Ground-fault protection of equipment (GFPE) protects large equipment from devastating arcing events and destructive burn-downs. GFPE requirements are provided in sections 210.13, 215.10, 230.95, 240.13 and 517.17, to name a few.
Electric arcs generate significant amounts of heat and, in a circuit of 277 volts (V) to ground, an arcing fault is readily sustained. A ground fault is typically not a solid or “bolted fault” condition, so dynamic arcing impedance is introduced in the circuit. This reduces the fault current seen by a standard overcurrent device and could increase the time the fault can exist, which allows arcing faults to manifest into destructive events. During an arc event, ionized gas is dispersed, creating a conductive gas or plasma in the atmosphere surrounding the busbars within the equipment. This condition often rapidly escalates from a phase-to-ground fault event to a phase-to-phase fault condition.
GFPE is generally required for solidly grounded wye services and feeders of more than 150V to ground but not exceeding 600V phase-to-phase for each disconnect rated at or above 1,000 amperes (A). GFPE is required for nominal 480Y/277V, three-phase, 4-wire, wye-connected systems. The maximum settings are 1,200A and not longer than 1 second for fault currents 3,000A or more.
As indicated in Section 210.13, GFPE is also required for large branch circuits of more than 150V to ground but not exceeding 1,000V phase-to-phase for each disconnect rated at or above 1,000A. Note that GFPE is not permitted for fire pumps or in systems where a non-orderly shutdown or interruption would introduce additional hazards. GFPE is optional for emergency and legally required standby systems.
It is important to understand that ground-fault protection (GFP) installed in service equipment provides protection only on the load side of the GFP. It does not provide protection on the line side of the GFP system in the equipment. A line-side ground-fault event is not detected by the GFP sensors and equipment can be severely damaged or destroyed by a line-side ground-fault event.
Coordinating the trip sequence of overcurrent and GFPE devices in power distributions systems is often necessary for system reliability and where selective coordination is required by the NEC . Selective coordination can be accomplished by various combinations of overcurrent and GFPE devices that are carefully applied and engineered into the power distribution system. Selective coordination can be achieved using circuit breakers, fuses or combinations of fuses and circuit breakers.
Where GFPE systems are installed, they should also be designed such that they are selective to the point where the offending ground-fault event opens only the closest upstream device from the fault event. The term “selective coordination” is defined in Article 100 of the NEC . Selective coordination of overcurrent devices in emergency systems is specified in Section 700.28, legally required standby systems in Section 701.27, and critical operations power systems in Section 708.54. Where overcurrent protective devices and GFPE are selectively coordinated, they provide the benefits of restricting outages to the circuit or equipment closest to the ground-fault or short-circuit event by operating the local overcurrent or GFPE device, rather than causing extended cascading outages.
The point at which a ground fault can occur in any system is never predictable or known, maximum protection and system coordination is about anticipating a ground-fault event and strategically leveraging GFPE or overcurrent devices that can effectively clear ground-faults close to equipment most likely to experience a ground-fault. Applying GFPE in multiple levels of feeders on the load side of the service GFP device can effectively localize ground faults and simultaneously provide power continuity. This is a requirement in Section 517.17 for healthcare facilities. A coordinated system of GFP in cascading feeder levels affords the ability to isolate an offending fault to one location or feeder level. GFPE can provide faster response times for currents, whereas standard overcurrent protective devices (OCPD) can respond to any overcurrent including ground faults with a magnitude high enough to be within the OCPDs’ trip curve characteristics. Ground currents can be high enough to trip a standard OCPD due to effective equipment grounding methods providing effective equipment protection.
New forms of arc-energy reduction technology can protect against arcing burndowns. It would appear that the NEC is positioned to evolve and recognize new technologies that can provide equivalent equipment protection that has long been provided by traditional methods of ground-fault protection of equipment.
About The Author
JOHNSTON is NECA’s executive director of codes and standards. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at [email protected].