Douglas Krantz - Technical Writer - Describing How It Works

Is There a Single Type of Ground Fault Tester?

By Douglas Krantz | Descriptions

A ground fault creates a complete loop between one of the wires in a building-wide fire alarm system, the building's ground, and the ground fault detection circuitry in the fire alarm panel.

Greetings Douglas,

The problem is really very serious, and I don't know why we didn't think of a device capable of detecting this earth fault problem on fire loops, which is very ''annoying''.

I know that some manufacturers have instruments only for their brands of equipment, and in the end, the equipment really only works with their own equipment.

Is there a single tester that will work with all types of fire alarm systems to find ground faults?

Thank You, PK

A ground fault is really a short to ground, or a leakage of electrical current to ground, somewhere in the building.

The problem is there isn't a single test tool or test instrument that can be used to find all ground faults because there isn't a single type of ground fault.

Ground faults can be categorized three ways:

Hard Ground Faults - Grounded-Metal-to-Fire-Alarm-Copper-Conductor:
  • When metal of the building ground system makes direct contact with one of the copper conductors of the fire alarm loop of wire, somewhere in the building

Soft Ground Faults - Grounded-Metal-Through-Degraded-Insulation-to-Fire-Alarm-Copper-Conductor:
  • When water drips onto wires or connections in the fire alarm system, the water degrades the insulation and conducts a little electricity, or the water itself conducts a little electricity

  • When a wire gets pinched so the insulation is very thin, the thin insulation is degraded and conducts a little electricity

  • When a wire's insulation is partially worn off, the insulation has become thin and therefore degraded, and starts to conduct a little electricity

Induced Ground Faults - Not-Wanted-AC-or-DC-Power-Introduced-into-Wanted-Fire-Alarm-Circuitry:
  • This would be AC (low or even high frequency) or DC power, usually low power (not always), being forced into the fire alarm system from something else in the building

Intermittent Ground Fault

Many ground faults are the Hard Ground Fault type: if the panel's ground fault light is on, the ground fault is truly there; if the ground fault light is off, the ground fault does not exist.

Some ground faults are the Soft Ground Fault type: if the panel's ground fault light is on, the ground fault is probably truly there; if the ground fault light is off, the ground fault may be there, but not a strongly, or the ground fault might not be there.

A few ground faults are the Induced Ground Fault type: for the purpose of troubleshooting, the panel's ground fault light is useless.


A cheap ohmmeter is all that is needed for detecting Hard Ground Faults.

An expensive ohmmeter, one that has a 9-volt battery, will work for some soft ground faults, but an insulation tester is needed for some soft ground faults.

A DC voltmeter, an AC voltmeter, an insulation tester, or on a few rare cases, an oscilloscope is needed to detect induced ground faults.

Be careful when using an insulation tester. Unless using one that is set to 40 volts or less, the fire alarm system may be damaged by the tester. BE CAREFUL.

Finding the Ground Fault

I've never used an instrument that will tell me where the ground fault is located in a building. I've always had to find the ground fault the hard way; divide-and-conquer troubleshooting. Sometimes, I've used shortcuts to troubleshooting, but knowing how the shortcuts work takes a lot of experience.

The problem with an instrument that shows the exact location of a ground fault is that the building's internal ground-network is almost a direct distance between the panel, whereas the wire-distance to the ground fault may be five times the direct distance. A few ground faults may be found using devices that use a speed of light time-based measuring system may help find a few ground faults, but most of the ground faults still take real troubleshooting techniques.

Another problem with a distance-to-the-ground-fault detector is the detector does not show which direction to the ground fault, or even which floor the ground fault is on.

The time-based detection instrument may work with hard ground faults, but it won't work with many soft ground faults, and it's useless for any induced ground faults.

Book: Make It Work - Hunting Ground Faults

Ground faults are usually easy to fix. Finding the ground fault is the hard part.

I do have two versions of a book that is written specifically written to help find ground faults.

Make It Work - Hunting Ground Faults - Laptop or Desktop Computer PDF


Make It Work - Hunting Ground Faults - Mobile PDF

Douglas Krantz
Life Safety
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