The National Electrical Code has been incrementally expanding some performance testing requirements for several years. Performance testing is required to confirm equipment functionality and verify it will function as intended to provide a degree of protection for equipment and property. The Code’s purpose is the “protection of persons and property” and refers to protecting the electrical equipment and the facility.
Ground-fault protection of equipment (GFPE) differs from ground-fault circuit interrupter (GFCI) protection, as it protects larger equipment. GFPE requirements are provided in NEC rules such as 210.13, 215.10, 230.95, 240.13 and 517.17. The NEC defines “ground-fault protection of equipment” in Article 100.
Electric arcs generate significant amounts of heat, and in a circuit of 277V 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 faulted circuit. This reduces the fault current seen by a standard overcurrent device and increases 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 short-circuit condition. This is why 230.95 requires equipment ground-fault protection. GFPE 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 1,000A.
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 of 3,000A or more. As indicated in 210.13, GFPE is also required for large branch circuits of more than 150V to ground but not exceeding 600V phase-to-phase for each disconnect rated at or above 1,000A. It is important to note that GFPE is not permitted for fire pumps or in systems where a nonorderly shutdown or interruption would introduce additional hazards.
Performance testing of GFPE verifies that the system will interrupt a ground-fault event at selected current pickup and time settings.
Types of GFP equipment
Two popular types of GFP equipment are ground-strap type and zero-sequence (residual). The main bonding jumper is implied by the term “ground strap or neutral ground strap.” Both types provide protection from load-side ground faults. Ground-fault protection installed in service equipment 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 the ground-fault event.
Section 215.10 requires GFPE for feeders of the same voltage and current ratings that qualify services for GFPE. The exceptions provided in 230.95 for services are also in 215.10 for feeders.
Each separately derived system with feeders at voltages and configurations requiring GFPE must be protected if it qualifies because of the ampacity level of the feeder and equipment supplied. Remember that GFPE is generally required for each disconnecting means rated at 1,000A or greater installed on 480Y/277V systems.
Branch circuit GFPE
Section 210.13 requires equipment ground-fault protection for branch circuits of the same voltage and current ratings that qualify services and feeders for GFP equipment. 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. An example of a large branch circuit is a single unit of industrial utilization equipment such as a furnace or motor. Branch circuits are the conductors from the final overcurrent device to the outlet where the equipment is connected.
GFPE must be performance tested when first installed on-site to ensure proper operation. The Code requires the performance testing to be done by qualified persons.
Performance testing of GFPE verifies that 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 and include primary current injection. The test records must be made available to the authority having jurisdiction.
As an aside, recent editions of the NEC contain new requirements for arc energy reduction such as in sections 240.67 for fuses and 240.87 for circuit breakers. Performance testing by qualified persons is also required when first installed on-site.
About The Author
Michael JohnstonNECA Executive Director of Codes and Standards
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]