Anyone who has ever serviced a fire alarm system knows that ground faults can be quite a challenge, especially if the fault is intermittent. Many don’t realize the importance of testing the ground-fault feature and the effect it can have on a system if it is not corrected. The worst case I ever discovered was due to a ground fault—the system did not shut down the air-handling equipment as it was supposed to do when the system went into alarm.
I have found fire alarm systems with the ground reference disconnected either at the fire alarm control unit or in a junction box above the ceiling. If the ground reference is not connected, the panel can still experience a ground fault, but it would not show up as a trouble condition.
Testing is not difficult and can be done quickly. You do not need to go to the devices in the field; you can test it right at the panel. The fire alarm panel monitors all the circuits, so remote testing is not necessary. Simply take a piece of wire, touch one end to a conduit or metal component of the building or insert it in the ground terminal of an electrical outlet. Touch the other end to an initiating device circuit or signaling line circuit terminal on the fire alarm panel. If it is connected properly, it will indicate a trouble condition. If you don’t get a trouble signal, check for a proper connection of the ground reference. Some panels have an adjustment to change the resistance to ground level.
Most ground faults occur when the conductor insulation is damaged from pulling wire through a raceway with rough edges. The conductor can also be damaged if it is exposed to water or pinched when installing a fire alarm device on its backbox. This is a common problem when monitoring equipment on a building’s exterior. A ground fault can occur on either the positive or negative side of a circuit, and it can even affect speaker systems when the metallic shield on the speaker cable is grounded.
Troubleshooting a ground fault
In most cases, troubleshooting a constant ground-fault indication is far easier than troubleshooting an intermittent fault. However, it is possible to have more than one ground fault at a time, which can be misleading. Each fire alarm panel has a threshold of a certain resistance to ground, and you can get a trouble condition when they are all present. If you disconnect a circuit and the ground-fault trouble clears, it may only have lowered the resistance to ground below the panel’s threshold. You troubleshoot a ground fault the same way that you would troubleshoot any other circuit fault.
Disconnect one circuit at a time and, using an ohmmeter, read the resistance between the circuit and ground. You may have to remove the end-of-line resistor temporarily. If you read any resistance to ground, you probably have a ground fault on that circuit. Now, simply keep breaking the circuit in half and testing in both directions to determine the fault’s location. Even if you find the fault and fix it, it is always a good idea to individually disconnect each circuit and test for a resistance to ground. If you don’t do that, you may have only corrected part of the problem and the trouble may return.
Intermittent ground faults are much more challenging than constant ground faults. If you have an addressable fire alarm system with an event buffer and a ground fault occurs, you should have information about which circuit reported the trouble.
Conventional fire alarm panels don’t have an event buffer to identify individual faults, which makes them more complicated than addressable panels. Try to find out if the fault always occurs at the same time of day, which may prove very useful. I once had to set up a video camera aimed at the fire alarm panel display. When a trouble occurred, I had the building staff note the time so I could look at the recorded video to see which circuit indicated a trouble. Obviously, it took a few days to locate and correct the problem.
It is important to test and correct ground faults, because they can keep your fire alarm system from working properly. Testing is also an NFPA 72 requirement. Table 126.96.36.199, item 3(3) requires testing the ground-fault circuitry at the initial acceptance test and then annually.
Understanding what causes ground faults and learning good troubleshooting skills will make your job much easier.