What is a GFCI or GFI?
The GFCI, commonly shortened to Ground Fault Interrupter (GFI), is an anti-shock device. It is most often seen in the form of a receptacle, also know as an outlet, with two pushbuttons on it marked TEST and RESET. It can also be built into a circuit breaker and installed directly in the electrical panel. The electrical code today requires that any outlet near a kitchen sink or bathroom fixture be protected by a GFI. They are also required on the exterior, in the garage and in an unfinished basement. A single GFI can protect just itself, or can it also protect other outlets downstream from it.
Why do you need a GFI? The regular circuit breakers in the electrical panel are designed to protect your wiring from overheating and burning. The smallest breakers normally found in a breaker panel are rated at 15 amps (Amperes). If the current in the circuit exceeds 15 amps significantly, the breaker trips and kills the circuit. Fifteen amps, however, is thousands of times more current than is needed to electrocute a person, so a regular circuit breaker will not protect you from serious shock or electrocution. Something much more sensitive is required, so some bright person invented the ground fault interrupter. The GFI is designed to trip at only 6 milliamps (that is 6 one thousandths of one amp). That is 2,500 times more sensitive than the regular 15-amp breaker.
How does the GFI work? The GFI monitors the current flowing in the black (hot) and white (neutral) wires in the circuit. If everything is normal, the amount of current moving through both wires will be identical. This is because the very same current is flowing through the black wire, then through the load (i.e. the appliance in use), and back to ground through the white wire.
If, however, in addition to the current flowing through the load (e.g. hairdryer, etc.), some current from the black wire is also finding its way to ground through some pathway other than the load (for example, your body), then more current will flow in the black wire than in the white wire, and that is what the GFI detects and responds to.
The mechanism in the GFI that actually detects the imbalance is a small torroidal (doughnut shaped) transformer. The black and white wires pass through the hole in the center of the transformer. The alternating current in the black wire produces a fluctuating magnetic field, which induces a small voltage in the transformer windings. The current in the white wire is flowing in the opposite direction from the current in the black wire, so the magnetic field it creates
induces a voltage of opposite polarity from the black wire. The two voltages cancel each other out, so that normally, the net output from the transformer is zero.
If the current in the black wire becomes more than the current in the white wire (i.e. current is going someplace it isn’t supposed to), then the transformer produces a small signal, which is then amplified and used to trip the relay that interrupts the circuit. It is the tripping of this relay that produces the audible “click” you here when a GFI trips.
Test your GFI regularly! Pushing the test button creates a small “ground fault” on purpose, so you can find out if the GFI is working. You should push the test button monthly to be sure the unit is tripping. If it doesn’t click off when you push the test button, the GFI should be replaced immediately, because it has gone bad and is no longer protecting you from shock. Inspectors, electricians and other professional use our GFI/GFCI Circuit tester which retails for less than $8 (price verified 6/13/2010).
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