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Fire Alarm -- Description

Measure Supervision Voltage before pressing reset or disabling any zones to find out what is happening in the circuit
To find out what's happening in the field, measuring Supervision Voltage at the panel can tell the technician whether it's a normal circuit (same voltage as a normal zone), an open wire (higher voltage than normal), water on the wires or smoke detector in alarm (lower voltage than normal), or a shorting device like a pull station or heat detector, or even a wire-to-wire short (zero or near zero voltage).
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

How Do You Troubleshoot Using Supervision Voltage?

By Douglas Krantz

Troubleshooting is like fishing; patience is the key, but not too much patience. If using one method, the fish is caught, go for it. But, if after a while no fish, change methods or you're wasting time.

It's the same with troubleshooting. If, using one method of troubleshooting you find the problem, go for it. But if the problem remains elusive, change troubleshooting methods.

Supervision Voltage Method

With Fire Alarm Systems, one method of troubleshooting is measuring supervision voltage. It's the voltage at the panel's IDC input terminals (Initiating Device Circuit for conventional detectors, pull stations, waterflow switches, etc.) or NAC output terminals (Notification Appliance Circuit for the horns, strobes, speakers, etc.).

Supervision voltage is generated by the fire alarm panel. When the fire panel is looking for trouble on the system, the panel uses the supervision to determine, among other things, whether the wires are still connected.

Using supervision voltage, the panel:
  • Supervises the wiring, making sure the wires and connections remain good at all times, in either Class B or Class A wiring systems
  • Powers the detectors
  • Detects if a switch is closed or a detector is in alarm
Supervision voltages can be:
  • Near zero volts
  • Low
  • Normal
  • High

Supervision Voltage Measurements

Normally the supervision voltage is lower than the open circuit voltage, but higher than the alarm or short circuit voltage.

For troubleshooting, one can use the manufacturer's maintenance manual to find out the normal supervision voltage. This is ideal, but most of us aren't so fortunate as to have this documentation with us while troubleshooting a system that has been in place for several years.

Instead, after checking the panel to see what zone is off-normal, make a comparison of the voltage between a normal zone and the zone in trouble or alarm.

Supervision Voltage Equaling Zero

On the off-normal zone, near zero volts indicates a pull/flow/heat detector/gatevalve switch in alarm, or the wires are shorted. Usually it's a switch in alarm.

Check what's on the zone. Spending a little time walking around the building and looking for the pull station in alarm or gatevalve off normal may save a lot of time troubleshooting.

One time I was outside a building in the parking lot when the alarms sounded. I waited for the fire department to arrive and entered the building with them. They didn't find anything, so they started to leave.

But, by checking the supervision voltage, I found a waterflow switch zone still showing zero volts, meaning meant water was flowing somewhere.

I then called the fire department back into the building. It wasn't until after they came back inside that they saw water flowing out of an unoccupied apartment in the basement.

Supervision Voltage Low

On the other hand, if the voltage is a little low to moderately low, the trouble may be a smoke detector in alarm.

One time, in a brightly lit stairwell, a smoke detector in alarm was missed by the fire department because the LED was washed out by sunlight.

Then again, the lower voltage may be water on the line, partially shorting the circuit.

More than once, letting the water out of the junction box has fixed that problem.

Supervision Voltage Normal

If the supervision voltage is the same as other zones, go on to using another method of troubleshooting.

Supervision Voltage High

If the voltage is higher than normal, then it's more likely an open circuit.
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Supervision Voltage

Reading the supervision voltage on the zone having problems and seeing whether it is too high, normal, too low, or near zero helps narrow down what to look for and where to look.

But, as I said at the start, this is just one method of troubleshooting. If reading supervision voltage doesn't help, go on to look for the fire alarm system problem using a different method of troubleshooting.


Douglas Krantz

Describing How It Works

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Electrical Flow

On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at