Douglas Krantz - Technical Writer - Describing How It Works

How Do I Find this Intermittent Ground Fault?

When hunting the cause of the ground fault, first make sure you are detecting the ground fault reliably. It's only after you can detect the ground fault that you can follow wires to find the ground fault.

How Do I Find this Intermittent Ground Fault?

How Do I Find this Intermittent Ground Fault?

Greetings Douglas,

I have been hunting an intermittent ground fault in an apartment building recently. It first occurred during the annual inspection and is connected to a loop that runs NAC wires through a section of apartments.

The real issue is any time we go to troubleshoot, the ground fault isn't there. We have had issues finding out exactly when they do occur other than the original time when we were on site.

One issue I have noticed is that the wires connected to the loop have a ground wire with distinct blackened look inside any of the boxes. Could this be caused by a bigger issue connected to the building ground system?

What we have tried already is taking an ohm reading of the loop at the panel while the wires were disconnected. We were able to get a correct ohm reading related to the resistor connected to end of the line.

Thank you, D F

There a number of issues described here.
  • The ground fault occurred right after the annual fire alarm inspection
  • The circuit runs through apartments
  • The ground fault only intermittently turns on the ground fault light
  • One of the wires seems to be blackened
  • The wires seem to be connected all the way through the circuit to the end of line resistor
  • The only test instrument you're using is a standard ohmmeter (1.5 volt internal battery? 3 volt internal battery? 9 volt internal battery?).

Intermittent Ground Fault

You say that the ground fault is intermittent. While sometimes, when the ground fault light on the front panel turns on and off, the intermittent ground fault could be a metal-to-metal electrical contact that connects and disconnects. That is an intermittent "hard" ground fault.

Then again, the ground fault light is.

This would be a "soft" ground fault. Usually, this is caused by:
  • Metal-to-water-to-metal contact
  • Metal-to-carbon-path-to-metal contact
  • Metal-to-thin-insulation-to-metal contact

More information on Soft Ground Faults can be found at: What is a Soft Ground Fault?

Soft ground faults are harder to detect with standard ohmmeters. If it is a soft ground fault, you may need an Insulation Tester to detect it. If you do use an insulation tester, make sure the testing voltage is less than 45 volts, or you may destroy the whole fire alarm system. Building your own insulation tester would be a safer bet than using most commercial insulation testers.

You can find information about building an insulation tester from:

When calling up that site, just by X-ing out of the subscription request drop-down, you can read most of the article. However, even if you're cheap like me, you still might want to subscribe. The subscription isn't all that expensive, it is a good magazine for the electronic hobbyist, and besides, you can read the full article.

Ground Fault Light

As far as using the ground fault circuitry in the panel for troubleshooting goes:

Don't Even Look at the Ground Fault Light

The ground fault light lies. See for more information on not following the ground fault light.

The article shows how the light lies, and the article shows what to look at instead.

Electrical Carbon Path

You indicate that one of the wires seems to be blackened. Could this be carbon paths? Carbon will conduct electricity, but not like the copper wire because, just like silicone, carbon is a semiconductor.

Lightning damage could have occurred. I have seen where a carbon-path ground fault showed up weeks after the original lightning strike. An ohmmeter, though, didn't detect it; an insulation tester (homemade - described above) did detect it. An insulation tester is really just a regular ohmmeter - on steroids; it uses a higher voltage battery than is in regular ohmmeter that will overcome the breakdown voltage on soft ground faults like carbon paths.

Mini-horn Circuit

"A loop that runs NAC wires through a section of apartments" is commonly used to turn on the mini-horns inside each apartment, and from the resident's point of view is a loud source of disturbing noise. May times, residents have disconnected their mini-horn so they would not be annoyed anymore. Usually, this disconnection occurs during the annual testing and activation of the fire alarm system.

You have indicated that the end of line resistor seems to be connected. If a resident did disconnect the mini-horn in their apartment, they either have reconnected it, or, by just connecting the wires together, they electrically bypassed it.

In either case, where the mini-horn was disconnected/reconnected, the wiring inside the backbox for the mini-horn was disturbed. This is something that could be a problem.

When putting the mini-horn back into the backbox, a soft ground fault could be made by pinching a wire against the box, or the carbon path described above could be a problem.

Detect Then Troubleshoot

Before trying to troubleshoot when hunting the cause of the ground fault, first make sure you are detecting the ground fault reliably. It's only after you can detect the ground fault that you can follow wires to find the ground fault.

Douglas Krantz
Life Safety
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