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.
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.
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.
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.
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.