Douglas Krantz - Technical Writer - Describing How It Works

How do I Troubleshoot this Turned-Off Fire Alarm System?

By Douglas Krantz | Maintenance

How do I Troubleshoot this Turned-Off Fire Alarm System?

How do I Troubleshoot this Turned-Off Fire Alarm System?

Greetings Douglas,

I have an old Fire-Lite MP-24 panel with 2-wire smokes and some horn strobes etc. It had been installed back in the 80s, when this was a group home. After that, a fire alarm system was no longer required, so the people owning the building shut down the fire alarm system.

Recently, I decided to reconnect the system. I replaced all the smoke detectors for it. It had been using only ESL 425Cs from decades ago, Now, it is using System Sensor 2W-Bs.

So I tested the field wiring from the panel for the detectors/pulls and getting 4.7k EOL as should be.

And before I replaced the smokes, I was getting 9.7k on the NAC EOL wiring as well, as it should be, and now after finishing up, I've noticed that testing the wiring shows 18k ohms, I traced back to the EOL and checked it, the resistor was good, but I went ahead and replaced it, because I had it down and had new ones anyways, so I moved to the next towards the panel, testing backwards to see if I get the EOL.

There were no issues with the strobes or horns before the replacement of the smoke detectors, and I've tested from the EOL back towards the panel.

So, I was just wondering if you had any theories on what could be causing the 18k ohms reading now compared to the correct 9.7k ohms that it was originally, I did run into an issue with one smoke detector having a bad wire probably from its original installation back in the 80s, but I didn't see any other issue.

I did also put a 10k resistor on the field wiring at the panel and tested back towards it in a couple of rooms on the end where I think the problem is, and I got the 9.7k reading going back to the panel from them.

All the Zone wiring for the smoke detectors tests completely fine, 4.7k EOL showing with resistance metering, just some odd issue with the NAC popped up, where its 18.0-18.8k ohms now, except for when I measure going away from the EOL the devices on that side seem to hit the EOL and it reads as expected.

Is it possible a NAC went bad (which I don't think one is bad when I was getting the correct EOL reading before), or is it more likely that the wiring has a short somewhere between where I'm getting correct readings and was measuring from the panel with the field wiring off getting 18.8k instead?

Thank You, GB

Supervising the Wires

Here's a quick tutorial on how the fire alarm panel supervises the loops.

The panel is supervising the building's wires of the fire alarm system. It doesn't check to see if the devices even work, only that the wires are still connected to everything.

The panel supervises the wires by running a continuity check of the wires. If a wire breaks, the panel says "Trouble" by turning on the trouble light and buzzer. Then again, if a wire comes loose from a device's terminal, the says "trouble".

The continuity check is the wires in a complete circuit. If the loop, for either the input circuit (IDC or Initiating Device Circuit) or the output circuit (NAC or Notification Device Circuit) just ends at nothing, the circuit isn't complete; the loop is just a pair of wires.

The ends of the wires have to be connected in order for the panel to check for continuity.

Input or Output Shorted Circuit

Remember, we are looking at both the IDC (input circuit) and the NAC (output circuit) from several directions at once.

IDC: The panel checks for continuity of the wires on a complete circuit, but at the same time, if the wires are shorted together, the panel considers that a fire alarm.

NAC: The panel checks for continuity of the wires on a complete circuit, but at the same time, if the wires are shorted, the panel won't apply power for the horns and strobes. The panel won't apply full power to a shorted NAC circuit because applying the full power from the panel to a shorted circuit will short out the entire panel, making the whole fire alarm system useless.

End of Line Resistor

The end of line resistor doesn't detect fires and it doesn't sound any alarm; it's a circuit completer. Remember, a circuit is a circle, it must be completed.

In any circuit, the electrons are continually reused. For the electrons to be reused over and over again, they have to travel in a complete circle. For the conventional fire alarm loop, the electron path around the circuit (circle) is:
  • Starting at the panel
  • Through one of the wires, end to end
  • Through the end of line resistor
  • Through the other wire, end to end
  • Back into the panel
  • Through the power supply of the panel
  • Back to the panel's starting point, to be reused

Stopping the current anywhere in the circle (circuit) is stopping the current everywhere in the circle.

In a conventional fire alarm system, the end of line resistor is a circuit completer, but it is also a current limiter; the end of line resistor prevents the free-flow of electrons.

To prevent false alarms from a shorted IDC, or else the panel's power not being applied to a shorted NAC, the end of line resistor is a small-current conducting, non-shorting device.

Get a Manual

Make sure you have a manual on the MP-24 panel. It has lots of information. The installation manual can be found at:

Start by pretending that the system is a brand-new system. Remove all the devices, including smoke detectors, pull stations, horns, strobes, etc. Also, disconnect the input and output loops from the panel. At this point, you can concentrate on the wires, and not have any interference from any devices or the panel.

Wire-nut all of the wires through, red wires to red wires, black wires to black wires; make the loops continuous from the panel to the end of the loop.

Then, connect the proper end of line resistors to the end of the circuit. This gives you a complete circuit.

Use you Ohmmeter

The only time you can use an ohmmeter to get an accurate reading on either the NAC or the IDC is when there are no devices on the loop. Horns, strobes, and smoke detectors have transistors and diodes inside them; horns, strobes, and smoke detectors are non-linear devices and can interfere with an ohmmeter's accuracy.

At the panel, measure the resistance of the wires for each loop. The ohmmeter readings should be the same as the resistance of the end of line resistors.

If the readings don't match the resistance of the end of line resistor, find out where the loop is open or shorted. Fix the open or short.

Once the ohmmeter readings match the resistance of the end of line resistor, the loop is a normal loop. That is exactly what the panel is looking for when it's supervising the wires of the circuit.

Connect the Devices

Once the ohmmeter says that the wires are a normal circuit, put the ohmmeter away, and connect wires to the panel.

Color of the Wires: Don't be concerned about the color of the wires. In a conventional fire alarm system, the red wire can be the one you measure as either positive or negative, and the black wire can be the one you measure as negative or positive.

This is especially true of the NAC circuit. Depending on whether the circuit is being supervised or the circuit is sounding the alarm, the NAC circuit will always flip voltage on the wires.

IDC: Use your voltmeter. Each time you connect a smoke detector, use your voltmeter to determine the polarity of wires. Connect the measured to be positive wire to the "+" terminal, and the measured to be negative wire to the "-" terminal.

NAC: Use your voltmeter. When you are connecting the NAC devices and they are not sounding the alarm, the polarity is backward.

Each time you connect a horn or strobe, use your voltmeter to determine the polarity of wires. Connect the measured to be positive wire to the "-" terminal, and the measured to be negative wire to the "+" terminal.

You read it correctly.

When the alarm is not sounding, the wires the horns and strobes are normally using the wrong polarity. Once the fire alarm control panel is sounding the alarms, the NAC output of the control panel will automatically change polarity, and the polarity on the voltage going into the screw terminals of the horn or strobe will be correct.

Test the System

At this point, you need to test the system to see if it works. Use smoke to see if the smoke detectors sense smoke, actually pull the pull stations to see if they can be used and see if they work, and listen to each horn and look at each strobe to determine if they work.

Douglas Krantz
Life Safety
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