Tracing a rain-related short circuit

[David51]

I have an intermittent short on a circuit with a GFCI breaker and some outdoor fixtures. The breaker pops after a significant rain and won't reset until after several dry days. I've tried to isolate the break by disconnecting the outdoor fixtures sequentially with no luck. Now I'm doing continuity tests on each part of the circuit.

With the power off, should I just be testing for continuity between hot and neutral, or also between hot and ground? Does the fact that the problem is related to moisture make a difference?

Sorry to ask such a basic question, but I couldn't find an answer on YouTube or Wikipedia.

Thanks very much!

 

[James]

An insulation test from live to earth will show it up normally

 

[David51]

An insulation test from live to earth will show it up normally

Thanks very much for the very quick reply, James. For future reference, if a short isn't intermittent and rain-related, would I test continuity across hot/neutral, or hot/ground, or both? What different information, if any, do they provide about the cause of the short? Thanks!

 

[Avo Mk8]

I have an intermittent short on a circuit with a GFCI breaker and some outdoor fixtures. The breaker pops after a significant rain and won't reset until after several dry days. I've tried to isolate the break by disconnecting the outdoor fixtures sequentially with no luck. Now I'm doing continuity tests on each part of the circuit.

With the power off, should I just be testing for continuity between hot and neutral, or also between hot and ground? Does the fact that the problem is related to moisture make a difference?

Sorry to ask such a basic question, but I couldn't find an answer on YouTube or Wikipedia.

Thanks very much!

Your fault is not really a short, but a leakage current to ground. I believe somewhere around 6 milliamps of leakage would trip the GFCI, so when looking for the problem, if you were trying to use say a multimeter, you would be looking for a resistance from live to earth of around 20k ohms.
Insulation Resistance is typically measured with an IR meter, or multifunction tester, that uses a high voltage to do the test (which encourages breakdown if there's a problem in the circuit)
You don't mention what you intend doing "continuity tests" with, but you really need to be doing resistance measurements around the value I mention, not continuity tests.
You will need to use a suitable meter that indicates resistance, not a continuity tester.
If you do the detective work during damp conditions (circuit OFF!), you may be lucky and trace the problem with a standard multimeter.
However if it's more elusive, or there's a lack of rain when you're looking, you may have more luck borrowing or hiring a proper insulation resistance meter, or getting an electrician in!

 

[David51]

Your fault is not really a short, but a leakage current to ground. I believe somewhere around 6 milliamps of leakage would trip the GFCI, so when looking for the problem, if you were trying to use say a multimeter, you would be looking for a resistance from live to earth of around 20k ohms.
Insulation Resistance is typically measured with an IR meter, or multifunction tester, that uses a high voltage to do the test (which encourages breakdown if there's a problem in the circuit)
You don't mention what you intend doing "continuity tests" with, but you really need to be doing resistance measurements around the value I mention, not continuity tests.
You will need to use a suitable meter that indicates resistance, not a continuity tester.
If you do the detective work during damp conditions (circuit OFF!), you may be lucky and trace the problem with a standard multimeter.
However if it's more elusive, or there's a lack of rain when you're looking, you may have more luck borrowing or hiring a proper insulation resistance meter, or getting an electrician in!

Thanks, Avo Mk8, very instructive and helpful. I have a multimeter with a separate resistance mode (10 ohm resolution +/-1.5% in the 40-400K range). The rain stopped yesterday and things have dried out to the point that the GFCI will reset. I'll shut it off and start testing different runs for leakage, not continuity. Thanks again!

 

[Avo Mk8]

Thanks, Avo Mk8, very instructive and helpful. I have a multimeter with a separate resistance mode (10 ohm resolution +/-1.5% in the 40-400K range). The rain stopped yesterday and things have dried out to the point that the GFCI will reset. I'll shut it off and start testing different runs for leakage, not continuity. Thanks again!

I would suggest any readings under 100k ohms suggest there's a problem with the cable or fixture In that area
Good luck hunting!

PS You may find resistance readings vary with time as you observe them (I think maybe due to electrochemical effects at the fault with the damp and applied dc voltage). Don't worry about it! Once you've found any value up to the 100k mentioned, you've found a problem!
There's another effect if you have a long cable, namely the capacitance between the cable conductors. That can cause a 'kick' in the measured value, which then settles (depending on your meter) after a second or so.
This is easier to read and understand with an analogue meter! But don't let all this put you off!
See what you can find

 

[David51]

I would suggest any readings under 100k ohms suggest there's a problem with the cable or fixture In that area
Good luck hunting!

PS You may find resistance readings vary with time as you observe them (I think maybe due to electrochemical effects at the fault with the damp and applied dc voltage). Don't worry about it! Once you've found any value up to the 100k mentioned, you've found a problem!
There's another effect if you have a long cable, namely the capacitance between the cable conductors. That can cause a 'kick' in the measured value, which then settles (depending on your meter) after a second or so.
This is easier to read and understand with an analogue meter! But don't let all this put you off!
See what you can find

Mapped the circuit connections, tested each segment, found the likely culprit - the underground branch to the shed. It shows 24K ohms three days after the rain stopped and one day after the breaker would reset. Rain is forecast for later today and the circuit is back on, waiting for the breaker to trip. I'll get a new resistance reading asap after that.

Thanks for all your very clear advice!