A GFCI is the outlet with the little TEST and RESET buttons, or the breaker version of the same thing in your panel. It is the one protective device in a house designed to save a person rather than the wiring, and the distinction is worth understanding because it explains why an ordinary breaker will not do the job.
What it actually measures
A GFCI watches the current going out on the hot conductor and the current coming back on the neutral. In a healthy circuit those match. If they do not, the difference has left the circuit somewhere, and the most common route out is through something that touched a live part.
When it sees an imbalance of a few milliamps it opens the circuit in a fraction of a second. It does not need a ground wire to do this, which is why a GFCI can be installed on an ungrounded circuit in an old house and still provide shock protection.
Why a normal breaker is not enough
A 15A breaker protects the wire in the wall. It trips when the current gets high enough to overheat that wire, and that threshold is enormous compared to what harms a person.
The current that can stop a heart is measured in milliamps, thousandths of an amp. A fault sending that through a person draws nothing a 15A breaker would notice. The breaker will sit there, entirely satisfied, throughout. That gap is the reason GFCIs exist as a separate device rather than a better breaker.
Outlet or breaker
The receptacle version protects itself and, when wired that way, everything downstream of it on the circuit — which is why one GFCI outlet near a sink often covers several ordinary outlets past it.
The breaker version lives in the panel and protects the whole circuit from the first inch. It costs more and it means the reset is in the basement rather than on the wall, which is a real consideration for a circuit that trips occasionally for benign reasons.
They wear out
The sensing electronics degrade, and a GFCI that has been in a damp garage for fifteen years may no longer trip when it should. It will still pass power and look completely normal, because failing to trip is a silent failure.
That is what the TEST button is for, and testing it monthly is the single cheapest safety habit available to a homeowner. If pressing TEST does not cut the power, the device is dead and needs replacing regardless of how new it looks.
How to spot it in your house
- Look for a receptacle with two small buttons between the sockets, usually marked TEST and RESET.
- In the panel, a GFCI breaker is wider than its neighbours and has a small test button on its face, often with a white pigtail wire.
- Press TEST: the power should cut immediately and RESET should restore it. If TEST does nothing, the device has failed.
- A dead outlet with no obvious cause is often downstream of a tripped GFCI somewhere else — check the bathrooms, garage and exterior outlets before assuming a breaker.
The next question
Where are GFCIs required in Canadian homes?Related terms
- AFCIA breaker that listens for the electrical signature of arcing, a loose or damaged connection sparking inside a wall, and cuts the circuit before it starts a fire.
- BreakerA switch in your panel that cuts a single circuit when it draws more current than the wire behind it can safely carry.
- Grounding and bondingThe system that gives stray current a safe path back instead of through you: grounding ties the system to the earth, bonding ties metal parts together so they sit at the same voltage.