As with so many other aspects of electrical systems, in order to ensure selective coordination in a system, the overcurrent protective device scheme must be properly engineered, installed and maintained. Even if a system is designed and installed properly, a lack of proper maintenance can negate the selective coordination scheme that may be vital for life safety or critical business reasons. The same applies to electrical safety for workers. (See Sidebar on page 44.)
What is selective coordination?
Th e way electrical distribution systems are laid can be likened to a tree. As depicted in Fig. 1, the building service is typically one large ampacity circuit (a tree trunk) that divides into a number of lower ampacity feeders (limbs) that further are divided into many still lower ampacity branch circuits (branches), which supply power to individual end-use equipment such as motors, lighting circuits, computers and HVAC units. Each segment of the electrical distribution system (service, feeder and branch) has a circuit breaker or fuse and sometimes a relay that provides overcurrent protection for its respective segment of the circuit. When overcurrent protective devices in an electrical distribution system are chosen by the engineer to be selectively coordinated, by design, whenever there is an overcurrent condition on a circuit, only the nearest upstream protective device should open for an overcurrent condition (Fig. 2). This is applicable for any overcurrent condition, whether an overload or short-circuit.
Most people assume that if there is a short-circuit condition on a 20A branch circuit, only the 20A circuit breaker or fuse will open and the larger 200A feeder circuit breaker or fuse will not open. This, however, is not always the case (Fig. 3). In some situations, if the engineer did not choose the proper overcurrent protective devices and/or specify the proper settings or amp rating, a feeder (limb) or even the service (truck) overcurrent protective device may unnecessarily open for a branch circuit fault. In the same manner, a short-circuit on a feeder circuit should only open the feeder overcurrent protective device and not cause the service overcurrent protective device to open.
Why is selective coordination important?
Selective coordination of all the overcurrent protective devices for the circuits supplying vital loads improves the reliability of the system to supply power. In today's buildings and manufacturing processes, there is a far greater dependence on electricity.
The ability of the electrical distribution system to provide continuous availability of power to vital loads is ever more important. Worker productivity, industrial process loads, critical computer business system loads and life safety emergency loads, such as exit lighting and healthcare essential electrical system loads, depend on a continuity of power. In some cases, the unexpected loss of power to some industrial processes can be a hazardous condition. When there is a dangerous overcurrent condition, the overcurrent protective devices are expected to open the circuit. But, if a lack of selective coordination unnecessarily opens higher level upstream overcurrent protective devices, critical loads could be disrupted needlessly.
Selective coordination can be either a design consideration or a mandatory National Electrical Code® (NEC) requirement. The National Electrical Code has mandatory selective coordination requirements in 517.26 Healthcare Essential Electrical Systems, 620.62 Elevator Circuits, 700.27 Emergency Systems, 701.18 Legally Required Standby Systems and 708 Critical Operations Power Systems. Typically, these systems already have periodical maintenance requirements for generators, automatic transfer switches and other key components. This article is advocating maintenance of the overcurrent protective devices wherever selective coordination is an important consideration for the system.
How is selective coordination achieved?
Although it is not within the scope of this article to fully explain how to select fuses or circuit breakers to achieve selective coordination, some mention of this process definitely is in order here. Suffice it to say, in most cases, selective coordination must be designed in up-front for an electrical system. This is especially important with circuit breakers. In some cases with fusible systems, after the installation there is more flexibility to merely change to different fuse types to achieve selective coordination. The best practice occurs during the system design phase, when the electrical engineer does a short-circuit current analysis and coordination analysis of the electrical system. In this process, the engineer must select the proper circuit breakers or fuse types and amp rating/settings to achieve selective coordination.
The engineer's specification should provide a selectively coordinated electrical system. Yet, merely installing a system with the proper circuit breakers, fuses and relays that provide a selectively coordinated system is not sufficient. Proper maintenance of the overcurrent protective devices will help ensure that the system will perform as specified over its lifetime.
What maintenance is required?
Now that you have an idea what selective coordination is and what impact it has on the reliability of an electrical distribution system to deliver power to the loads, let's focus on what maintenance needs to be performed on overcurrent protective devices.
The internal parts of modern current-limiting fuses do not require maintenance. There are no adjustments or settings necessary—or possible. However, like all electrical components, the integrity of fuses can be negatively affected by the surrounding environment and components. That's why it is important to periodically check fuse bodies, fuse mountings/ clips and adjacent conductor terminations for signs of overheating, poor connections or insufficient conductor ampacity.
