Douglas Krantz - Technical Writer - Describing How It Works

How do you Troubleshoot an Addressable System?

Divide and Conquer troubleshooting involves dividing a circuit in half, and determining which half has the trouble.

Greetings Douglas,

I have a problem on the Fire Alarm Control Panel (FACP) where an addressable device does not appear on the control panel. The number of addressable devices are around 72, bit the FACP shows me more than 190, or 200, or something like that. There is no short circuit, open circuit, or ground fault. What do you think the problem is?

Thank you, M N

No matter what the panel says, or what a multimeter says, all faults are either an open (or partial open) or a short (or partial short). Both causes could be in the building wide fire alarm system at once, and there may be more than one problem to contend with. Shorts could be inside a device, in the building wiring, or in the panel; opens could be inside a device, in the building wiring, or in the panel.

A ground fault is a short, and ground faults require two shorts in order for a ground fault to affect the fire alarm system.

At all times when troubleshooting, keep an open mind. Never assume that you know what the problem is until the problem is fixed.

When working on fire alarm systems, do the easy parts first. Maybe you will find the problem without getting a tall ladder out. If you do the easy parts first, and the problem seems to be in a more difficult location, at least you will have a better idea of which hard direction to go to get at the problem.

Take Pictures with your Cell Phone

Before you disconnect any wires, take pictures of the wires so that when you put them back exactly the way they were. That is the only way you will keep from making the system worse and much harder to troubleshoot.

The following can't be used as a step-by-step instruction manual, all troubles in fire alarm systems have different causes. This can only be used to get general ideas of what to do. You have to make all the decisions and decide which direction to go at any time.


Keep in mind that the display on the panel doesn't show what any trouble on the system is, the display on the panel will only show you that there is trouble on the system. The first thing that you do, though, is read what the display says, and figure out what it means.

Is the panel saying that it is having troubles with 190, 200 devices? If so, the panel is probably not able to detect these devices. If this is what the panel is showing, the only thing you can know is that the panel isn't able to detect those devices.

Now you need to look at the voltage on the Signaling Line Circuit (SLC or Addressable Loop) itself. If there are that many devices that the panel can't see, then you need to figure out why.

The screw terminals in the Fire Alarm Control Panel (FACP) is where to start.

Normal Unsteady Voltage

On the Signaling Line Circuit, what is normally there is a very unsteady 10 to 20 Volts DC. The voltage will bounce around there every half of a second or so, and not be steady at all. Get used to it. This unsteady voltage is what is normal.

Zero Volts on the Screw Terminals with Wires Connected

Go into the panel and measure the voltage on the addressable loop (Signaling Line Circuit or SLC).

When measuring the screw terminals, if the voltage is zero or close to zero, then either something on the circuit is pulling the voltage down to zero, or the panel isn't providing voltage. Don't assume where the problem is located, confirm whether the problem is with the panel or with the wiring in the rest of the building. Check out which is causing the zero volts.

Normal Unsteady Voltage on the Screw Terminals with the Wires Disconnected

Disconnect the addressable loop wires from the screw terminals. Tighten down the screws on the screw terminals to make sure you are making an accurate reading.

If there's 15 or 20 volts showing on the screw terminals, that means that when the wires are connected to the panel, something on the addressable loop, somewhere in the building, is pulling down the voltage on the loop. Reconnect the wires to the screw terminals.

Do the same test for voltage on the devices elsewhere in the building - go to the Divide and Conquer section.

Zero Voltage on the Screw Terminals with the Wires Disconnected

First, make sure the screws are tightened. Again, wait a minute to make sure the panel will restart the loop, then check the voltage on the screw terminals again. If it's still zero, try a restart on the panel. If after a minute there's still no voltage, power down the panel (disconnect the batteries as well as turning off the circuit breaker) and power up the panel again. Wait 5 minutes to allow the panel to stabilize.

If there's still zero volts on the screw terminals for the Signaling Line Circuit (SLC) after a minute, there may be something wrong with the panel itself.

Divide and Conquer

Divide means to disconnect wires so the addressable loop is in two halves, and check each half. Conquer means that after figuring out which half of the addressable loop has a problem. Divide the half that's bad and check which one is bad.

Repeat this division and check which half is bad until the divided half of a loop is short enough to troubleshoot.

Often, addressable loops have T-Taps. If you find a T-Tap, that makes it easier to divide because that makes three even shorter loops.

Divide the Addressable Loop

One or more devices could be connected plus to minus causing a partial short, wires could be connected plus to minus in the back-box behind a device on in a junction box causing a partial short, a device somewhere could be bad causing a partial short, there could even be a wire-to-wire short somewhere.

Something is pulling the normal voltage put out by the panel to zero volts.

What you need to do is divide the working part of the fire alarm system from the non-working part of the system.

If I or anyone else just worked on the system, that is where I would start there because that may be the source of all the problems.

Guess and Confirm

All troubleshooting is a guess and confirm process. Guess where the wires run in the building and then confirm, guess where to divide the system and then confirm, guess which side of the division has the problem and then confirm.

Voltage Before Disconnecting

At any location, before disconnecting any wires, check the voltage. If there's voltage, look at the voltage very carefully. Is it steady? If so, exactly what is the voltage? Write down the exact voltage, including the numbers after the decimal point.

Is the voltage unsteady? If so, write down the approximate voltage, including the high voltage readings and the low voltage readings.

Go back to the panel and measure the voltage on the addressable loop measuring at the screw terminals. If the voltages are different, then you might be on the wrong loop. If the voltages are the same, check the display on the panel to see what it says.

Something may have changed. Try and figure out what changed.

Voltage After Disconnecting

If there's no voltage on the wires where you're at, take pictures and disconnect the wires from each other.

Does the voltage return on one of the pairs of wires? If so, the problem you are having is with the part of the circuit that is farther from the panel because now the voltage from the panel can at least get that far.

If the voltage has returned on a pair of wires, reconnect the wires using the pictures you just took. Confirm that the voltage goes away. If the voltage stays after reconnecting the wires, check the panel. Something may have changed. Try and figure out what changed.

If the voltage goes back to zero, leave the wires connected and divide the loop somewhere farther from the panel.

Zero Volts after Disconnecting

If there's no voltage on the wires where you're at, that means that the problem is on the wires somewhere nearer the panel. At this point, leave the wires disconnected until later so you don't get confused about where you're at. After finding and fixing the problem, you can return to reassemble the parts you took apart.

Check for Troubles on the Panel

Remember, there may be multiple troubles, check the panel each time you correct anything.

Also, keep in mind that in order to troubleshoot, you have caused more troubles. Reconnect everything the way it was.

Check the panel. Make sure there is no yellow light on the panel. The panel should only have a green light. If there's still a yellow light, find the cause and fix it.

Closing Up

Do a walk-through of the building. Make sure everything that was taken apart is put back. Look at all detectors, all covers, anything that was disconnected to make sure everything is returned to normal. Do this even if you plan on returning the next day; never count on returning right away.

Douglas Krantz
Life Safety
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