tested for a short-circuit condition. Melted arc chutes will not interrupt fault currents. If the breaker cannot interrupt a second fault, it will fail and may destroy its enclosure and create a hazard for anyone working near the equipment. This could result in an arc flash incident.
To further emphasize this point the following quote is provided:
“After a high level fault has occurred in equipment that is properly rated and installed, it is not always clear to investigating electricians what damage has occurred inside encased equipment. The circuit breaker may well appear virtually clean while its internal condition is unknown. For such situations, the NEMA AB4 ‘Guidelines for Inspection and Preventive Maintenance of MCCBs Used in Commercial and Industrial Applications may be of help. Circuit breakers unsuitable for continued service may be identified by simple inspection under these guidelines. Testing outlined in the document is another and more definite step that will help to identify circuit breakers that are no longer suitable for continued service.”
A circuit breaker may require replacement just as any other switching device, wiring or electrical equipment in the circuit that has been exposed to short-circuit current. Questionable circuit breakers must be replaced for continued, dependable circuit protection. The condition of the circuit breaker must be known to ensure that it functions properly and safely before it is put it back into service.
ARC FLASH HAZARD CONSIDERATIONS
Maintenance and testing are essential to ensure proper protection of equipment and personnel. With regard to personnel protection, NFPA 70E requires an arc flash risk assessment be performed before anyone approaches exposed energized electrical conductors or circuit parts that have not been placed in an electrically safe work condition. NFPA 70E, Section 130.5 states that the arc flash risk assessment must take into consideration the design of the overcurrent protective device and it’s opening time, including its condition of maintenance. In addition it requires an arc flash boundary to be established.
All calculations for determining the incident energy of an arc and for establishing an arc flash boundary require the arc clearing time of the overcurrent protective device. This clearing time is derived from the settings on the divice along with the time-current curves. This information can also be obtained from a current engineering protective device coordination study, which is based on what the protective devices are supposed to do. If, for example, a low-voltage power circuit breaker had not been operated or maintained for several years and the lubrication had become sticky or hardened, the circuit breaker could take several additional cycles, seconds, minutes, or longer to clear a fault condition. The following are specific examples:
Two incident energy analyses will be performed using a 20,000-amp short-circuit with the worker 18 inches from the arc:
- Based on what the system is supposed to do:
- 0.083 second (5 cycles) arc clearing time
- Due to a sticky mechanism the breaker now has an unintentional time delay:
- 0.5 second (30 cycles) arc clearing time
The example calculations use the NFPA 70E equations found in Annex D of the 2015 edition.
= maximum 20 in. cubic box incident energy, cal/cm2
= distance from arc electrodes, inches (for distances 18 in. and greater)
= arc duration, seconds
F = short circuit current, kA (for the range of 16 kA to 50 kA)
= 18 in.
= 0.083 second (5 cycles)
(3) F = 20kA
- 0.3453F + 5.9675]
= 1038 x 0.0141 x 0.083[0.0093 x 400 - 0.3453 x 20 + 5.9675]
= 1.4636 x [2.7815]
= 3.5 cal/cm2
NFPA 70E, 130.5(C)(1) requires arc-rated clothing and other PPE are to be selected based on this incident energy level exposure. Therefore the arc-rated clothing and PPE must have an arc rating of at least 3.5 cal/cm2
= maximum 20 in. cubic box incident energy, cal/cm2
= distance from arc electrodes, inches (for distances 18 in. and greater)
= arc duration, seconds
F = short circuit current, kA (for the range of 16 kA to 50 kA)
= 18 in.
