Ground Fault Circuit Interrupter (GFCI) is a safety device that cuts power within a fraction of a second when it detects current leaking to ground, protecting workers from fatal electric shock.
A ground fault circuit interrupter (GFCI) is a fast-acting device that cuts power when it senses current leaking out of a circuit along an unintended path to ground, such as through a person's body. It is designed to protect people from electrocution, not to protect wiring or equipment.
Shock deaths on job sites often follow the same pattern: a damaged extension cord, a wet floor, a drill with a cracked housing, and a worker who becomes the path to ground. A GFCI interrupts that circuit within a fraction of a second, usually before the current reaches a level that stops the heart. It does not prevent the shock entirely. The person still feels it. What it prevents is the sustained current that kills.
In a healthy circuit, the current flowing out on the hot conductor equals the current returning on the neutral. The GFCI runs both conductors through a sensing coil and watches for a difference. If some current goes missing, it has found another way home, usually through the equipment grounding conductor, water, or a human body. When the imbalance passes the trip threshold, the device opens the circuit.
Personnel-protection GFCIs in North America are Class A devices listed to UL 943. A Class A GFCI must trip when ground current reaches 6 milliamps and must not trip below 4 mA. That band sits below the level at which most adults lose the ability to let go of an energized conductor. Industrial equipment sometimes uses special-purpose GFCIs with higher thresholds, listed under UL 943C, which are not interchangeable with Class A devices for general receptacle use.
Under 29 CFR 1926.404(b)(1), employers must protect workers from ground faults in one of two ways. The first is GFCIs on all 120-volt, single-phase, 15- and 20-ampere receptacle outlets that are not part of the building's permanent wiring and are in use by employees. The second is an assured equipment grounding conductor program (AEGCP).
An AEGCP replaces GFCIs with a documented testing regime for cord sets, receptacles and cord-and-plug equipment. The program needs a written procedure and at least one competent person to run it. Each item is tested for continuity and correct grounding before first use, after any repair, after any incident suggesting damage, and at intervals of no more than three months. Fixed cord sets and receptacles not exposed to damage may go six months. Tests must be recorded, often with colored tape by quarter. Many contractors find GFCIs simpler than keeping AEGCP records current, and some use both.
In general industry, OSHA's 1910.304(b)(3)(ii) applies similar protection to temporary receptacles used during construction-like work such as maintenance, remodeling or repairs. The National Electrical Code, Section 210.8, lists permanent locations needing GFCI protection, such as bathrooms, kitchens, outdoor outlets, garages, basements and areas near sinks. The list has expanded in recent editions, so check which NEC edition your jurisdiction has adopted. In Australia and the UK, the equivalent device is the residual current device (RCD), with its own testing rules.
A GFCI is a narrow tool. Workers who think it makes a circuit "safe" can get hurt in ways it was never designed to stop.
Put simply: a standard breaker protects the wire from overheating, an AFCI protects the building from arcing fires, and a GFCI protects the person from shock. Combination devices offer more than one function, but each function still has the limits above. For the wider set of shock, arc flash and burn risks, see electrical hazard.
A drywall contractor on a mid-rise residential build ran temporary power from a spider box on each floor. Rain had leaked through an unfinished window opening, pooling water on the slab. A laborer dragging a cord through the puddle grabbed a plug whose insulation had been crushed under a pallet jack. The portable GFCI at the spider box tripped immediately. He felt a jolt and dropped the cord, uninjured.
The incident review found that the floor below was running power through a plain extension cord plugged into a permanent building receptacle with no GFCI, because "it was the closest outlet." The contractor bought GFCI cord sets for every crew, added a pre-shift test of each GFCI to the foreman's checklist, and tagged out and removed crushed cords instead of taping them.
Press the test button and confirm the device trips, then reset it. Manufacturers generally recommend monthly testing for installed GFCIs. On construction sites and for portable units, test before each use or at the start of each shift, because cords and devices take a beating. A GFCI that will not trip, or will not reset, should be taken out of service and replaced.
Nuisance trips usually point to a real problem: moisture in a connection, a damaged cord, a tool with deteriorated insulation, or several tools whose small normal leakage adds up. Long cord runs can also contribute. Isolate the cause by plugging in one item at a time. Never bypass the GFCI to keep working. A tripping device is doing its job.
Yes. A GFCI detects imbalance between hot and neutral, so it can trip without an equipment grounding conductor. That is why the NEC allows GFCIs as a replacement option for old two-prong receptacles. The outlet must be marked "No Equipment Ground," and it still lacks the fault path a grounding conductor provides to clear faults on metal equipment.
Yes, and on construction sites it is often needed. Many portable generators have built-in GFCI receptacles. OSHA's 1926.404(b)(1) has a narrow exception for some small generators meeting specific conditions. If you are unsure whether yours qualifies, use a portable GFCI or GFCI cord set at the generator, which removes the question.
Part of SafetyIQ's EHS glossary: plain-English definitions of workplace health and safety terms.
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