During a recent National Electrical Code (NEC) training program, inquiries about ground-fault protection of equipment (GFPE) were raised. Two questions were: Why is ground-fault protection required for large equipment rated at 1,000 volts (V) or less, and what are the requirements for GFPE when the equipment is installed in healthcare distribution systems? The third question related to required performance testing for installed GFPE. This article addresses each of these questions.
First, let’s look at the reason GFPE is required. It protects large equipment from devastating arcing events and destructive burn-downs. If a phase-to-ground voltage is 120V, GFPE is generally not an NEC requirement. In a circuit of 277V to ground, electric arcs generate significant amounts of heat and 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 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 an initial phase-to-ground fault event to a phase-to-phase short-circuit condition. This is why the NEC requires GFPE. Sections 210.13, 215.10, 230.95, 240.13, 517.17 and others provide GFPE requirements.
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. 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. Note that GFPE is not permitted for fire pumps or in systems where a nonorderly shutdown or interruption would introduce additional hazards.
GFPE in healthcare facilities
The second questioner sought input on GFPE rules for healthcare facilities, such as hospitals. Special rules for GFPE of electrical systems apply to these facilities. Section 517.17(B) indicates, when GFPE is provided at the service or feeder disconnecting means, as specified by Section 230.95 or 215.10, an additional level of GFPE is required in the next level of feeder-disconnecting means downstream toward the load. The primary purpose for this is to ensure that a ground-fault event in the electrical system does not open the service GFPE disconnecting means but opens the feeder device closest to the fault, thus isolating the offending circuit while maintaining continuity of power to the rest of the healthcare facility. Section 517.17 clarifies that, regardless of whether the healthcare facility is supplied by a service or feeder, where GFPE is provided at the service or feeder disconnecting means, a second level of GFPE is required downstream. This requirement helps ensure that, when there is a ground fault at the feeder level, only the GFPE in the feeder will open, and the upstream feeder and/or service GFPE devices will remain closed.
Performance testing of GFPE
The third question dealt with the performance-testing requirements for GFPE; each level of the system must be tested, when installed, in accordance with the manufacturer’s instructions. Specific procedures depend on the type of GFPE. Testing GFPE verifies that the system will interrupt ground-fault events at predetermined and coordinated current pickup and time settings.
Section 230.75 provides the requirement for a means to disconnect the grounded conductor (usually a neutral) within the service equipment. This is for testing purposes. Although the term “neutral disconnect” link is not used in the NEC, this term is used by manufacturers of equipment that is suitable for use as service equipment. It is critical to GFPE system-performance testing that the grounded (neutral) conductor be completely isolated from any grounding connections downstream from the service. This is a general requirement in NEC Section 250.24(A)(5). Removal of neutral disconnect link is a vital element of the performance testing and is a specific requirement of the manufacturer’s installation and testing instructions for the GFPE. This is performed before the system is energized. The link can be in the form of a busbar or, in smaller equipment, can be the terminal to which the neutral conductor is connected. Once the link is removed, a test can verify that the neural is isolated from grounding connections on the load side of the service disconnect.
Once the test is completed, the testing organization typically field-labels the equipment with the results and date listed. The test records must be made available to the authority having jurisdiction.
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].