Douglas Krantz - Technical Writer - Describing How It Works

How do I Find the Ground Fault?

How do I Find the Ground Fault?

Greetings Douglas,

I need your help and tips for hunting ground faults. One of our fire panels is showing a ground fault on loop one.

On that loop, there are a total 32 devices and 27 modules. I tried removing the wires on the cable one by one. However, the SLC loop fault is gone only when all wires are removed.

If I remove wring from one detector, the fault is not cleared. When I remove detector #27 wring and any other detector, then the fault goes away.

Please suggest some tips to easily hunt ground fault with multi meter.

Thank you, MS

First off, DON'T LOOK AT THE GROUND FAULT LIGHT. It will only confuse you; it is there to tell the building owner to call you. See: Can the Ground Fault Light be Used for Troubleshooting?

Next, to get a general idea of what a ground fault may be, look up the article What is a Ground Fault?

Then, look up the articleHow do I Troubleshoot a Ground Fault? The article shows you some ideas for troubleshooting.

There are no "tips" to use that will easily find a ground fault. Just standard troubleshooting procedures.

Douglas Krantz

Mr. Krantz

I tried to remove 1 wire of loop one. If I remove all 4 loop wires, then the ground fault is clearing. I checked by removing 1 wire, but I'm not able to find ground fault on incoming loop or outgoing loop.

For Divide and conquer method, can you please be more specific? Do we have to remove only one device or two devices? Only when I am removing two devices from the loop wires does the ground fault clear.

I'm guessing problem is on detector #27, if I remove detector #27 wires as well as detector #26 wires, then I'm clearing the ground fault. If I remove only #27, I am not clearing the ground fault.

How do I find the ground fault?

Thank you, MS

Before getting into the ground fault or the fire alarm system, we have to understand some things about electricity. Remember that the building's fire alarm system, even the circuit wires and the detectors and modules, is a single electrical device.

Electricity follows rules. These rules are not made up by anyone, these rules have been figured out. The more you understand the rules that electricity follows, the easier it is to figure out what is wrong with an electrical circuit.


Take pictures. Use your cell phone camera as a memory device. Many wires are going to be disconnected, and the only way of making sure the wires are returned to the right connectors is to look at the pictures you've taken before disconnecting each connection. Don't assume that the wire colors are the same at each location. You didn't install the wires, don't trust that there aren't changes in color somewhere else.

Electrical Circuit

The most important thing about electricity is that it requires a circle to have electrical current. We can't lose the electrons as they flow, the electrons have to return to the power source in order to flow again. That means there has to be a complete electrical path for the electrons to flow from the power supply or battery, through the circuit, and back into the power supply or battery. If the wire is cut, or the switch is opened, the electricity stops because the electrons can't return to the power source.

The opposite is also true.

Electricity won't be able to flow if there isn't a pathway for the electrons to flow back to the power source. If an extra pathway is put in so the electrons can return to the power supply, then there is a complete electrical circuit and the electrons flow.

Multiple paths are each individual paths. If a wire is cut or the switch is opened in one path, the other path still has some electricity flowing through it. The total electricity is reduced because one path is blocked, but there is still electricity flowing in the other path.

Ground Fault Detector

Inside the fire alarm panel there is an electrical path to the grounded metal of the building. The ground fault light on the panel comes on anytime that electricity flows through this connection to ground.

Normally, nothing else in the entire fire alarm system, anywhere in the building, is supposed to be connected to the metal of the building. That means that there is no complete electrical path from the power supply of the fire alarm panel, through the circuitry with the devices, to building ground, and back to the panel's power supply through its ground fault detection.

If a wire on the circuit with the detectors and modules touches metal of the building, that touch completes the electrical path from the power supply, through the circuit, through the building ground metal, and back into the panel through the ground fault detection. Even though the wire is only touching the metal, the touch is now including the metal of the building in the electrical circuit (electrical circle).

Because current is flowing through the ground fault detection circuit, the trouble light and the ground fault light turns on.