Always have spare fuses of the correct type and ampere rating readily available. For critical circuits, it is best to keep ample spares in a cabinet marked "spare fuses" near the electrical equipment. Also, label the equipment with the proper fuse type and ampere rating. Fuses from different manufacturers should not be mixed in usage since the claims for selective coordination by individual manufacturers are applicable only to their respective products. A continuity tester can verify that a fuse has not opened.
In addition, a resistance check of fuses can be made using a sensitive four-wire instrument such as a Kelvin bridge. The ANSI/NETA MTS 2007 ANSI/NETA MTS-2007, Standard for Maintenance Testing Specification has a guideline of 15% variance for fuse resistance. If the resistance of fuses checked in a disconnect deviates by more than 15%, further investigation should be made. Fuses that are within the manufacturer's resistance tolerance and meet industry standards for performance may have resistance values that deviate by more than 15%. Highly skilled electrical maintenance contractors, though, have told this author that the 15% guideline works well for maintenance purposes. Generally, when they find a fuse out of tolerance, the percentage difference is far greater than 15% (an order of magnitude or greater).
Circuit breakers are mechanical devices that require periodic maintenance to ensure proper operation. A popular misconception is that if a circuit breaker has not tripped due to an overcurrent it is in original condition. In fact, a circuit breaker that sits without opening over long periods can have performance issues. The lubrication of the mechanism, which is vital for its proper operation, can degrade or dry over time and affect the circuit breaker's ability to operate properly. A circuit breaker also can be damaged or degraded after interrupting a fault.
Good preventive maintenance for circuit breakers should include periodic exercise of the operating mechanism. A better practice is to exercise the trip latch mechanism since it can seize due to lack of use. The trip latch mechanism can be exercised by primary injection testing or, if a circuit breaker is equipped, by pushing the Push-to-Trip or similar button (usually red in color), which directly operates the trip latch. It is recommended that periodic circuit breaker maintenance include exercise (every six to 12 months), visual and mechanical inspections and calibration tests.
In addition, periodically check conductor terminations for signs of overheating, poor connections and/or insufficient conductor ampacity. The "as found" and "as left" records should be retained and trended for each circuit breaker's condition and maintenance tests. When a circuit breaker is nearing the point where it requires repair or replacement, the trending of the test data tends to escalate.
For fuse and circuit breaker assemblies, as well as other electrical components, infrared thermographic scans are one method for monitoring conditions where loose connections or other situations may cause intolerable thermal conditions.
Relays also should undergo period maintenance. Such maintenance should entail calibration; checking the relay power source; ensuring that all wiring schemes are correctly installed; and maintaining the disconnecting means that the relay will signal to open if there is an overcurrent condition that must be removed.
How often, etc.?
One consideration in maintenance of overcurrent protective devices is the frequency of maintenance. This is dependent on many factors including the device type, environment of the installation, usage, loading, age of equipment, prior maintenance data trends and necessary reliability. Your first source of information should be the device manufacturer's maintenance manual. Other industry sources include NFPA 70B, Recommended Practice for Electrical Equipment Maintenance and Appendix B, "Frequency of Maintenance Tests" of ANSI/NETA MTS 2007 ANSI/NETA MTS-2007, Standard for Maintenance Testing Specification. The "Frequency of Maintenance Tests" provides a very useful matrix and schedule table for determining which maintenance activities should occur, as well as the frequency based on the condition of the equipment and its criticality.
The next question usually is what tests should be performed. Again the device manufacturer's maintenance manual is the first place you should go for answers. Another excellent resource is ANSI/NETA MTS-2007, Standard for Maintenance Testing Specification for the prescriptive tests for all types of electrical equipment in an electrical distribution system. MT
Tim Crnko is manager, Training & Technical Services with Cooper Bussmann, where he has spent 22 years focusing on the application of overcurrent protective devices. Crnko received his B.S.E.E. and M.S.E.E. from the University of Missouri at Columbia. He is a member of IEEE, NEMA, NFPA and IAEI, as well as a committee member of the NFPA 70B, Recommended Practice for Electrical Equipment Maintenance. For additional information on selective coordination, visit www.CooperBussmann.com; for information on NFPA 70B, visit www.NFPA.org; for information on ANSI/NETA MTS-2007, visit www.NETAworld.org