How a GFCI protects a person from electric shock

Electric current normally flows out through a hot conductor and returns through a neutral conductor. If some current escapes that intended path—for example, through damaged insulation, wet equipment or a person touching an energized part—the difference can indicate a ground fault. A GFCI is designed to detect that imbalance and shut off power quickly.

How the device detects a fault

A GFCI monitors the outgoing and returning current. Under ordinary operation, those amounts should be essentially equal. A leakage current creates an imbalance. When it reaches the device's trip threshold, the GFCI opens the circuit and interrupts power. OSHA describes construction GFCIs as fast-acting devices that can shut off power within a fraction of a second.

What it protects against—and what it does not

A GFCI reduces the likelihood and severity of shock by limiting how long current can continue through an unintended path. It does not make contact with electricity safe, replace safe work practices, or guarantee that every shock is prevented. A GFCI also is not the same as an overload breaker: an overload device protects wiring from excessive current, while a GFCI responds to current escaping its intended circuit path.

Construction-site requirement

OSHA's construction standard requires employers to use either GFCIs under §1926.404(b)(1)(ii) or an assured equipment grounding conductor program under §1926.404(b)(1)(iii) to protect employees on construction sites. The GFCI option covers specified 120-volt, single-phase, 15- and 20-ampere receptacle outlets that are not part of permanent building wiring and are in use by employees, subject to the standard's stated exception.

Test and reset controls

Portable GFCIs and protected outlets commonly have test and reset buttons. Workers should follow the manufacturer's instructions and site procedures to test equipment. If a device trips, do not repeatedly reset it without finding the cause. Remove damaged equipment from service and have a qualified person investigate persistent trips, moisture or wiring defects.

GFCI or assured grounding program

The construction rule provides two compliance methods. An employer may use required GFCIs or establish and implement the specified assured equipment grounding conductor program. The program has written procedures, inspections and continuity testing requirements. The alternatives protect against different electrical hazards through different controls; an employer cannot simply ignore ground-fault protection because a receptacle appears grounded.

Exam traps

• A GFCI does not prevent all electrical contact; it interrupts power after detecting a fault.
• It is not interchangeable with ordinary overcurrent protection.
• Construction employers must choose an allowed protection method for the covered hazards.
• The construction GFCI requirement describes specific outlet ratings and locations; do not apply it to every outlet without checking scope.
• A tripping device indicates a condition to investigate, not a nuisance to bypass.

Key takeaway

A GFCI detects a difference between outgoing and returning current and rapidly interrupts the circuit. On construction sites, OSHA permits GFCIs or a compliant assured equipment-grounding-conductor program for required ground-fault protection.