GFCI: what it protects, and where you need one

What it is, and what it isn’t

GFCI, ground fault circuit interrupter, the outlet with “test” and “reset” buttons: different words for the same thing. It’s also the most common electrical safety device in a home, and the most misunderstood.

An ordinary breaker watches how much current flows. When the circuit draws more than its capacity, it trips. It protects the wire from overheating, so it protects the house from fire.

A GFCI watches something else entirely: it compares the current going out with the current coming back. If the difference exceeds a tiny threshold, current is escaping somewhere: through a faulty appliance, through water, or through a person. It cuts the power in a fraction of a second.

The Code states that a GFCI is a Class A device, meaning it acts on a leakage current whose threshold doesn’t exceed 6 milliamperes, at the branch circuit level. Six thousandths of an ampere: below what injures.

The practical consequence: you need both, and neither replaces the other. A circuit can have a perfect breaker and kill someone; a circuit can have a perfect GFCI and overheat a wire.

How it works

A GFCI constantly monitors the current flowing through the circuit. It compares the current going in with the current coming back: if the two differ by as little as 0.006 amperes, it cuts the power faster than a blink of an eye, according to the U.S. Consumer Product Safety Commission. It’s designed to act before the current can affect the heartbeat.

What it looks for is a ground fault: an unintended path between the power source and a grounded surface. The classic example: a bare wire inside an appliance touches its metal case, and the case becomes live. If you touch the appliance while another part of your body touches a grounded metal object, such as a faucet, the current goes through you; if the appliance is plugged into an outlet protected by a GFCI, the power is cut before a fatal shock can occur.

Because it detects ground faults, it can also prevent some electrical fires. A GFCI breaker, for its part, protects against both a ground fault and an overload.

The same source also recommends testing it after a power failure: outages and other power disturbances can sometimes keep it from working properly.

Where you need one

The logic is consistent: wherever electricity and water can meet, and everywhere outdoors. Bathrooms, kitchen counters, outdoors, garage, unfinished basement, around a pool or a hot tub.

A specific example, confirmed in the edition in force: a 120-volt CSA 5-20R receptacle installed outdoors, less than 2.5 m above the ground, must be GFCI protected.

What to take from this, in practice: don’t rely on a list to know whether your installation complies. That’s the master electrician’s job, with the edition in force in front of them, and it costs nothing to ask for it in a quote.

How to test it

It’s the only safety device in your home you can check in ten seconds, without a tool. And almost nobody does.

1. Press the TEST button. The device should cut the power immediately: the outlet goes dead, or the breaker moves to its middle position.
2. Press RESET to restore power.
3. Do it again every month, as manufacturers recommend. The first of the month is a handy reminder.

If nothing happens when you press TEST, the device is dead. The outlet keeps supplying power; it simply no longer offers any protection. That’s a replacement, not a repair.

When it trips for no obvious reason

“For no reason” is almost always wrong. A GFCI that trips has detected something. The causes, from most to least common:

• Moisture in an outdoor outlet. The number one cause in spring and during storms. A box whose seal has aged lets water in, and the GFCI does exactly its job.
• An appliance that leaks current. An old freezer in the basement, a pool pump, a workshop tool. To find out: unplug everything on the circuit, reset, then plug the appliances back in one at a time.
• Too many appliances on the same protected circuit. Every appliance has a tiny, perfectly normal leakage current. Added together, these leaks can cross the threshold.
• The device itself at the end of its life. It happens, especially after fifteen or twenty years outdoors.

Adding one to an existing home

As long as nobody touches it, an installation that met the rules when it was built remains acceptable. So many Québec homes have no GFCI anywhere.

Adding them is one of the most worthwhile electrical jobs there is, because the cost bears no comparison to the risk it removes. Two ways to do it:

The right choice depends on how your circuits are wired, and that shows when the panel is opened, not over the phone. If your panel is already full, it’s also the time to look at the bigger question: see our guide on replacing an electrical panel.

Frequently asked questions