What is a GFI?

First off, the correct acronym for a GFI (ground fault interrupter) is a GFCI (ground fault circuit interrupter). The purpose is to act as a safety device to protect from electrical shock.

GFCIs can be built into outlets, circuit breakers, and even extension cords and are generally used for safety in wet environments like bathrooms, kitchens, and outside.

A GFCI measures the difference in current between the line (hot) and the neutral. When even a slight difference exists between neutral and hot, the GFCI will trip. A difference between neutral and hot indicates that some of the current is “leaking” to ground instead of being carried properly on neutral.

An example would be an electric drill plugged into an outlet outside, and the cord plug falls into a mud puddle. If there is no GFCI, some of the current will go out of the plug to ground through the puddle, causing hot to carry more current than neutral and making the puddle a potential shock hazard. If the circuit were protected with a GFCI, it would trip immediately when the imbalance was detected.

Another nice thing about a GFCI is that it can help protect a circuit that does not have an equipment ground, such as tools and appliances with two-prong cords or two-conductor outlets.

—Bryan

P.S. – If you want to read a follow-up to this article where I respond to a technician's question, you can check it out HERE. Many other techs have chimed in on the matter in the comments, too.

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3 responses to “What is a GFI?”

  1. Hey Bryan,

    I have had a few instances where we are firing off a furnace in a new build with a temporary power pole outside with gfci outlets installed that are tripping the gfci on blower motor start up. (In order to get temporary heat for drywall, we run an extension cord to the gfci pigtailed to the furnace). I’ve never had issues doing this with PSC motors, but everytime I’ve tried to use a gfci with a variable speed or true ECM motor, it trips the gfci. I’ve tried multiple motors, different furnaces.

    What gives?

    Sincerely,
    Frustrated in Fort Collins

    • I have not tried this so I haven’t experienced it but it does make some sense. The GFCI is just watching for a difference in current on hot and neutral. When you add in a variable frequency drive (Which is essentially all an ECM or X13 motor is) the circuitry / housing is exposed to a lot of “induced” electromagnetic fields. Some of this induction will be routed to ground causing a tiny imbalance. Here is a thread in my buddy Mike Holt’s forum that talks about this very phenomenon http://forums.mikeholt.com/showthread.php?t=92263

      Thanks for participating. Great stuff!

  2. GFI are for equipment protection, trip at 30mA, GFCI are for personnel protection, trip between 5&7mA i think, they DO NOT prevent electrical shock, they prevent electrocution, you still feel a shock, but it shuts off so fast it could save your life.

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