Ground-fault protection of equipment (GFPE) provides protection from devastating arcing events and destructive arcing burn-downs. National Electrical Code (NEC) sections 215.10, 230.95, 240.13 and 517.17 provide GFPE requirements, and Article 100 defines it.

Electric arcs generate significant amounts of heat. At phase-to-ground voltages above 150 volts (V), such as 277V, 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, reducing the fault current seen by a standard overcurrent device and increasing the time the fault can exist. This enables 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 bus bars within the equipment. This condition often escalates from a phase-to-ground fault event to a phase-to-phase short-circuit condition.

This is why the NEC requires GFPE. Protection is generally required for solidly grounded wye services and feeders of more than 150V to ground but not exceeding 1,000V 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. GFPE is not permitted for fire pumps or in systems where a nonorderly shutdown or interruption would introduce additional hazards.

Types of GFP equipment

Two types of GFP equipment are ground-strap type and zero-sequence. The main bonding jumper (MBJ) is what is meant by the terms “ground strap” or “neutral ground strap.” The ground-strap type of GFP equipment monitors the MBJ for high inrush current triggered by a ground-fault event. Both types provide protection from load-side ground faults. GFPE installed in a service does not provide protection on the line side of the GFP equipment. A line-side ground fault is not detected by the GFP sensors, and equipment can be severely damaged or destroyed by the ground-fault event.

A zero-sequence GFP equipment is often referred to as residual because this type of equipment sums up all of the current in the phases and grounded conductor, and any excess current is residual or left over. The GFP equipment operates when the amount of residual current exceeds predetermined set values. Like the ground-strap type, this type of system is equipped with a control relay, one or more sensing current transformers, and a shunt-trip breaker or disconnecting means. 

GFPE system coordination

Where overcurrent and GFP devices 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 GFP device, rather than causing the entire system to suffer a failure. The point of a ground fault in any system is never known, so system coordination is all about anticipating a ground-fault event and strategically locating GFP equipment at each level desired in the system.

The NEC defines selective coordination, and applying GFP equipment in multiple levels of feeders on the load side of the service GFP device can effectively localize ground faults and simultaneously provide power continuity. A coordinated system of ground-fault protection in cascading feeder levels affords the ability to isolate an offending fault to one location or feeder level. The coordination often incorporates zone-selective interlocks or differential relay settings. 

Zone-selective interlocking involves installing signaling circuit wiring between each level of GFP equipment. Where necessary or desired, the tripping function of the service GFP device can be delayed while the feeder GFP closest to the offending fault can be activated. GFP equipment coordination involves analyzing the system characteristics and applying appropriately selected GFP equipment in a way that effectively provides a deliberate separation of the tripping bands from the lowest-rated to the highest-rated GFP system device.

Testing GFPE

Equipment ground fault protection systems must be performance-tested by qualified people when installed to ensure proper operation. The test must involve a primary current injection process. The requirements for qualified people and primary current injection are new in the 2017 NEC. Testing verifies the system will interrupt a ground-fault event at selected current pickup and time settings. 

The performance testing must be in accordance with the manufacturer’s instructions. Section 230.75 requires a neutral disconnecting means to be provided within the service equipment for testing purposes. Although the term “neutral disconnect link” is not used in the NEC, it is used by manufacturers of equipment that is suitable for use as service equipment. Once the link is removed, a test can verify that the neutral is isolated from grounding connections on the load side of the service disconnect. Complete test records must be available to the authority having jurisdiction.