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I want to test if my oven can trip the GFCI. The leakage current should not be a problem but I worry about the GFCI would trip at high frequency. So I bought three circuit breaker and tested oven on them individually. They all didn't trip.

But I just realize that my lab has a circuit breaker too. Will the circuit breaker on another circuit breaker impacts my results?

The oven is a commercial oven. I am not familiar with how commercial kitchen do this. Do you think I should buy some portable GFCI and receptacle GFCI to test as well?

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    I'd suggest to the OP adding more info to the questing indicating why you want to test the breaker. Do you not trust the manufacturer to have produced a suitable product? Does the GFCI have a UL (or other regulatory/governing/standards body) stamp on it? If so, do you not trust them to have certified that it's safe and will do its job? If you don't know how to go about testing, what do you think you can prove that they haven't already proved? If you're wondering if the oven can trip it, open the back of the oven, short circuit it and see what happens - NOT recommended.
    – FreeMan
    Commented Jun 25, 2020 at 15:39
  • It could be "relatively" safely tested using a low wattage (say 25 watt) incandescent light bulb using a socket with wires. Connect one wire to the hot and the other to the ground. Don't touch anything during what should be a very brief test. Make the connections with the power off. turn on the breaker, it should trip instantly. This is not an approved method, but would probably satisfy your concern about the GFCI functioning properly. Commented Jun 25, 2020 at 15:50
  • I can understand wanting to test , I am required to verify personal and equipment GFCI’s for our insurance company , since I could not find one like the plug in ones I have for 120 v receptacles I built a simple adapter that works for a 3 lamp plugin receptacle tester that has the test button or my expensive one with the 10 turn potentiometer.
    – Ed Beal
    Commented Jun 25, 2020 at 16:14
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    I am trying to understand the goal. Is the goal here to "make sure that the GFCI will function if ever needed (i.e., to be safe)?" Or is the goal "make sure the GFCI only trips for a true problem and not due to normal operation of the oven?". Or to put it another way, are you trying to "make sure the GFCI works" or "make sure you won't get nuisance trips"? Commented Jun 25, 2020 at 16:53
  • I just noticed the last portion , portable GFCI have additional safety’s for an open neutral the standard fault circuit is the same an imbalance will in excess of 5 ma will cause a trip an open neutral will also cause a trip.
    – Ed Beal
    Commented Jun 25, 2020 at 18:23

3 Answers 3

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They do make testers but it depends on where you live to the model used. I will guess you are in the US since this is a new requirement. The good thing about us power is the max voltage to ground is 120v 240v is 2 120v lines that are out of phase so the max voltage to ground is 120v where other places in the world have 240v to ground. I have a fancy one that plugs into a wall socket and a precision 10 turn pot so I can verify from 3ma to 100 ma depending on the type of protection required (personal or equipment), According to the NEC commentary in the handbook GFCI’s for personal protection is required the set points are from 4-6 ma you can do like I did with my 180$ tester and turn it into a 4 wire tester with a receptacle and a plug mounted in a box (ok the first few times I did it I just used wires on the prongs but you can use a standard tester with an adapter to test Connecting the smaller prong (hot) to one of the hot(s) and the ground round pin on the tester to the ground on the stove receptacle When the test button is pressed a 15.5k resistor is connected from ground to a hot this is above the 4-6 at almost 8 ma but a good test unless you want to purchase the pro model and record the exact trip point I have a switch on my setup so I can test both hot’s after checking 5 or 6 they all trip at the exact same level no matter which hot is connected.

I used a standard 120v receptacle single a single pole double throw switch And a 50 a 4 wire range power cord , plug my tester in and push the button on the cheap home owner one or dial the resistance until it Tripp's this fit in a single gang box with a 3/4 cord grip. I have not seen a commercially available tester so I made one that will work with either of my standard receptacle testers. If you really want to test this was the cheapest way I could find to do it.