Crooked Electrical Pathways

Lifting a single wire on the circuit won't help you find the ground fault. There are three-more wires that the electrons can use to flow through. Each detector and each module also conduct the electrons through them, so there are many pathways for the electrons to pass from one wire to another in the circuit.

All four wires of the circuit have to be removed from the panel in order to divide the circuit from the panel.

Divide and Conquer / Guess and Confirm

Once you have removed all four wires of the circuit from the fire alarm panel, you have divided the circuit from the panel.

Another way of looking at what you have done is that you've guessed the first step in the guess-and-confirm process. Now, you have to confirm whether or not you have guessed right.


To confirm your guess, use your ohmmeter.

The ohmmeter has a battery (power source) and two leads. Connect one lead to the metal ground of the building and the other lead to the wires of the circuit that has been disconnected from the panel.

The ohmmeter will show you if there is a ground fault. To show a ground fault, the electrical pathway for the ohmmeter is:
  1. From the battery inside the ohmmeter
  2. Through the lead that has been connected to the fire alarm circuit in the building
  3. Through the circuit wires
  4. Through the ground fault itself
  5. Through the metal of the building
  6. Back to the other lead of the ohmmeter
  7. Back to the ohmmeter's battery

If the ohmmeter says that there is an electrical path, you have confirmed your guess that the ground fault is on the wires, somewhere.

Detector #27

Detector #27 is housed in plastic. By itself, it will not conduct electricity and by itself, it won't be the ground fault.

However, at detector #27 you have a good starting point for the second guess, and that guess will have to be confirmed.

Remember, the ground fault is a wire touching ground somewhere in the building, and the only way of finding the ground fault is to divide the system, guess which side of the division has the ground fault, and then confirm the guess.

To divide the circuit, you have to disconnect all the wires so there will be two different circuits. Then use your ohmmeter again in the same way to confirm which set of wires has the ground fault.

Leave the wires disconnected so you can further guess and confirm at another point you've divided the circuit.

Divide and Conquer / Guess and Confirm

Go in the direction that shows a ground fault on the circuit. Guess a good halfway point. Divide the circuit at a convenient place. Confirm your guess with the ohmmeter.

Follow the Wire

Keep guessing and confirming until you find the ground fault, or at least get very close. At some point, the ground fault may be somewhere between detectors or modules. When this happens, change tactics to following the wire. Just don't change to follow-the-wire too early in the divide-and-conquer process, or you'll spend too much time following wire and not finding the ground fault.


Once you have fixed the ground fault, confirm that it is really fixed. When your ohmmeter shows that the ground fault is fixed, then you have to reconnect everything. This is the time you'll be glad you took pictures of every connection before you disconnected it.

Douglas Krantz

Mr. Krantz

I hope you doing well. At last, today, I cleared the ground fault. It was in a manhole.

Trouble Shooting

As I told you at detector #27, after removing wires I cleared the ground fault. For a week my head was blown thinking about the fault.

First, I found out which device is first from panel in the drawing. It was showing device Number 1, but in reality, the first device on the wires was a module. Then devices on were on the roof. The roof for #31, then again #29, then it's going to rest of devices.

I followed the divide and conquer method, dividing the loop until I found a device which is not clearing the ground fault. Module #27 on an under-ground cable is going to detector #27, then through the manhole, which is filled with water, an ending on the panel loop card input.


Due to bad installation, the fire cable insulation is damaged, and manhole is filled with rain water. As soon we removed cable and cleaned it with a dry cloth, ground fault was cleared.

As Built Drawings

The drawings cannot be trusted. I can only trust the Multimeter.

I thank you very much for supporting and briefing me on troubleshooting.

Thank you, MS

Good for you. You sound like you now know how to look for ground faults in a fire alarm circuit.

Now you can use your experience to find and fix more ground faults, and to find and fix many other troubles in a fire alarm system.

Keep up the good work. After getting more experience, you will become the expert that everyone else comes to for help.

Douglas Krantz
Life Safety
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