I setup the above to make a temporary gfci tester or tripper. The white button works manually, but when I plugged in a load to the hot and neutral output of the gfci 2-pole breaker, why didn't it trip it? I understand the GFCI monitors the current between the 2 hots, so when there is a leakage, it trips. Why can't the hot to neutral output be considered a leakage?

Original message:

I have searched the archive, but couldn't find question or answer to this supposedly common question.

I have successfully connected 2 US Siemens 2-pole GFCI breaker and the outlet has 240v. Can I use the ordinary 120v GFCI tester for this 240v outlet? Is the circuit inside the tester only for 120v and what would fry if the source is 240v?

I can't use the European RCD tester because their test leakage current is high 30mA. US GFCI tester has 5mA trip rating and test signal.

If no product can be found for this. Any way to test if the 2-pole gfci breaker is working (besides testing its self test button)? I want to test the outlets connected to it 1 floor away of leaking 5mA can trip the gfci breaker.


No. A standard US 120V GFCI tester has a NEMA 5-15 plug. It is physically unable to plug in to a 240V circuit.

Receptacles used on 240V circuits are keyed differently, precisely to keep that kind of mistake from happening. Obviously it is a mistake because it would wreck any equipment plugged into it (including, possibly, a GFCI tester). Even if it happens to inconvenience you right now.

Anyone using a NEMA 5 on any voltage other than 100-132V should not be playing with electricity. They should use Euro style plugs, unless they have the unique center ground of some parts of the Philippines, in which case NEMA 6 would be just right... but given Philippine ambitions, I would think they'd be standardizing the country on one of the Euro plugs.

Someone who should be playing with electricity may note that the GFCI tester is simply a resistor that allows a calibrated amount of current between hot and ground, inducing a ground fault just above detection level. Now if you were doing this in Europe, or Euro-wired sections of the Philippines with 240V between hot and ground, the Resistor would be seeing 240V instead of 120V, and flowing twice the current, possibly to its destruction.

However, if you are in a US-legacy section of the Philippines where ground is centered between the two hots, then the resistor is of the correct value. One could use a custom cheater cord to use the GFCI tester as intended. However, that would only test one "hot". To test the other hot, one would need a second cheater cord with the second one cross-wired. These cheater cords would be harmless; you wouldn't connect the cheater's NEMA 5 socket neutral to anything at all, and only connect one of the plug hots, so if someone accidentally connected a 120V appliance it would not power up.

If your socket is flippable Euro style, then you wouldn't need 2 cheater cords, you could just flip the cheater cord over.

What you just drew in your recent edit, is not a ground fault. It is the GFCI working as intended. Currents through the GFCI device on L1 and neutral will cancel out, with nothing on L2, so currents sum to zero and no trip. Note that the GFCI is not comparing currents on the two wires you want it to. It is comparing currents on all three. If they add up to zero (all current going out comes back), no trip.

The GFCI tester trips because its current is coming out L1, which does go through the GFCI, and going back on ground, which does not go through the GFCI. Hence it does not see the return current, and sees currents as not adding up to zero, and correctly trips.

Do not ask more than 25 watts out of either side of that transformer, or you will overload it. One iPad is fine. Two would be at transformer limits.

  • See picture of my whole panel and added Siemens GFCI 2-pole breakers at diy.stackexchange.com/questions/150196/… Each pole of 16 2-pole breakers is 120v. There is no neutral or ground bar. So using a GFCI tester, can it work if I connect it to one pole (120v) and to the centertap of the autotransformer which also powers the Siemens GFCI breakers? – Samzun Nov 8 '18 at 21:10
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    @samzun I was describing how to wire the cheater cord. You missed what I was saying. – Harper - Reinstate Monica Nov 11 '18 at 1:53
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    Yes, any leakage, whether to ground, neutral, a human, your dog or Subic Bay, will result in currents not being equal and snap! – Harper - Reinstate Monica Nov 11 '18 at 4:10
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    If you connect a load to hot and neutral of the load side of the GFCI, that is not a fault current. If you connected it to hot of the load side, and neutral of the line side, that would be. That is to say, the breaker doesn't know it's in the Philippines and doesn't know it's naughty to use 120V loads :) – Harper - Reinstate Monica Nov 12 '18 at 2:50
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    They are connected continuous through a sense coil. Check its inductance, it's non-zero. – Harper - Reinstate Monica Nov 12 '18 at 3:04

In general, devices designed for 120V should not be used at 240V. At a minimum I'd try to contact the manufacturer of the tester before using it in that way.