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  • Doesn't the built-in test button do the same thing with a fixed 10k resistor? (in case people are unsure of the fitness of the test).
    – dandavis
    Commented Jun 25, 2020 at 16:29
  • 10 k is a bit low but yes this is the test required by FM insurance current is current the system is only measuring an imbalance.
    – Ed Beal
    Commented Jun 25, 2020 at 16:38
  • It depends on the GFCI safety standards for trip current. N.Amer. Levels are much lower than EU Commented Jun 25, 2020 at 16:54
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I have a NEMA 10 outlet

To lay out an exception here, if this is a NEMA 10 (hot-hot-neutral) receptacle, this is a train wreck waiting to happen: GFCI provides far worse than no protection to a NEMA 10 socket: it provides zero shock protection while creating the illusion of protection. NEMA 10-wired appliances bootleg their chassis to neutral, which defeats the purpose of GFCI. This bootlegging must be removed as part of the procedure for converting to a NEMA 14 outlet, and a NEMA 14 outlet must be fit without ground.

I have a NEMA 14 outlet, but it’s not grounded

Now, if you have a NEMA 14 outlet, you should look seriously at legally retrofitting a ground wire, because that will make this go much, much better.

If you can’t do that, then temporarily provide an equipment ground to your ungrounded NEMA 14 recep; drape it in an obvious, hokey way as to remind yourself to remove it when the test is done.

My NEMA 14 is grounded (even if illegally)

Now, obtain one of the illegal “cheater cords” that plug into a NEMA 14 outlet and provide 2 separate banks of NEMA 5-15 receps.

Plug any common-as-dirt GFCI tester in to one of the receps. Push the “TEST” button. The GFCI should trip.

Reset and move the GFCI tester to a recep on the other “leg”. (If you don’t know which that is, test more than 50% of them). Push TEST again. Should trip.

I have a NEMA 6 outlet, and it’s grounded.

You will have to build your own “illegal cheater cord” of the above type. Obtain a NEMA 6 plug, and two 3-prong extension cords. On the extension cords, lop off the plug end, you won’t be using that.

Both cords’ grounds go to ground.
Both cords’ neutrals are insulated (even from each other!!).
Each cord’s hot goes to one of the NEMA 6 hots.

Then you do the GFCI tests as above. This cable is useless for anything else. Never connect the neutrals to ground!

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  • Why would a GFCI provide zero protection with a NEMA 10? If the chassis somehow becomes hot (e.g. due to a broken neutral) and someone touches it, the resulting current through their body to ground should trip the GFCI.
    – TooTea
    Commented Jun 25, 2020 at 20:01
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    @TooTea the resulting current to where? Remember the dryer bootlegs ground. This would only work if contact was between there and an external ground. Commented Jun 25, 2020 at 22:47
  • Right, but isn't that the most likely way to get nailed in such a situation? I mean, where else could the current go? It doesn't feel likely that someone would simultaneously touch the hot chassis and the unbroken part of a broken neutral, which is the only case in which the GFCI wouldn't trip. It's fairly common to have your body grounded in some way, so that you get a jolt if you touch a hot wire.
    – TooTea
    Commented Jun 26, 2020 at 7:46
  • As long as you make sure that nearby appliances with bootlegged grounds aren't all on the same GFCI (I suppose NEMA 10 appliances like dryers are typically on a dedicated circuit), the GFCI should still provide a decent level of protection (though obviously nowhere near having a proper safety ground).
    – TooTea
    Commented Jun 26, 2020 at 7:56
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Why would a GFCI provide zero protection with a NEMA 10? If the chassis somehow becomes hot (e.g. due to a broken neutral) and someone touches it, the resulting current through their body to ground should trip the GFCI.

I guess GFCI breaker would trip immediately, because the 120V control unit puts current on neutral wire (thus imbalance between the hots). Unless the GFCI has all three conductors (hot, hot, neutral) running through the current sensing core. Then it would protect against shock current due to, say, neutral (and chassis) being at different potential from the water pipe or garage floor near the appliance.

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