Ground-Fault Protection Explained

Ground-fault protection of equipment (GFP or GFPE) detects current flowing to ground and opens the circuit before an arcing fault can destroy the equipment. It isn't a people-protection device. It's set in hundreds of amps, not milliamps, and its job is to stop a low-level arcing fault that ordinary overcurrent protection can't see.

Why overcurrent protection isn't enough

A bolted fault draws enormous current and trips a breaker almost instantly. An arcing ground fault is different: the arc itself has voltage across it, and the fault path has impedance, so the current can be a small fraction of the bolted value. On a 480Y/277 V system there's enough voltage to sustain that arc.

Take a 3,000 A service main and an arcing ground fault drawing 2,000 A. That's below the breaker's rating, so its phase overcurrent protection will never trip. The arc keeps burning, destroying the switchgear, until something else gives out. A GFP relay set at 1,200 A sees the 2,000 A going to ground and opens the breaker within its time delay.

Lower-voltage systems such as 208Y/120 V are less prone to sustained arcing faults, which is why the NEC requirement applies above 150 V to ground.

What NEC 230.95 requires

• Where: solidly grounded wye services with more than 150 V to ground, for each service disconnect rated 1,000 A or more. In practice, that means 480Y/277 V services. Recent editions extend the voltage range up to 1,000 V phase-to-phase; earlier ones stopped at 600 V.
• Settings: a maximum pickup of 1,200 A, and a maximum time delay of one second for ground-fault currents of 3,000 A or more.
• Testing: the system must be performance-tested when first installed on site, following the manufacturer's instructions, with a written record kept.
• Exceptions: fire pumps are excluded, so that a ground fault doesn't shut down fire protection. So are continuous industrial processes where a non-orderly shutdown would add hazards.

Similar requirements apply to large feeder disconnects (NEC 215.10) and building disconnects (240.13) where there's no GFP upstream. Health care facilities need an additional level of ground-fault protection on feeders, so that a downstream fault doesn't take out the whole service.

Emergency systems work differently. The NEC doesn't require equipment GFP on an emergency system's alternate source, because tripping the generator on a ground fault defeats the point of having it. Instead, it requires ground-fault indication, so the fault is alarmed and can be found.

How GFP senses a ground fault

The neutral CT is the part most often wired wrong. If it's missing, reversed or on the wrong side of the neutral-to-ground bond, normal unbalanced neutral current looks like a ground fault and the system trips on load. Or, just as bad, a real fault is missed.

Coordination: why GFP on the main alone is a problem

GFP is required at the service. If that's the only level of ground-fault protection, the main is the only device that can clear a low-level ground fault anywhere downstream. A fault on one branch circuit takes down the entire service. Selectivity needs GFP on the feeders as well, with settings and time delays graded so the device nearest the fault trips first. The coordination study should include the ground-fault curves, not just the phase curves.

GFP is not GFCI

A ground-fault circuit interrupter (GFCI) protects people. It trips at around 5 mA, and UL 943 requires Class A devices to trip at 6 mA or less, far below the level that could stop a heart. Equipment GFP protects equipment, with pickups in the hundreds of amps. One can't substitute for the other.

Data center considerations

• Multiple sources are normal. Utility, generators and tie breakers mean the neutral and bonding arrangement has to be designed for ground-fault sensing from the start, not fixed at commissioning.
• Separately derived systems reset the rules. Transformers and some UPS configurations create new grounded systems downstream, each with its own bonding point, which the grounding and bonding guide covers.
• Nuisance trips are expensive. A GFP trip on a service main is a site-wide event. Performance testing and a documented ground-fault coordination study are cheap by comparison.

Common mistakes

• Relying on phase overcurrent protection for arcing ground faults. They can burn below the breaker rating.
• Installing GFP on the main only. Any downstream ground fault then trips the whole service.
• Mis-wiring the neutral CT, or placing it on the wrong side of the bond.
• Using a simple residual scheme on a multi-source system. It needs a scheme designed for the neutral paths.
• Skipping the on-site performance test. It's required, and it's where wiring errors are found.

Frequently asked questions