You’re reading an outdated article. Please go to the recent issues to find up-to-date content.
Distribution system capacity continues to increase in commercial, industrial, and institutional facilities. Arcing ground faults can seriously damage distribution equipment, causing fires, which damage facilities and endanger personnel. They also cause extended downtime during system repair. Ground fault protection is the first line of defense. Once installed, ground fault protection systems stand by until needed to protect services and feeders. However, if these systems malfunction when a ground fault occurs, the distribution system and facility will be as damaged as if no systems were installed. Ground fault systems must be installed properly and tested and maintained regularly. What is a ground fault? A ground fault is any short circuit that results in an unintended connection between an energized ungrounded phase conductor and ground. Ground faults are the most common type of fault on power distribution systems. They result from the unintentional grounding of an ungrounded phase conductor or insulation failure that brings the ungrounded phase conductor into contact with ground. Unintentional grounding of a phase conductor can occur when a small animal enters a piece of equipment and contacts both an ungrounded phase conductor and the grounded enclosure. Insulation failure resulting in a ground fault can occur when busbar insulator contamination results in a flashover or when age or other environmental factors degrade the conductor insulation. For a solidly grounded distribution system, a ground fault results in current flowing back to the source through the equipment grounding conductor, which includes the metallic raceway enclosing the circuit conductors, separate equipment grounding conductor if installed, or both. In addition, the ground fault current can also flow back to the source through other paths, including grounded metal piping, structural steel, and the ground itself. The amount of ground fault current flowing through alternate paths outside the distribution system ground path will depend on the relative impedance between the distribution system ground return path and the alternate parallel paths. If the return impedance for a ground fault were as low as for a phase-to-phase fault involving two or more phase conductors, the fault current resulting from a bolted ground fault would be comparable to that of a phase-to-phase fault and the normal overcurrent protective device would operate quickly to open the circuit and safely clear the ground fault. However, the ground return path impedance is typically higher than that of a bolted phase-to-phase fault, resulting in smaller fault current and delayed clearing of a bolted ground fault. This is due to the inverse time characteristic of the overcurrent device protecting the circuit conductors. This is further compounded by the fact that most ground faults start out as high-impedance arcing faults, which result in a significantly lower fault current than the normal overcurrent device can detect. After the start of the arcing ground fault and before it becomes a bolted fault that the normal overcurrent device can detect, a great deal of damage can be done. This damage may not be confined to the distribution system and associated equipment. NEC-required ground fault protection Ground fault protection is needed in commercial, industrial, and institutional facilities. The National Electrical Code (NEC) addresses the need for ground fault protection in NEC Section 215-10 for feeders, NEC Section 230-95 for services, and NEC 240-13 as part of overcurrent protection. Specific ground fault protection requirements in health care facilities that exceed the general NEC requirements are provided in NEC 517-17. In general, the NEC requires that ground fault protection be installed on services and feeders that meet all of the following criteria: * The distribution system is a solidly grounded wye system. * The nominal voltage to ground is greater than 150 volts and less than or equal to 600 volts line-to-line. * The disconnecting means is rated 1,000 amperes or more. The above NEC criteria specifically include high-capacity 480Y/277-volt distribution systems that are common in modern commercial, industrial, and institutional facilities. The NEC singles out high-capacity 480Y/277-volt distribution systems for mandatory ground fault protection, because once initiated, an arcing ground fault will typically sustain itself on a 480Y/277-volt system by restriking on each half cycle after the phase voltage passes through zero. This occurs because the intense heat generated by the arc vaporizes metal and degrades the insulation strength of the surrounding air. On 208Y/120-volt systems, the arc typically extinguishes itself after the phase voltage passes through zero and does not restrike. Exceptions to the ground fault protection requirements in NEC Sections 215-10, 230-95, and 240-13 include: where a non-orderly shutdown of an industrial process will increase hazards, where ground fault protection is not required for fire pumps, and on feeders where it is provided upstream from the feeder’s point of supply. In addition, ground fault protection is not required for emergency and legally required standby systems per NEC Sections 700-26 and 701-17, respectively. However, a ground-fault indicator is required where practicable for emergency systems in accordance with NEC Paragraph 700-7(d). The NEC criteria above only determine when ground fault protection is mandatory. The NEC does not restrict the use of ground fault protection on other systems or parts of the system that do not meet the above criteria. Ground fault protection may be provided on services and feeders that NEC does not require to have for added protection, system coordination, or other operational considerations. For example, feeders fed from a 480-volt, three-phase, four-wire switchboard may not be required to have ground fault protection, even though the NEC requires the main circuit breaker protecting the switchboard. In this case, providing