= 0.5 second (30 cycles)
(3) F = 20kA
- 0.3453F + 5.9675]
= 1038 x 0.0141 x 0.5[0.0093 x 400 - 0.3453 x 20 + 5.9675]
= 7.3179 x [2.7815]
= 20.4 cal/cm2
NFPA 70E, 130.5(C)(1) requires arc-rated clothing and other PPE are to be selected based on this incident energy level exposure. Therefore the arc-rated clothing and PPE must have an arc rating of at least 20.4 cal/cm2
If the worker is protected based on what the circuit breaker is supposed to do (0.083 second or 5 cycles) and an unintentional time delay occurs (0.5 second or 30 cycles), the worker could be seriously injured or killed because he/she was under protected. As can be seen, maintenance is extremely important to an electrical safety program. Maintenance must be performed according to the manufacturer’s instructions in order to minimize the risk of having an unintentional time delay in the operation of the circuit protective devices.
In order to protect electrical equipment and personnel, proper electrical equipment preventive maintenance must be performed. Several standards and guides exist to assist users with electrical equipment maintenance. When the overcurrent protective devices are properly maintained and tested for proper calibration and operation, equipment damage and arc flash hazards can be limited as expected. Unfortunately many in industry think that just because the lights are on or the machines are running that everything is okay and that maintenance is not needed, because the circuit breaker is working just fine. No, the circuit breaker is not working, it is closed. Working is when an overload, ground-fault, or short-circuit occurs and the circuit breaker opens automatically in the time specified or when it is manually opened or closed. Maintenance of overcurrent protective devices is critical to electrical equipment and systems reliability, as well as for safety of personnel.
- NFPA 70B, Recommended Practice for Electrical Equipment Maintenance, 2013 Edition
- IEEE Std. 3007.2-2010, IEEE Recommended Practice for the Maintenance of Industrial and Commercial Power Systems
- NEMA Standard AB-4, Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers Used in Commercial and Industrial Applications
- IEEE Standard 493-2007 (the Gold Book), Recommended Practice For The Design Of Reliable Industrial And Commercial Power Systems
- IEEE Standard 1015-2006 (the Blue Book), Recommended Practice For Applying Low-Voltage Circuit Breakers Used In Industrial And Commercial Power Systems
- NFPA 70E-2015, Standard for Electrical Safety in the Workplace
- IEEE Standard 1584-2002, IEEE Guide for Arc Flash Hazard Calculations
- ANSI/NETA MTS-2015, Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems
- National Equipment Manufacturer’s Association (NEMA) Vince A. Baclawski, Technical Director, Power Distribution Products, NEMA; EC&M magazine, pp. 10, January 1995
- Manufacturer’s Instruction Books
Dennis K. Neitzel, CPE, CESCP, is Director Emeritus of AVO Training Institute, Inc., Dallas, Texas. He has over 48 years of experience in electric utilities, industrial facilities, and commercial electrical equipment and systems maintenance and testing, with an extensive background in electrical safety and power systems analysis. He is an active member of IEEE, ASSE, AFE, IAEI, SNAME, and NFPA. He is a Certified Plant Engineer (CPE), Certified Electrical Safety Compliance Professional (CESCP), and a Certified Electrical Inspector-General. Mr. Neitzel is a Principle Committee Member and Special Expert for the NFPA 70E, Standard for Electrical Safety in the Workplace
; Working Group Chair for IEEE Std 3007.1-2010 Recommended Practice for the Operation and Management of Industrial and Commercial Power Systems
, 3007.2-2010 Recommended Practice for the Maintenance of Industrial and Commercial Power Systems
, 3007.3-2012 Recommended Practice for Electrical Safety of Industrial and Commercial Power Systems
; and IEEE Std 45.5-2014 Recommended Practice for Electrical Installations on Shipboard - Safety Considerations
; He is a co-author of the Electrical Safety Handbook
, McGraw-Hill Publishers. Mr. Neitzel earned his Bachelor’s degree in Electrical Engineering Management and his Master’s degree in Electrical Engineering Applied Sciences. He has authored, published, and presented numerous technical papers and magazine articles on electrical safety, maintenance, and training. For more information, contact Mr. Neitzel by e-mail at firstname.lastname@example.org