There are three issues involved in finding any ground fault: What Type of Ground Fault, Detecting the Ground Fault, and Hunting Down the Ground Fault.
What Type of Ground Fault
In a building-wide fire alarm system, there are three types of ground fault.
Hard Ground Fault
- A metal-to-metal electrical contact is made between a conductor in the fire alarm system and building ground.
Soft Ground Fault
- A metal-to-(water, thin insulation, degraded insulation)-to-metal electrical contact is made between a conductor in the fire alarm system and ground.
More information on Soft Ground Faults can be found at: https://www.douglaskrantz.com/BlogSoftGroundFault.html
Induced Ground Fault
- An outside source of AC or DC power is induced into the fire alarm system, either through electrical contact, magnetic coupling, or electrostatic coupling.
Detecting the Ground Fault
Don't bother trying to use the ground fault light on the panel. The ground fault light is a very fickle indicator. It can be on a delay so you have to wait for it to turn on or off, it can fail to indicate the ground fault even though the fault is still there, it can latch on-to showing the ground fault even after the ground fault is fixed.
In other words, the ground fault light can show false positives and false negatives. This false reading is especially a problem when the electrical contact varies its resistance as the voltage on the contact changes (non-linear resistance).
- Using the voltmeter to get two separate battery voltage readings is a better way of detecting a ground fault. The first reading is to measure the voltage on the battery between the plus battery contact and ground. The second reading is to measure the voltage on the battery between the minus battery contact and ground.
Each model of panel is going to be different, so get used to seeing each model's voltage readings.
If there are any questions, contact the manufacturer's technical support team. Often, they can provide a better ground fault detection method.
Hard Ground Fault
- The ohmmeter is usable in the field for most hard ground faults because the hard ground fault is a low ohmage, linear resistance type of electrical contact. The ohmmeter shows an electrical short.
Soft Ground Fault
- The internal battery for the ohmmeter is critical in the field when trying to detect a soft ground fault.
- When the internal battery for the ohmmeter is 3-volts, the ohmmeter won't detect most soft ground faults . . . at all. That's because the voltage applied to the electrical contact (water, thin insulation, degraded insulation) doesn't cause any electrical current to flow. The ohmmeter only shows an electrical open.
- When the internal battery for the ohmmeter is 9-volts, the ohmmeter often will show a very high resistance short. This is high resistance short is hard to recognize, and often sends a repair technician in false directions. Because the ground fault is through a non-linear resistance, sometimes the 9-volts isn't enough voltage to show any conduction through the ground fault at all. The ohmmeter only sometimes shows an indeterminant ground fault.
- When the internal battery for the ohmmeter is above 27-volts, the ohmmeter will detect all soft ground faults. The 27-volts is usually higher than the battery voltage of the panel.
OK, the higher voltage ohmmeter isn't called an ohmmeter, it's called an insulation tester. An insulation tester is really an ohmmeter on steroids; its internal battery has a much higher voltage.
An insulation tester also doesn't show the resistance of the circuit, it shows the current leakage of the insulation. For the technician, low resistance means high current, and high current means low resistance. There is no real difference between low resistance and high current.
Electricians may know of an insulation tester by its brand name: Megger.
Fire alarm technicians don't use an insulation tester very often because the insulation tester can use an internal battery of 100-volts to 15,000-volts. NEVER connect these higher voltages to any fire alarm circuit because these higher voltages can easily destroy the entire fire alarm system.
Yes. There are some insulation testers on the market that have a switchable testing voltage of 10-volts to 100-volts, but I am reluctant to use these because the switch might accidently be left at 100-volts, and the higher voltage could possibly damage the fire alarm system.
Before there were the lower voltage insulation testers, I built my own tester. More information on how to build this tester can be found at: http://nutsvolts.texterity.com/nutsvolts/201009/?folio=42&pg=42#pg42
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's a good magazine for the electronic hobbyist, and besides, you can read the full article.
Induced Ground Fault
- There is a whole cluster of possible causes for this type of ground fault. This kind of problem requires the right type of equipment, and the equipment could be an AC voltmeter, a DC voltmeter, an oscilloscope, etc.
- Fire alarm cable placed in the same conduit as AC current, like utility power, thermostat wiring, speaker wiring, can produce symptoms similar to ground faults.
- Unintended connections to AC or DC equipment may produce symptoms similar to ground faults.
- Nearby radio and TV stations can produce symptoms similar to ground faults.
Hunting Down the Ground Fault
I have been told that there are meters that can be used to determine the length of wire between the panel and the ground fault. There are so many caveats to consider when using this kind of equipment that I find tried-and-true methods easier.
First, find out if there has been any remodeling that has gone on, or any water damage in any part of the building. Intuition sometimes helps here, especially when water has spilled and trickled down inside the walls three or four floors above the ground fault itself.
Second, use the proper ohmmeter, insulation tester, AC meter, DC meter, etc. to divide and conquer. That's how I hunt down the ground faults.
Yes, I have a whole book on the subject called "Make It Work - Hunting Ground Faults" at https://douglas-krantz-s-fire-and-life-safety.myshopify.com/collections/desktop-and-laptop-computer-versions-pdf/products/make-it-work-hunting-ground-faults-desktop-version
The best I can do over Email is to give a summary of what to do; the book covers ground faults more thoroughly.