ground fault protection on each feeder improves system selectivity and continuity by preventing a ground fault on any one feeder from interrupting power to all feeders when the feeder ground fault trips the main circuit breaker. What won’t ground fault protection do? Ground fault protection is meant to protect property and personnel from the hazards that result from both arcing and bolted ground faults where the magnitude of the resulting fault current magnitude is less than what the normal phase overcurrent protective device will detect and clear quickly. Ground fault protection will not protect personnel from shock or electrocution. Ground fault protection is typically set to detect fault current magnitudes greater than 5 amperes and up to hundreds of amperes. Ground fault circuit interrupters (GFCIs) are used to protect personnel from electrocution and are designed to interrupt the circuit when a ground fault current of about 5 milliamperes (mA) is detected. Ground fault protection system elements NEC Article 100 defines ground fault protection of equipment as, “A system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent device.” System components The ground fault sensor senses current that is flowing outside the normal distribution circuit through the equipment grounding conductor or other alternate ground path. Current transformers are typically used as sensors to detect ground fault current. Current transformers are used in three ways Ground return current sensing method. The ground return sensing method uses a current transformer installed in the ground return path of a wye-connected source as shown in Figure 1. Ground current must flow back to the source through the main bonding jumper no matter what path it takes. When ground fault current returns through the main bonding jumper to the source neutral point, it results in current being induced in the current transformer secondary that is detected by the ground fault relay. Zero sequence current sensing method. The zero sequence or core balance current sensing method uses one current transformer that encompasses all three-phase conductors and the neutral if used as a current-carrying conductor. Just as the sum of the currents in each phase plus the neutral if used adds to zero, so does the magnetic flux that results from the flow of these currents. When there is no ground fault, the residual magnetic flux is zero, resulting in no current flow in the secondary of the current transformer. However, if there is a ground fault and current flows back to the source through the equipment grounding conductor or other alternate ground path, the sum of the currents in the phases and neutral is no longer zero. This results in a residual magnetic flux that, in turn, induces current in the secondary of the current transformer, which the ground fault relay will detect. Residual current sensing method. The residual or differential current sensing method involves placing a current transformer on each phase of the service or feeder being protected along with the neutral, if it is used as a current-carrying conductor. This method resembles the zero sequence method, except that it uses multiple current transformers as a detector instead of just one. The sum of the currents in each phase plus the current in the neutral conductor always sums to zero when there is no ground fault. Likewise, the sum of the currents in the secondary of the associated current transformers will add to zero when these current transformers are properly connected. However, during a ground fault, neither the sum of the phase and neutral currents nor the sum of the current transformer secondary currents will add to zero. The ground fault relay will detect the resultant secondary current due to a ground fault. The ground fault relay (GFR) acts on the information the ground fault sensor provides. The ground fault relay can either be installed remotely from the sensors or located with the sensors, as in the case of a circuit breaker that has an integral ground fault unit. When the level of the ground fault current exceeds the preset relay pickup setting and persists longer than its time delay setting, the ground fault relay initiates a trip signal, which instructs the disconnect device to open and clear the faulted circuit. The resulting signal can either be a set of normally open contacts that close to complete a control circuit and cause the disconnect device to open or a solid-state circuit designed to operate a low-energy shunt trip in a molded case circuit breaker. NEC Paragraph 230-95(a) specifies the maximum ground fault protection setting to be 1,200 amperes and the maximum time delay to be 1 second at 3,000 amperes. A short circuit and coordination study should be used to establish actual ground fault relay settings to ensure reasonable and coordinated ground fault protection while optimizing service continuity and minimizing nuisance tripping. Nuisance tripping occurs when the upstream ground fault protection trips when a downstream device should have cleared the fault and minimized the interruption to the facility. Power loss from an oversensitive ground fault relay setting can cause safety problems that may be more serious than those caused by low-level ground fault damage. Additionally, NEMA Publication PB 2.2 entitled Application Guide for Ground Fault Protective Devices for Equipment recommends that during construction, when equipment is being installed and accidents are likely, minimum settings should be used to maximize ground fault sensitivity. The NEC has no requirements regarding multiple levels of ground fault protection or the coordination of ground fault protection for any facility other than health care. NEC Paragraph 517-17(a) requires an additional level of ground fault protection in health care facilities whenever ground fault protection is provided on services or feeders to improve selectivity and avoid the unnecessary interruption of loads on unfaulted feeders. NEC Paragraph 517-17(b) requires full selectivity between main and feeder ground fault protection and provides minimum setting