How can I Find a Non-Linear Ground Fault?

By Douglas Krantz | Maintenance





One of my stories about finding a ground fault after the end of the day was at a large chain-store distribution center. I'd never been there before. Until that night, our company didn't have anything to do with that store chain. Our dispatcher, though, asked if, before going home, I could stop there and fix a ground fault.

A few years earlier, another company had installed a system, so they were the company that did the service (until then, at least).

It was a four-loop addressable system, but the building itself was a several-block-long warehouse. The problem was there was an intermittent ground fault that, when anyone was there to find it, the ground fault was not showing up on the panel. After several weeks of failing to find the problem, the other company quit even showing up.

OK, because I know some things about electronics that aren't covered very well in electronic courses, I could troubleshoot a problem that apparently few other fire alarm technicians know much about: non-linear resistance.

Non-Linear Resistance

A common resistor has the same resistance, no matter what the voltage; the current through the resistor exactly tracks the voltage. When the voltage doubles, the current doubles. That's a linear relationship.

Water does not have the same resistance, no matter what the voltage. Neither does a surge suppressor. When the voltage on either is low enough, the resistance is very high. There is almost no current at all through water or through a surge suppressor as long as the voltage stays low.

With water, there is a voltage threshold, or knee, of 5 volts to 8 volts; with a working 25-volt surge suppressor, there is a voltage threshold, or knee, of about 25 volts.

Doubling the voltage across water from 4 volts to 8 volts will often increase the current by 10 to 20 times. Doubling the voltage on a working 25-volt surge suppressor from 15 volts to 30 volts will increase the current by at least 50 times. When the voltage doubles across the knee on either water or a surge suppressor, the current increases many times what the voltage has increased. That is a non-linear relationship.

Keep in mind that the 24-volt (nominal) battery voltage is split up by the ground fault detection circuitry in the fire alarm panel. The real ground fault detection for a fire alarm panel is closer to 14 volts.

Back to the Story

I arrived on site, and, you guessed it, the panel was normal. Checking history, I found which loop the ground fault was on. I knew that this was an "intermittent" ground fault, and the panel wasn't detecting it, so I whipped out my "Ground Fault Detector" and confirmed that the ground fault was truly existing.

Fire Alarm Specifications: Check the maintenance manual for most fire alarm systems. In there they specify the resistance or current level for the ground fault detection threshold. Listed for this specification, I've seen 15K Ohms or 45K Ohms.

My ohmmeter, having 3volts for its internal battery, isn't capable of detecting current when the knee on non-linear resistance is 5 volts or higher. However, my "Ground Fault Detector" is a regular, cheap ohmmeter that has 36 volts (four 9-volt batteries) in series with one of the leads. It also has a resistor in series to keep the meter reading full scale.

The technical name for this device is "Insulation Tester", but at the time, the only insulation testers on the market used a voltage that was too high for fire alarm circuit safety, so I had to make it myself.

Having never seen the building before, and no access to anything like an as-built diagram, I used the technique of disconnecting parts of the loop, and looking for the troubles on the panel. I combined that troubleshooting technique with my "Ground Fault Detector", and found a surge suppressor that had decreased its protection voltage from 25 volts to around 15 volts. The surge suppressor was on the Signaling Line Circuit (SLC) because the SLC went to an out-building electric fire pump about a block away.

Even though the panel didn't show it, I had found and fixed the ground fault in about three and a half hours.

The other service company had used their expensive ohmmeters, with internal 9-volt batteries, and couldn't find the 15-volt surge suppressor problem. Using my homemade "Ground Fault Meter", I fixed it. We got a new service contract.

Ground Fault Meter

The Ground Fault Meter is actually an "Insulation Tester". These are commonly used by electricians for testing the insulation on higher-voltage equipment or even power lines. Search the web to find these. There are at least a dozen manufacturers that make them.

There are a number of insulation testers on the market that can be switched to a low enough voltage to be used on fire alarm circuits. Any voltage produced by the tester below 45 volts is generally safe for fire alarms. Above 45 volts, and there is a remote possibility of some damage to the fire alarm system.

And yes, a lot of commercial meters can be set to use a low enough voltage to be safe, but they come with an unwritten caveat: whoever uses the switchable-voltage insulation tester has to remember to use a low enough voltage setting. Failure to remember to make sure of the setting can damage the system.

I don't trust myself to always, and every time, remember to make sure the voltage is low enough. Because I'm not absolutely perfect, and might not remember to set the voltage low enough on the insulation tester, I use a homemade tester, it can't exceed the 45-volt damage-threshold.

Build Your Own Tester

To build an insulation tester for use on fire alarm systems, use the instructions printed in the Nuts and Volts Magazine September 2010 Issue, Page 42 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 is a good magazine for the electronic hobbyist, and besides, you can read the full article.

The advantage to building your own tester is that you learn more about soft ground faults, and more about the testing for soft ground faults. Not only that, but without spending much money, you can possess a very useful test instrument.

The disadvantage is it really is a one-person test meter. It will never be an industrial strength meter to be shared with other technicians around the shop; unlike most store-bought meters, it won't stand up to abuse.