Douglas Krantz - Technical Writer - Describing How It Works
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How Can I Find this Intermittent Ground Fault?

On a fire alarm system, a ground fault is an electrical leakage to ground. Usually, the devices themselves aren't where the electrical leakage is occurring, it is usually the wiring that is leaking the electrical current to ground.

How Can I Find this Intermittent Ground Fault?


How Can I Find this Intermittent Ground Fault?


Greetings Douglas,

I'm having problems with an intermittent ground fault on a Wilson Fire Equipment Monitoring system, on a GE Power plant. I have isolated each discrete devise, individually, and I am unable to find the culprit. Could this be coming from one of the analog inputs (LEL Detectors)? This is becoming very frustrating. Any advice would be appreciated.

Thank you, RC

A ground fault is an electrical leakage to building ground on a fire alarm system. The ground faults are most commonly in the wiring. Devices themselves, unless there's water damage, are usually electrically isolated so they won't be the cause of a ground fault.

The ground fault shown on the panel can be caused by:
  • Faulty ground fault circuitry inside the fire alarm control panel (very rare)
  • Hard Ground Fault: A wire conductor from the fire alarm system, somewhere in the building, making contact with building ground (the most common type)
  • Soft Ground Fault: Cracked batteries inside the control panel leaking electricity (it happens once in a while)
  • Soft Ground Fault: Water on the wires or devices conducting current to ground (somewhat common), not always detectable by an ohmmeter but often detectable by the control panel or an insulation tester
  • Soft Ground Fault: Worn, pinched, or damaged insulation on a fire alarm copper conductor (not very common), not always detectable by an ohmmeter but often detectable by the control panel or an insulation tester
  • Induced Ground Fault: Power induced into the fire alarm wiring from another source (not very common), not detectable by an ohmmeter but sometimes detectable by an AC Voltmeter or an oscilloscope
  • Induced Ground Fault: The building's ground system could have different electrical potentials between two different parts of the building, causing a small current in the grounding between the control panel and an auxiliary panel (it happens once in a while), not very detectable by an AC voltmeter - usually requires an oscilloscope

Not very often: Intermittent ground faults can be caused by a physical hard electrical contact, but only intermittently contacting building ground. Usually, though, this kind of ground fault is constantly shown on the panel.

More often: A soft ground fault is often conducting a small amount of electricity to ground at all times. It might look intermittent because with a soft ground fault, sometimes it might conduct enough electricity to activate the ground fault light in the panel, sometime it might not conduct enough electricity to activate the ground fault light.

Once in a while: With induced ground faults, the fire alarm wires could be picking up varying magnetic induction because they are inside the same conduit as AC power wires. The ground fault would be showing up only when certain equipment, using power through the same conduit as the fire alarm wires, is turned on. Then again, RF from radio transmitters could cause the ground fault, or cross-wiring could also cause the ground fault light to intermittently turn on.



Note: Be extremely careful with commercial insulation testers, like Meggers that use higher than 50 volts for testing. That can destroy fire alarm systems. Make sure the insulation tester is using more like 20 to 30 volts for its testing.

Douglas Krantz

facpdoug@douglaskrantz.com
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