By Douglas Krantz
A ground fault is an electrical leakage from the fire alarm system to building ground -- somewhere in the building.
The ground fault light on the fire alarm system front panel is a good indicator that something needs to be fixed, but the indicator light often gives false positives or false negatives while troubleshooting.
Ground Fault Detection Instrument
Often, the method is to disconnect one loop or pair of wires at a time from the fire alarm panel until the ground fault light on the front turns off. Once the pair of wires with the ground fault is found the service technician uses the ohmmeter to look for the ground fault on the disconnected loop of wire.
Sometimes this doesn't work, and the service technician becomes frustrated.
The issue is that the ground fault light is not a good ground fault detection instrument.
On the other hand, if a fire alarm technician is experienced with the model of fire alarm panel and knows how to connect and read a voltmeter to detect the ground fault, the ground fault light on the front of the panel becomes a secondary indicator and can usually be ignored while troubleshooting.
The voltmeter is a much better ground fault detection instrument.
Problems with the Ground Fault Light
First, the light on the front of the panel saying "Ground Fault" is fickle.
The customer sees the light and calls for service. However, by the time the service technician arrives on site, the light might be on or it might be off again.
False Negative Indication
The light being off on arrival may mean it's an intermittent hard ground fault; sometimes it's making a solid connection with ground and sometimes it's making no connection at all.
The fire alarm system doesn't show a ground fault at the moment, but the potential for the ground fault still exists. The light being turned off on the technician's arrival is a false negative.
Then again, the light being off on arrival may mean the fire alarm system has a soft ground fault; partially leaking current all the time. The amount of current its leaking varies up and down.
The ground fault is still there, but at the moment, the leakage isn't quite enough to turn on the ground fault light. In the case of a soft ground fault, the ground fault light being off when the technician arrives is a false negative.
False Positive Indication
The ground fault light may be on when the technician arrives on site, but the ground fault itself has totally gone away forever.
The issue here is that the panel keeps showing the ground fault after the ground fault is removed. In other words, the ground fault light is latched on. Only a few models of fire alarm panels latch the lights on when there have been troubles. It helps if the service technician is familiar with that aspect when servicing these panels. The latching of the trouble light would be a false positive.
The ground fault light is still on while troubleshooting by disconnecting loops, even after the affected loop with a ground fault (A Signaling Line Circuit or SLC, ZAS, Data Loop, or whatever it's called) has been disconnected.
The problem here is the module connected to that loop has reported the ground fault to the panel using data over the Signaling Line Circuit (SLC). The panel obediently turns on the ground fault light. While troubleshooting, once that loop has been disconnected from the panel, the panel can't receive any signals over the SLC from the module. The inability of the module to report that the ground fault is disconnected from the panel means the panel's ground fault light remains on. This is a false positive.
Ground Fault Detection Delay
Be aware, some panels have a delay in their ground fault detection circuitry. With a delay, the panel only checks for the ground fault condition about once a minute; most of the time the panel isn't even connected to building ground. Just to be sure the ground fault is disconnected, the technician has to stand at the panel for a minute after disconnecting a loop. Once the loop is reconnected, the technician has to wait another minute to make sure the panel is back to the condition it was in before the loop was removed.
This slows down the technician tremendously.
During the minute between ground fault checks, the light on the panel may show a false positive or a false negative.
Internal Reference Voltage
Voltage is what the panel is looking at; voltage is what the technician should look at.
The panel, using its ground fault detection circuitry, compares an internal reference voltage to the voltage of the building ground.
The panel is looking for one of two conditions:
- Current is leaking to building ground somewhere else in the fire alarm system; the building ground voltage doesn't match the reference voltage. That's when the panel turns on the ground fault light.
- There is no current leaking to building ground somewhere else in the fire alarm system; the building ground voltage matches the reference voltage. That's when the panel turns off the ground fault light.
Finding the Reference Voltage
For the service technician, finding this reference voltage on the circuit board of the fire alarm system is almost impossible. There is no test point to connect a voltmeter to and there is no documentation to tell the technician where to find this point.
This is where the service technician needs to be familiar with the brand and model of the fire alarm panel being worked on. The battery voltage compared to building ground can tell the technician a lot.
Internally, on the circuit board, the standby batteries are connected to the + and - of the power supply. Knowing that, to find the power supply + and - voltage, use the battery terminals.
The panel is using a reference voltage, somewhere in between the + 24 volts and the - 24 volts (nominal) of the power supply, and comparing that reference voltage to the building ground voltage. As long as the panel can detect that the two voltages are the same, the panel is not detecting a ground fault.
To see if there is a ground fault, the service technician has to also compare these voltages.
Exact Reference Voltage isn't Important
The exact voltage isn't important --- it's the comparison between the reference voltage and the building ground voltage that's important. Remember, the batteries are connected to the + and - of the power supply.
No matter what the ground fault light on the front panel says, if the + and - voltages as compared to building ground are balanced (the + and - voltages are somewhat the same) there probably is not a ground fault.
If the voltages are quite a bit unbalanced, there probably is a ground fault.
Being familiar with the make and model of the panel will tell the service technician just where this balance should be between the + and -.
Providing a unique problem, though, is any panel that has a delay. Most of the time, it's not connected to building ground.
If there's no ground fault, no current leakage to building ground somewhere in the building, the battery voltage will "float".
No matter whether the voltmeter is connected to the + or - of the batteries while comparing battery voltage to building ground voltage, the meter is going to show less than the battery voltage, and this voltage is going to drift toward zero.
This is an indication that there is no ground fault.
On the other hand, if unbalanced and steady voltage is shown when comparing the battery voltage to building ground voltage, there is a ground fault.
Every minute or so the panel will also be checking for ground, just ignore what is seen during that time.
Ground Fault Lights Lie, Voltmeters Tell the Truth
While troubleshooting ground faults, use the voltmeter to compare building ground voltage with the voltage on the standby battery's terminals to test for a ground fault.
Using this comparison will show a truer indication of the existence of a ground fault than the ground fault light on the front of the panel.