Testing ungrounded receptacles on GFCI circuit

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Steven B

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Clarksburg, MD
I am currently working in an older home with ungrounded 2 wire circuits. I have installed a GFCI receptacle to protect 3 wire ungrounded receptacles downstream. I know that a plug tester with a GFCI test button will not trip the GFCI receptacle but I assumed I could test the downstream receptacles by shorting the hot side of the receptacle to a nearby piece of grounded metal. I thought that it would simply trip the upstream GFCI well before the breaker would trip. To my suprise, I got a nice flash and the breaker tripped. Aside from any criticisms of my test methods, can anyone tell me why the breaker tripped instead of the GFCI? I am pretty sure that everything is connected properly. The test button on the GFCI works and kills power to the downstream receptacles.

Thanks for any help.
 
Certain testers like an old Wiggy will trip the gfci when you test from hot to a grounded object. A DMM usually will not.
 
ActionDave said:
The GFCI should be faster, yes. You never gave the GFCI a chance to see any imbalance between the hot and neutral going to ground. You left the neutral out of the circuit.
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But any time a GFCI trips, the current is going to ground and the neutral is out of the circuit. That's why it trips
 
Steven B said:
But any time a GFCI trips, the current is going to ground and the neutral is out of the circuit. That's why it trips
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Not true, the reason a GFCI trips is because there is an alternate path for current besides the ungrounded and grounded conductors. The current flow between the hot and neutral must add to zero, if there is a difference of more than .004 to .007 amps the GFCI (class A) should trip.

Roger
 
roger said:
Not true, the reason a GFCI trips is because there is an alternate path for current besides the ungrounded and grounded conductors. The current flow between the hot and neutral must add to zero, if there is a difference of more than .004 to .007 amps the GFCI (class A) should trip.

Roger
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Sorry to be so dense but it seems to me that since the current to the downstream receptacles is being fed through the upstream GFCI, the GFCI is measuring the current on the both the hot and neutral for everything downstream and should know when something downstream is shorted to ground (which means there is less current on the neutral coming back upstream) and should be able to react much faster than the breaker. What part am I getting wrong?
 
Steven B said:
Sorry to be so dense but it seems to me that since the current to the downstream receptacles is being fed through the upstream GFCI, the GFCI is measuring the current on the both the hot and neutral for everything downstream and should know when something downstream is shorted to ground (which means there is less current on the neutral coming back upstream) and should be able to react much faster than the breaker. What part am I getting wrong?
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Ok in the case of a hot to ground short, yes the GFCI will see a current imbalance but it will be seeing this at exactly the same time as the breaker sees an overload condition.

There is no reason to think a GFCI can physically open the circuit faster than the breaker, that is not the purpose of GFCI.
 
Steven B said:
... why the breaker tripped instead of the GFCI?
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Many people are amazed at how fast circuit breakers actually are, when tripping in their instantaneous region, like with a direct short.
Likewise, many are surprised that GFCI's actually have a time delay in their operation.
 
roger said:
Not true, the reason a GFCI trips is because there is an alternate path for current besides the ungrounded and grounded conductors. The current flow between the hot and neutral must add to zero, if there is a difference of more than .004 to .007 amps the GFCI (class A) should trip.

Roger
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I thought it was >6mA? What you are saying is not incorrect since 7>6. I thought anything over 6mA and the GFCI should trip.

To the OP, I am not objecting to your test method but will offer some other test methods. You could try a load that the breaker should not trip with such as a light bulb, a radio or otherwise. A load closer to the trip threshold of the GFCI would not give you too much objectional current through the grounded metal objects used as a return path. A high current fault could create a fire or shock hazard when finding it's way through normally non current carrying objects on its way back to the source. I recently made a GFCI tester with a cheap neon voltage detector. I soldered 3 55kohm 1/2 Watt resistors across the input terminals. With this I can use the probes to select my current path as opposed to the limitations of a plug in tester.
 
MD84 said:
I thought it was >6mA? What you are saying is not incorrect since 7>6. I thought anything over 6mA and the GFCI should trip.

To the OP, I am not objecting to your test method but will offer some other test methods. You could try a load that the breaker should not trip with such as a light bulb, a radio or otherwise. A load closer to the trip threshold of the GFCI would not give you too much objectional current through the grounded metal objects used as a return path. A high current fault could create a fire or shock hazard when finding it's way through normally non current carrying objects on its way back to the source. I recently made a GFCI tester with a cheap neon voltage detector. I soldered 3 55kohm 1/2 Watt resistors across the input terminals. With this I can use the probes to select my current path as opposed to the limitations of a plug in tester.
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Good idea about using a different test method. The resistance of a light bulb would do the trick. I just was not expecting the GFCI to be slower than the breaker. I am going to try to find the trip times for each.
 
MD84 said:
I'm not satisfied until I trip the GFCI from a downstream protected receptacle.
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There is no need of that.

The accepted method is to put a load in at the downstream receptacle and push the trip button at the GFCI device.

If this is some hobby type exercise at your house that is great but if you are doing this on the job the tests you are describing will not count for anything if comes down to that.

Nothing you cobble together will be looked at as a reliable test.
 
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