requirements for coordination. The disconnect device receives the signal from the ground fault relay, it disconnects the ground fault from its supply by deenergizing the service or feeder supplying it. The disconnect device can be a molded case circuit breaker, power circuit breaker, bolted pressure (Pringle) switch, or other suitable disconnecting device with a shunt trip mechanism. A test panel is not mandatory. It typically provides an easy method of testing the ground fault protection system and also provides system status indicators. As noted in NFPA 70B, Paragraph 12-3.5.1, if the ground fault protector is provided with a test panel, then a formal program for periodic testing should be developed and followed. If there is no test panel, then ask the manufacturer for test instructions. Once you know the parts of the ground protection system and the NEC requirements governing its use, then you can test the system. NEC Article 230-95 requires performance testing upon installation. The NEC requires this test to be performed in accordance with manufacturer recommendations and that a written record of the test be made available to the local authority having jurisdiction. While not mandatory, test panels are an easy method of testing and providing system status indicators. If none is included with the system, however, ask the manufacturer for test instructions. Manufacturers recommend visually inspecting the ground fault protection system to ensure that everything was installed and connected properly, mechanically inspecting all connections for tightness, measuring neutral insulation resistance to ensure that no shunt ground path exists, and using a simulated fault current to test both operation and ground fault relay settings. UL 1053 entitled Ground-Fault Sensing and Relaying Equipment requires that manufacturers provide information sheets describing system testing instructions. As a minimum, UL requires the following performance testing for manufacturers’ test requirements: * Have a qualified individual inspect the ground fault protection system to ensure that it was installed correctly in accordance with manufacturer’s recommendations. - Verify that the location of sensors and the polarity of their connections are correct. * Identify system grounding points to make sure that no ground paths exist that would bypass the sensors. * Test the ground fault protection system using either a simulated or actual controlled ground fault to determine that the system settings are correct and that the system is operating as intended. * Record the results of the performance testing on the manufacturer-provided test form. In addition, NEMA Publication PB 2.2 requires the following for performance testing: * Manufacturer’s installation and instruction literature should be reviewed and understood prior to performance testing. * Performance testing should follow manufacturer’s recommendations. * Performance testing should be limited to those tests that determine that the ground fault system has been installed correctly and is operational. First test If an existing facility hasn’t had a comprehensive testing program including regular testing, it should be recommended that the first test of the ground fault system be performed in accordance with manufacturer recommendations for performance testing. This is true whether or not a record of the initial performance test exists or not. Over time, power distribution systems age and get modified and changed, which may impact the operation or settings of the ground fault protection system. In addition, parts of the ground fault protection system may have been disconnected, rewired improperly, or relay settings changed during maintenance and repair work. Doing a full-performance test on the ground fault protection system will ensure that it is still installed and operating properly. Ongoing testing The NEC does not explicitly require ongoing testing of ground fault protection systems. However, NEC Paragraph 110-3(b) does require listed and labeled equipment to be used in accordance with any instructions included in the listing or labeling. Therefore, any ongoing manufacturer requirements for inspection and testing of ground fault protection systems should be followed, to comply with the NEC. Most recommend testing the ground fault protection system monthly using the test button. In addition, ground fault protection systems should be tested after the disconnect device has experienced a fault of any kind. This testing can usually be accomplished without power interruption by using the test panel supplied with the ground fault protection system. However, it is best to periodically schedule a full test on the ground fault protection system, which involves the operation of the trip mechanism and the interruption of power. Keep a log of all testing, describing each test, when it was performed, the results, and any corrective actions taken. Maintaining ground fault protection Ground fault protectors are very reliable if installed properly and require little ongoing maintenance. However, NFPA 70B recommends that current transformers be inspected and cleaned on a regular basis, depending on their environment. In addition, terminal connections should be checked for tightness and cleanliness. Similarly, maintenance on the mechanical operating mechanism of the disconnect device and electronic trip units is recommended to be carried out in accordance with manufacturer recommendations. Much of this can be accomplished during the customer’s normally scheduled maintenance on switchgear, unit substations, switchboards, and other distribution equipment that includes ground fault protection. Acknowledgement This article is the result of ongoing research into the development of service contracting business by electrical contracting firms sponsored by the Electrical Contracting Foundation, Inc. The author would like to thank the foundation for its continuing support. Dr. GLAVINICH is Chair and Associate Professor of Architectural Engineering at The University of Kansas. He can be reached at (785) 864-3435 or [email protected].