If your 120V tester is a plug-in tester, it shouldn't fit into a 240V outlet. (e.g. a NEMA-5 plug shouldn't fit a NEMA-6 receptacle)

  • no problem about the Nema-5 or 6, will just use adaptor.. – Samzun Nov 8 '18 at 11:42

Put a small load on the circuit on the floor nearer to the panel. Go to the floor away and measure the voltage between the neutral and a ground, and calculate what resistance you would have between the two to give some test current. Connect that resistance across the two and if the breaker trips you have tested it to that current level.

As a first simple test to see if the GFCI is working at all, just put a wire from the neutral to a ground. The GFCI should trip, but you won't know the leakage current that caused the trip.

Note: Pay attention to what you are doing and be certain not to mistakenly insert a lead connected to a low resistance into the hot slot!

Example: Suppose you get 10 mV between neutral and gnd and want to test at 8 mA. R = V / I = 10 mV / 8 mA = 1.25 ohm

You could get a variable resistor and put it in series with an ammeter. Connect this from neutral to a ground. Sweep the resistance down from a high valve to lower and lower resistance while observing the reading of the ammeter. At some resistance the GFCI will trip and you have the current reading that caused the trip.

  • This is the reason I just wanted to get a GFCI tester so can just plug it into any outlet and there is reading immediately without getting through all the trouble above :) So can the US 120v Gfci tester can work for 240v? – Samzun Nov 8 '18 at 13:58

The problem is at 120v 5ma trip would not be the same current level so the test would not be accurate it would be similar to using a European tester if it held up but would not be accurate. What could happen? If the neon light type it could cause the neon lamps to explode. Doubling the voltage could overheat the load and cause parts to melt in your hand so for these reasons I would say NO it will not work and if it works the trip level would be wrong. If the tester creates a 5ma current on 120v circuit E/I = R the load would be 24K resistance , now use the 24k resistance with 240v and the 24K. Resistor E/R= A or 10ma so you can see that the current level would be wrong it would require a 48k resister to test a 5ma trip level on a 240v circuit.

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    I have a variable load that I use but because of cost probably not what a home owner would ever purchase +150$ with multiple adapters for 15a, 30a and 50a 240v adapters. I believe it is a high wattage wire wound resistor, you start at a higher resistance and turn down until it trips then the printed scale shows the resistance level it tripped at. – Ed Beal Nov 8 '18 at 14:21
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    I will dig it out and get back with you I have had it for many years it actually is for 500v and below and you won't need to purchase all the adapters just the one or a clip set for testing in the panel. – Ed Beal Nov 8 '18 at 14:32
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    Samzun I found my tester the model number is ARL-500-50 the name of the company is worn off and I think that is the entire number there was a space but the rest of the label is gone. It is put together so it can not be opened, it has a warning not to excede 150ma for more than 30 seconds. It has a reset probably a thermal safety. It measures 4k ohms at the maxim current and 100k ohms at the minimum. I could not find it on Google , I think I purchased it ~ 1999. I might have the book that came with it with my old code books but many of those haven't been unpacked since my last move. – Ed Beal Nov 8 '18 at 21:06
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    Although in a transformer power in the primary is aprometly equal to power in the secondary there is no direct connection between the 2 coils so you could have leakage on the primary circuit with none on the secondary. This is why we use isolation transformers in areas that are sensitive. An isolation transformer is usually a 1 to 1 ratio ie 120v in 120v out. Or we use a dedicated transformer with its own planer just powering loads for the sensitive equipment. – Ed Beal Nov 10 '18 at 17:24
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    Absoutly there is no direct connection between the primary coil and the secondary coil the 2 sets are in close proximity but insulated from each other the primary will have 2× the number of wraps or turns as the secondary around a lamented iron core for the step down with no connection. – Ed Beal Nov 11 '18 at 21:27

A 240V US outlet is still going to have 120V to ground, so the GFCI tester part should still work correctly.

However, any other functionality intended to have 120V L-N will see 240V. This could include other things like test lamps, circuit tracers etc.

Do not connect the 'neutral' on the tester; ensure it is safely insulated.

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