GFCI and AFCI: two devices for two different ways to get hurt

A GFCI compares the current going out on the hot with the current coming back on the neutral and opens in a fraction of a second if about 5 mA is missing, because missing current is going through something else, maybe a person; an AFCI listens for the electrical signature of an arc, the sparking at a damaged cord or loose connection that starts fires.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

A 20 A breaker will not save a person: it is thousands of times the current that can stop a heart. It protects the wire. The GFCI is the only thing in the circuit that is there for the person, which is why it is required near water. The AFCI is there for the house, in the bedrooms where a cord under a rug smoulders.

How to think about it

Ask what the hazard is. Water and people: GFCI, on the receptacle or the breaker. Living areas and fire from damaged wiring: AFCI, at the breaker. Then test it: press TEST, confirm the load is dead with a tester, press RESET, confirm it is live again. A device that does not trip on TEST is not protecting anything.

Worked example

Hot carries 6.000 A out; neutral returns 5.996 A

The GFCI measures both.

Missing: 6.000 − 5.996 = 0.004 A = 4 mA

Four milliamps are going somewhere else. Below the 5 mA trip point: it holds, and that is a marginal, wet, dangerous situation.

Missing: 6 mA

Over the trip point. It opens in about 25 thousandths of a second; the person feels a jolt and lets go.

A 15 A breaker trips at 15 A; the GFCI trips at 0.005 A

15 / 0.005 = 3000 times more sensitive. Different device, different job.

Your turn

Hot carries 10.000 A and neutral returns 9.993 A. Write the missing current in milliamps.

(10.000 − 9.993) × 1000 =  mA

The trap

Thinking a GFCI needs a ground wire to work. It compares hot and neutral; it works on a two-wire circuit with no ground at all, and that is why it is the legal way to replace an ungrounded receptacle. What it cannot do is protect against a hot-to-neutral shock, where the current comes back on the neutral as normal.

Practise GFCI and AFCI on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.