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Friday, December 12, 2014
Bases of Maintenance and Testing of Protective Devices
The NEC Articles 210-20, 215-3, 240-1, and 240-3 specify requirements for the protection of electrical equipment and conductors. The Fine Print Note (FPN) to Article 240-1, Scope, states, “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.”
To protect against overcurrent conditions, the only way to ensure that circuit breakers, overcurrent relays, and protective devices are working correctly is through regular maintenance and testing of these devices. There are several steps that must be taken in order to establish an effective maintenance program for the breakers and overcurrent protective devices. The first step in correctly maintaining electrical equipment and overcurrent protective devices is to understand the requirements and recommendations for electrical equipment maintenance from various sources.
Examples of such sources include, but are not limited to, the manufacturer’s instructions, NFPA 70B, IEEE Standard 902 (Yellow Book), NEMA AB-4, NETA Specs, NFPA 70E and this book.
The second step in performing maintenance and testing is to provide adequate training and qualification for employees.
NFPA 70E, Section 205.1 states, “Employees who perform maintenance on electrical equipment and installations shall be qualifi ed persons … and shall be trained in and be familiar with the specific maintenance procedures and tests required.” The NEC defines a qualifi ed person as “One who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training on the hazards involved.” It is vitally important that employees be properly trained and qualifi ed to maintain electrical equipment in order to increase the equipment and system reliability, as well as the employee’s safety.
The third step is to have a written, effective EPM program. NFPA 70B makes several very clear statements about an effective EPM program. These statements include
1. Deterioration of electrical equipment is a normal process, but that does not mean that equipment failure is eminent. If unchecked, deterioration will eventually cause equipment malfunction or complete failure. There are several factors that can accelerate the deterioration process, such as the environment, overload conditions, or severe duty cycles. An effective EPM program will help to identify and correct any or all of these conditions.
2. In addition to the deterioration problem, there are several other potential causes of equipment failure. These causes include, but are not limited to, load changes, circuit alterations, improper or misadjusted settings of protective devices, improperly selected protective devices, and changing voltage conditions.
3. With the absence of an effective EPM program, management assumes a greater responsibility for and an increased risk of a serious electrical failure, as well as the consequences.
4. An effective EPM program, that is administered properly, will reduce costly shutdowns and outages, reduce accidents, and save lives.
These programs will identify impending troubles and apply solutions to correct them, before they become major problems that require time consuming and more expensive solutions.
IEEE Standard 902 states: “In planning an 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.”
The forth step is that all maintenance and testing of electrical protective devices must be accomplished in accordance with the manufacturer’s instructions.
NFPA 70E adds to this by stating: “Protective devices shall be maintained to adequately withstand or interrupt available fault current.” It goes on to state, “Circuit breakers that interrupt faults approaching their ratings shall be
inspected and tested in accordance with the manufacturers’ instructions.”
In the absence of the manufacturer’s instructions, the NETA Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems is an excellent source of information for performing the required maintenance and testing of these devices. However, the manufacturer’s time–current curves would also be required in order to properly test each protective device.
The fifth and fi nal step that will be addressed here is the arc-fl ash hazard considerations. One of the key components of the fl ash hazard analysis, which is required by NFPA 70E and OSHA, is the clearing time of the protective devices, primarily circuit breakers, fuses, and protective relays. Fuses, although they are protective devices, they do not have operating mechanisms that would require periodic maintenance. However, fuses should be inspected to verify that they are in good working condition.
We will address some of the issues concerning maintenance and testing of the protective devices, according to the manufacturer’s instructions. We will also address how protective device maintenance relates to the electrical arc-fl ash hazard.
Molded-case circuit breakers:Generally, maintenance on molded-case circuit breakers is limited to mechanical mounting, electrical connections, and periodic manual operation. Most lighting, appliance, and power panel circuit breakers have riveted frames and are not designed to be opened for internal inspection or maintenance. All other molded-case circuit breakers that are Underwriters Laboratory (UL) approved are factory-sealed to prevent access to the calibrated elements. An unbroken seal indicates that the mechanism has not been tampered with and that it should function as specified by UL or its manufacture.
A broken seal voids the UL and the manufacturers’ warranty of the device.
In this case, the integrity of the device would be questionable. The only exception to this would be a seal being broken by a manufacturer’s authorized facility.
Molded-case circuit breakers receive extensive 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. Molded-case circuit breakers, 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.
The Professional Electrical Apparatus Recyclers League (PEARL) companies follow rigid standards to recondition low-voltage industrial and commercial moldedcase circuit breakers. It is highly recommended that only authorized professionals recondition molded-case circuit breakers. 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. The overcurrent device 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 fi res, damage to equipment, or injuries to personnel.
All too often, a circuit breaker fails because the minimum maintenance (as specified by the manufacturer) was not performed or was performed improperly.
