Residual-current device

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GFCI receptacle with red button for Test and black button for Reset

A ground fault circuit interrupter (GFCI), or Residual Current Device (RCD) is a circuit breaker. This device reduces the risk of electric shock. It shuts off an electric power circuit when it detects that current is flowing in a way it is not meant to, such as through water or a person. Electric shock may cause the heart to stop or cause burns. The device can also prevent some fires, for example, those caused when a live wire touches a metal conduit.

How it works[change | change source]

A GFCI device uses Kirchhoff's current law. This law states that everywhere on an electrical circuit that is in series, the current must be exactly the same as every other point on that same circuit. House wiring normally has a "hot leg" and a "neutral leg". The GFCI device watches the current "going out" on the hot leg and measures it against the current "coming back in" on the neutral. If there is a difference between the two, even as little as 4-5 milliamps, then that is considered "abnormal". The GFCI device will trip and stop all the current from flowing.

This is because a difference between the two currents (the one on going out and the one coming in) on that one circuit is usually caused by a short circuit. A short circuit is when current leaves the path it normally follows and goes a different way. This could be through water, or through a person who has come into contact with a live wire; this may send current through the human body, causing harm or death.

Death caused by electricity (Electrocution) can happen when as little as 30 milliamps of current flows through the heart for just a fraction of a second. The GFCI device protects at a level which is much lower than what is needed to cause harm.

One GFCI wall outlet can provide protection to other standard outlets without a GFCI that are wired on the same circuit after (further down from) the GFCI device.

If a GFCI device trips, and the problem is fixed, then the GFCI device is reset by pushing the reset button . If the problem is not fixed, the GFCI device keeps the circuit shut off and will not reset. There is also a test button, which will cause the GFCI to trip if it is working properly. GFCI outlets should be tested at least once a month.[1]

Types[change | change source]

GFCIs are available in two types: the circuit breaker that installs in an electrical panel, and the receptacle type that installs into an electrical box. GFCIs that attach to appliance cords, or are built in to extension cords, are also available. Newer hair dryers may have them also, appearing as a small box at the end of the power cord or on the handle itself.

Since a GFCI only monitors current on the hot leg compared to the neutral, the GFCI can be used to upgrade older two-prong (non-grounded) outlets to three-prong (grounded/earthed) outlets without installing any new wire. A circuit with a GFCI device without a ground is far safer than a two-prong outlet without a ground. A GFCI installed this way must be labeled "No equipment ground."

When a GFCI is installed in the electrical box without connecting the ground screw (as there is no ground wire), a label that says "No Equipment Ground" is placed on the GFCI outlet and all downstream outlets. Several of these labels are usually included with the GFCI. In some parts of the world, "ground" is called "earth".

Regulations[change | change source]

GFCIs are a suitable replacement for two prong outlets without a grounded wire. The National Electric Code requires GFCI protection on kitchen counter tops, in bathrooms, unfinished basements, garages, outdoor circuits and near water.

GFCI protection is required on construction sites.

References[change | change source]