#128 – Wiring Ground Faults

by Shawn Lee

Ground faults can be quite troublesome when dealing with fire alarm system malfunctions. While ground faults may not be the most numerous issue one encounters, it is one of the most problematic. The reason that ground faults cause such issues is because the fault can affect more than one circuit at a time, and the grounded conductor or component could be anywhere. This article discusses the what, where and why of ground faults, and gives information on identifying and diagnosing what to do in the situation of a ground fault issue. Two-Wire Detectors

What and Where is a Ground Fault?

A ground fault is defined as the unwanted grounding of one or more conducting wires. This can occur in several places. It can happen inside the fire alarm control unit enclosure, metal raceway, metal junction box, or any other location in which conducting wires and an earth ground source are close in proximity. Ground faults can potentially cause circuits to operate improperly due to the current being “bled” off to ground before making its way to devices.  Depending on the circuit type, ground faults must be indicated at the fire alarm control unit. NFPA 72, Chapter 12, Circuits and Pathways (Sections 12.3) will provide useful information. Hopefully the fire alarm system will indicate which circuit has the ground fault condition, making it easier to locate and repair the fault. If it is a system that only alerts you to the presence of a ground fault condition, but not the circuit, then the job will take longer to troubleshoot.

Why Does a System Develop a Ground Fault Condition?

There are different reasons why your system may develop a ground fault condition. For example, in newly installed systems, it may have to do with how the conductors were installed in the metal conduit. Sometimes conductors are skinned as they are pulled in the conduit and the copper conductor becomes exposed. If it is not realized that the conductor insulation has been damaged, but the system is otherwise correctly installed and tested, the condition may be evident when the control unit is first powered up. In this case, the problem can be corrected so the Authority Having Jurisdiction (AHJ) will accept the system.
There are times when a ground fault is not apparent when the unit is powered up. For instance, if there is a skinned conductor(s) but it is not in contact with the metal conduit or any metal junction box right away, then the ground fault may not be evident. There is technically no ground fault yet. When the system is tested everything checks out and is accepted by the AHJ. However, at some point later, the problem will show itself. Usually this occurs if the system is subjected to ambient conditions, has settled over time, or undergone some type of movement which causes the skinned conductor to connect with metal.

Ohm Meter

Use an Ohmmeter

Remember, if fire alarm circuit conductors are in contact with the grounded metal raceway or metal junction boxes, the problem will eventually be found using an ohmmeter. One or more circuit conductors will have continuity between it and a reliable grounding point. Finding the ground fault is only a matter of time and patience — and relatively easy to repair.

A Ground Fault Indication That Has Nothing to Do with a Grounded Circuit

There are times when a ground fault indication on the control unit has nothing to do with a grounded circuit. The following is an example a Fire Tech Instructor encountered while working in the field.

● Ground Fault Example

A fire alarm system with an intermittent ground fault condition had been giving us fits for two weeks. This was an older conventional system and did not indicate where the ground fault was. In the middle of trying to figure out which circuit had the problem the ground fault indication would go away. This made troubleshooting the problem even more difficult. We inspected and tested each field circuit looking for any indication of a ground fault and found nothing. We inspected inside the junction boxes for bad or damaged conductors. We looked for moisture in the conduit. We still found nothing. Then we even tried to narrow it down to a certain time of the day, but there was no consistent time of the day for the problem.
After two weeks, I happened to remove one of the batteries to clean the enclosure. I noticed there was a wet spot and paint had peeled up leaving a small area of bare metal. I checked the bottom of the battery. The battery had a tiny crack and was leaking a small amount of electrolytic fluid. The fluid would contact the bare metal. There was a complete circuit from the battery charger – through the battery – to the leaky fluid – to the grounded enclosure, which created a ground fault condition. We cleaned it up, replaced both batteries and did not have another issue from that fire alarm system. Luckily, we did not replace the control unit circuit board, as that would have not solved the problem. It was by chance that the problem was resolved.


Troubleshooting ground faults isn’t much different than troubleshooting any other electrical fault. Use a systematic approach to isolate the problem. In most cases there will be one or more conductors, or even whole circuits, that have made contact with a grounded piece of metal. Always keep in mind when it comes to fire alarm control units indicating a ground fault condition – sometimes a ground fault indication has absolutely nothing to with a grounded circuit. Every now and then there could be another reason — and that reason might be a little unusual – electrically speaking!

To learn more about basic fire alarm functionality, check out Fire Alarm and Detection Equipment NFPA 72 2016, L1