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I have a food truck with a 60 amp service installed. When I plug into a GFCI with an ungrounded cord it works fine. But when I plug into GFCI with grounded plug it trips immediately. Please help.

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    Is this a 120/240v generator? Many times the case is bonded to the frame and depending how it is wired could be the root of the problem.
    – Ed Beal
    Commented Sep 25, 2017 at 22:03

3 Answers 3

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GFCI's work by measuring the current going out on the hot wire and the current coming back on the neutral. If there is more than ~5ma of current difference they trip.

So, if it does not trip when you use the 2 wire cord and then trips with a grounded cord, that tells me your equipment or cord has a problem. If it trips with just the cord with nothing attached to it, then the cord seems to be the problem and you should try another cord.

If a piece of a equipment has a ground fault problem and is leaking current to the ground wire, then a two wire cord continues to work, while a three wire cord allows the current to leak to ground and trips the GFCI.

This is a dangerous situation and should be corrected immediately. That is the purpose of the GFCI. It tells you when your equipment has a problem. Repair it now.

Good luck!

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Tripping with the ground present, but not tripping when it's absent, means there is a hot-ground fault or neutral-ground fault within the wiring and appliances.

Turn all the breakers off and plug it in. See what happens. If it trips immediately, you have a neutral-ground bond somewhere on the truck. I'd start looking at the service panel, because most panels come default configured as a "main panel", with neutral and ground intentionally bonded. If you are downstream of a GFCI you must operate in the mode of a "sub-panel" with neutral and ground scrupulously separated.

If there's no neutral-ground bond, and breakers off, and it still fails, then lift all your neutrals off your neutral bar. It must surely clear at that point (or it's still in the panel somewhere). Re-add neutrals one at a time until you find the culprit. It could be that someone "helpfully" bonded neutral to ground in a junction box or at an appliance; that once was the official way to wire ranges and dryers. (still is, technically, but it's bad.)

OK. If it doesn't trip with all the neutrals attached, start throwing hots on one at a time, and powering up everything on that circuit. This is more of that "process of elimination".

As Sigmund Freud once said, sometimes a ground-fault-detector trip is actually a ground fault. There is such a thing as defective appliances; that's the whole point of using GFCI detectors. Usually commercial grade appliances are repairable, and often a plain old teardown/cleaning will suffice.

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That means that somewhere on your truck, neutral (or worse, hot!) is connecting to ground. That means that a patron could get injured if they touch your truck!

Add a picture of the 60-amp service panel with the cover off to be more clear.

But I suspect the two are bonded together in there, and you need to disconnect the bonding strap in the panel.

Bonding ground to neutral should only be done at the facility's main service panel, or in your generator.

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