I use a hairdryer with a 2-prong US plug (no ground). Suppose I plug it into a GFCI-protected outlet and drop it into a sink full of water. Will the GFCI still trip even though my hairdryer doesn't have a 3-prong (with ground) plug?

  • \$\begingroup\$ "GF" in GFCI stands for "ground fault" - where, by not having a ground connection at all, could be considered a fault. \$\endgroup\$
    – JYelton
    Jun 26, 2014 at 21:41
  • \$\begingroup\$ Curious about the downvote. I looked at the on/off-topic lists in the tour and this question seemed appropriate; please tell me if it is not. \$\endgroup\$ Jun 26, 2014 at 21:45
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    \$\begingroup\$ I think some may think this question is more suited for DIY, since it's about home mains wiring and appliances. But I find it to be on-topic since it's about how a particular circuit breaker works, despite the apparent DIY context. +1 \$\endgroup\$
    – JYelton
    Jun 26, 2014 at 21:52
  • \$\begingroup\$ Should I edit the title to not include "outlet" and "appliance"? (I didn't know the DIY site existed, and if the answer was "it's not safe" I'd buy a new hairdryer, not rewire the outlet.) \$\endgroup\$ Jun 26, 2014 at 21:56
  • \$\begingroup\$ I'd consider a title of "How does a GFCI breaker protect me if my device has no connection to ground?" or something like that. :) \$\endgroup\$
    – JYelton
    Jun 26, 2014 at 21:58

2 Answers 2


The name "Ground Fault Circuit Interrupter" is a bit misleading.

There are three wires called Line, Neutral and Safety Ground. The Safety Ground pin on a GFCI outlet is not connected to the GFCI protection circuit in the outlet.

A GFCI measures the difference in current flowing in the line and the current flowing in the neutral wires, and if that difference exceeds 5 mA (if I recall correctly) it will trip, and remove power from the outlet.

The GFCI doesn't care why there is a difference in current between Line and Neutral. The excess or missing current could be due to a connection to another circuit, not necessarily to ground.

  • \$\begingroup\$ In the UK these are called "RCD" - Residual Current Device. \$\endgroup\$
    – Majenko
    Jun 26, 2014 at 23:39

Yes a GFCI will protect 2 pronged plugs.

A GFCI works by measuring input and output amperage. If there is a big enough difference it triggers the reset.

  • \$\begingroup\$ So you're saying it trips if current goes to any ground (e.g., through a sink's plumbing), not just the ground on the plug? \$\endgroup\$ Jun 26, 2014 at 21:40
  • \$\begingroup\$ No it does not matter were the current goes. It only detects the difference of input and output amperages and sees if there is a significant difference. The difference is mainly caused by a short. \$\endgroup\$ Jun 26, 2014 at 21:42

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