Small things, like failing to properly clean and/or lubricate a circuit
breaker, can lead to operational failure or complete destruction due to overheating of the internal components. Common sense, as well as manufacturers’ literature, must be used when maintaining circuit breakers. Most manufacturers, as well as NFPA 70B, recommend that if a molded-case circuit breaker has not been operated, opened, or closed, either manually or by automatic means, within as little as 6 months time, it should be removed from service and manually exercised several times. This manual exercise helps to keep the contacts clean due to their wiping action and ensures that the operating mechanism moves freely. This exercise, however, does not operate the mechanical linkages in the tripping mechanism (Figure ).
The only way to properly exercise the entire breaker operating and tripping mechanisms is to remove the breaker from service and test the overcurrent 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-fl ash incident energy level to a value in excess of the rating of PPE. There will be more on incident energy later.
Another consideration is addressed by OSHA in 29 CFR 1910.334(b)(2) which states:
Principle components of a molded-case circuit breaker. (From Neitzel, D.K., Principle Components of a Molded-Case Circuit Breaker, AVO Training Institute, Inc., Dallas, TX (revision 2007, p. 14).With permission.)
Reclosing circuits after protective device operation. After a circuit is de-energized 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 and 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
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 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.
Any time a circuit breaker has operated and the reason is unknown, the breaker must be inspected. 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.
To further emphasize this point the following quote from the NEMA 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 identifi ed 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 not suitable for continued service.
After the occurrence of a short circuit, it is important that the cause be investigated and repaired and that the condition of the installed equipment be investigated. A circuit breaker may require replacement just as any other switching device, wiring or electrical equipment in the circuit that has been exposed to a short circuit. 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 back into service.
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 show these circuit breakers have durability beyond the minimum standards requirements. All of these factors give these circuit breakers a very high reliability rating. 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 checks must be made on a regular basis. Several studies have shown that low-voltage power circuit breakers, which were not maintained within a 5-year period, have an average of a 50% failure rate. Maintenance of these breakers will generally consist of keeping them clean and properly lubricated. The frequency of maintenance will depend to some extent on the cleanliness of the surrounding area. If there were very much dust, lint, moisture, or other foreign matter present then obviously more frequent maintenance would be required. Industry standards for, as well as manufacturers of, low-voltage power circuit breakers recommend a general inspection and lubrication after a specifi ed number of operations or at least once per year, whichever comes fi rst. Some manufacturers also recommend this same inspection and maintenance be performed after the first 6 months of service regardless of the number of operations.
If the breaker remains open or closed for a long period of time, it is recommended that arrangements be made to open and close the breaker several times in succession, preferably under load conditions. Environmental conditions play a major role in the scheduling of inspections and maintenance.
If the initial inspection indicates that maintenance is not required at that time, the period may be extended to a more economical point.
However, more frequent inspections and maintenance may be required if severe load conditions exist or if an inspection reveals heavy accumulations of dirt, moisture, or other foreign matter that might cause mechanical, insulation, or electrical failure. 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. The manufacturer’s instructions must be followed in order to minimize the risk of any unintentional time delay. Figure 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.
Medium-voltage power circuit breakers: Most of the inspection and maintenance requirements for low-voltage power circuit breakers also apply to medium-voltage power circuit breakers. Manufacturers recommend that these breakers be removed from service and inspected at least once per year. They also state that the number and severity of interruptions may indicate the need for more frequent maintenance checks. Always follow the manufacturer’s instructions because every breaker is different.
1. Shunt trip device
2. Trip shaft
3. Roller constraining link
4. Trip latch
5. Close cam
6. Stop roller
7. Spring release latch
8. Spring release device
9. Oscillator pawl
19. Reset spring
20. Closing spring anchor
21. Pole shaft
23. Emergency charge handle
24. Motor crank and handle
25. Moving contact assembly
26. Insulating link
27. Main drive link
10. Ratchet wheel
11. Hold pawl
12. Drive plate
13. Emergency charge pawl
15. Crank shaft
16. Emergency charge device
17. Crank arm
18. Closing spring
Power-operated mechanism of a cutler/hammer “DS” circuit breaker. (Courtesy of Cutler Hammer Corp.) (From Neitzel, D.K., Circuit Breaker Maintenance, Module 2, AVO Training
Institute, Inc., Dallas, TX ( revision 2007, p. 34). With permission.)
Above Figures illustrate two types of operating mechanisms for medium-voltage power circuit breakers. These mechanisms are typical of the types used for air, vacuum, oil, and SF6 circuit breakers. As can be seen in these figures, there are many points that would require cleaning and lubrication in order to function properly.
