Unlike traditional circuit breakers and fuses, which only open the circuit when the "hot" current exceeds a fixed value for a fixed time, a GFCI device will interrupt electrical service when more than 4 to 6 milliamperes of current in either conductor is leaked to ground (either directly or through a resistance, such as a person). A GFCI detects an imbalance between the current in the "hot" side and the current in the "neutral" side. Most receptacle outlets with GFCI have the added advantage of protecting other receptacles 'downstream' of them, so that one GFCI receptacle can serve as protection for several conventional receptacles, whether or not they are grounding-type receptacles. GFCI devices come in many configurations including circuit-breakers, portable devices and receptacles.
A GFCI receptacle typically has a pair of small push buttons between its two receptacles: one labeled 'test' and the other 'reset' (or T and R). Pressing 'test' will place a small imbalance in the line sensor, which will trip the device, resulting in an audible "snap". Pressing 'reset' will allow the socket to function normally after a test, or after a faulty appliance has been removed from the circuit or insulated from ground. If a GFCI receptacle fails to trip when the test button is pushed (and the GFCI had been previously armed by first pressing in the reset button), it means the GFCI receptacle must be replaced because it is no longer providing protection against ground faults.
Like fuses and circuit breakers, a GFCI receptacle has a finite number of uses. It must be replaced when a test fails to trip the device.
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