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Can a load on the line side of a GFI ever trip that GFI?

Niall C.
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John
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2 Answers2

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Although in theory a device on the line side of the GFCI will not trip the device, it can in practice.

In theory, a GFCI will only trip when it detects the current is not balanced on its load terminals. Unfortunately, the design of a GFCI makes them susceptible to interference. They can pick up electromagnetic signals, and also may also trip with wild swings of line voltage. There was a recent presentation at DEFCON that demonstrated how a (strong) radio transmission can cause the GFCI to trip, and even self-destruct.

I have personally experienced unwanted GFCI tripping from motors in my home: an exhaust fan and a garbage disposal. The GFCI tripped as the motor was turned off. In these instances, they were older motors and the circuit did not carry a safety ground (two-conductor NM cable).

Pigrew
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No. A ground-fault on the line side of a GFCI should never cause the GFCI to trip.

A GFCI uses a current transformer (CT) to measure for current differences on the ungrounded (hot) conductor, and the grounded (neutral) conductor. In a normally functioning circuit, the CT will always read 0 amperes (or close to it). It is only when the CT reads above a specific tolerance, that the GFCI will open the circuit.

If there's a ground-fault in the line side wiring, it will not cause a current difference through the CT. So it should not cause the GFCI to trip.

Tester101
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  • He's asking about a "load" on the line side. I take it you are assuming he means a ground fault. – Speedy Petey Nov 16 '14 at 02:14
  • I'm assuming a "load" is a device, and they're asking about a ground-fault in; or caused by, the device. Maybe they're asking if an overload on the line side could cause a GFCI to trip. Since GFCI devices don't react to overloads, I'm assuming that's not what they're asking. – Tester101 Nov 16 '14 at 04:14
  • And this is why I asked for clarification at the beginning. The question as asked is simply too vague. – Speedy Petey Nov 16 '14 at 13:12
  • I figured explaining how a GFCI works, would be good enough to answer the OP's question – Tester101 Nov 16 '14 at 13:15
  • In many clock radios they wrap a wire around the incoming cord as an antenna, the coil in a GFCI should be a much better antenna. Which means that EMI could cause it to trip. – Brad Gilbert Nov 17 '14 at 00:41