Protective relays: Relays must continuously monitor complex power circuit conditions, such as current and voltage magnitudes, phase angle relationships, direction of power fl ow, and frequency. When an intolerable circuit condition, such as a short circuit (or fault) is detected, the relay responds and closes its contacts, and the abnormal portion of the circuit is de- energized via the circuit breaker. The ultimate goal of protective relaying is to disconnect a faulty system element as quickly as possible. Sensitivity and selectivity are essential to ensure that the proper circuit breakers are tripped at the proper speed to clear the fault, minimize damage to equipment, and to reduce the hazards to personnel. A clear understanding of the possible causes of primary relaying failure is necessary for a better appreciation of the practices involved in backup relaying. One of several things may happen to prevent primary relaying from disconnecting a power system fault:
1. Tripping magnet
2. Tripping latch
3. Center pole unit lever
4. Main contact
5. Main link
6. Closing cam
7. Closing cam
8. Crank shaft
9. Tripping cam
10. Tripping trigger
11. Tripping cam
12. Front panel
13. Mech back plate
15. Dolly bracket
16. Tripping cam
17. Locking nut
18. Trip latch roller
Operating mechanism of stored energy air circuit breaker. (From Neitzel, D.K., Circuit Breaker Maintenance, Chapter 4, AVO Training Institute, Inc., Dallas, TX (revision 2006, p. 4-18). With
Solenoid-operated mechanism. (From Neitzel, D.K., Circuit Breaker Maintenance, AVO Training Institute, Inc., Dallas, TX (revision 2006, p. 1-10). With permission.)
Primary relaying for an electric power system. (From Neitzel, D.K., Protective Relay Maintenance, Module 3, AVO Training Institute, Inc., Dallas, TX (revision 2006, p. 5). With permission.)
Current or voltage supplies to the relays are incorrect
• DC tripping voltage supply is low or absent
• Protective relay malfunctions
• Tripping circuit or breaker mechanism hangs up
There are two groups of protective relays: primary and backup. Primary relaying is the so-called first line of defense, and backup relaying is sometimes considered to be a subordinate type of protection. Many companies, however, prefer to supply two lines of relaying and do not think of them as primary and backup. Figure illustrates primary relaying. Circuit breakers are found in the connections to each power system element. This provision makes it possible to disconnect only the faulty part of the system. Each element of the system has zones of protection surrounding the element. A fault within the given zone should cause the tripping of all circuit breakers within that zone and no tripping of breakers outside that zone. Adjacent zones of protection can overlap, and in fact, this practice is preferred, because for failures anywhere in the zone, except in the overlap region, the minimum numbers of circuit breakers are tripped. In addition, if faults occur in the overlap region, several breakers respond and isolate the sections from the power system. Backup relaying is generally used only for protection against short circuits. Since most power system failures are caused by short circuits, short-circuit primary relaying is called on more often than most other types.
Therefore, short-circuit primary relaying is more likely to fail.
Voltage and current transformers play a vital role in the power protection scheme. These transformers are used to convert primary current and voltages to secondary (120 V) current and voltages, and to allow current and voltage sensing devices, such as relays, meters, and other instruments to be isolated from the primary circuit. It should be clearly understood that the performance of a relay is only as good as the voltage and current transformers connected to it. A basic understanding of the operating characteristics, application, and function of instrument transformers is essential to a relay technician.
Some overcurrent relays are equipped with an instantaneous overcurrent unit, which operates when the current reaches its minimum pickup point (see Figure ). An instantaneous unit is a relay having no intentional time delay. Should an overcurrent of sufficient magnitude be applied to the relay, the instantaneous unit will operate and will trip the circuit breaker.
The instantaneous trip unit is a small, AC-operated clapper device. A magnetic armature, to which leaf-spring-mounted contacts are attached, is attracted to the magnetic core upon energization. When the instantaneous unit closes, the moving contacts bridge two stationary contacts and complete the trip circuit. The core screw, accessible from the top of the unit, provides the adjustable pickup range. Newer designs also feature tapped coils to allow even greater ranges of adjustment. The instantaneous unit, like the one
Instantaneous trip unit. (From Neitzel, D.K., Protective Relay Maintenance, Module 3, AVO Training Institute, Inc., Dallas, TX (revision 2006, p. 24). With permission.)
shown in Figure, is equipped with an indicator target. This indication shows that the relay has operated. It is important to know which relay has operated, and no relay target should be reset without the supervisor’s knowledge and permission, or after it has been determined which relay operated to clear the fault. As can be seen, several things can go wrong that would prevent the instantaneous unit from operating properly. These things include an open or shunted current transformer, open coil, or dirty contacts. Protective relays, like circuit breakers, require periodic inspection, maintenance, and testing to function properly. Most manufacturers recommend that periodic inspections and maintenance on the induction and electromagnetic type relays be performed at intervals of 1–2 years. The intervals between periodic inspection and maintenance will vary depending upon environment, type of relay, and the user’s experience with periodic testing. The periodic inspections, maintenance, and testing are intended to ensure that the protective relays are functioning properly and have not deviated from the design settings.
If deviations are found, the relay must be retested and serviced as
described in the manufacturer’s instructions.