Douglas Krantz - Technical Writer - Describing How It Works

Why Won't the Added Horns and Strobes Work?

When measuring voltage on the terminals of the horn or strobe when the panel is not in alarm, the positive terminal should measure negative and the negative terminal should measure positive

Hi Douglas,

We have a 2 wire, 24 volt dc ammonia detection circuit system with 4.7K ohm End of Line (EOL) Resistor supervision. For the Notification Appliance Circuit (NAC), the devices are strobes and horns only.

We attempted to add two horns and 2 strobes to the NAC circuit by extending the circuit and relocating the EOL resistor to that new end of line. When we did this, we get a trouble light. The cause of the trouble that has been isolated by troubleshooting to the NAC circuit.

The new added strobes had a different power level than the ones already there, so we ordered identical ones.

Still having the trouble light lit on the panel.

Any suggestions?

PLease Advise, M D

There are several possibilities, but one is very common, even for professional fire alarm installers; the horns or strobes may be installed with the incorrect polarity.

For many people, this is a problem because they used a voltmeter to check polarity on the wires. Then they connected the wire that measured positive to the "+" terminal on the horn or strobe and the wire that measured negative to the "-" terminal. Connecting the horns and strobes that way is wrong.

Now wait a second. Why is that wrong?

The problem is that when the wire polarity is being measured, the panel is not in alarm. The voltage on the wires at this time are for the panel's supervision of the wires.

The only time it's checking continuity (supervising the wires) is when the panel is not in alarm. That checking continuity is what the end-of-line resistor is for at the end of the circuit; it completes the circuit and allows the panel to check continuity through all of the wires.

When the panel goes into alarm, using an internal relay the panel reverses the voltage. Then, when it's in alarm, the panel should be applying positive voltage to the "+" terminal, and negative voltage to the "-" terminal of the horn or strobe.

The reason for this voltage reversal is so the horn or strobe won't conduct current while the panel is not in alarm; when the panel is checking continuity. There's a diode inside the horn or strobe to prevent the current from going through.

It's only when the panel is in alarm that the wire marked positive has "+" voltage and the wire marked negative has "-" voltage. That's when electrical current should go through the horn or strobe so that it sounds off or flashes.

Bottom line, for a conventional NAC circuit, when the panel is not in alarm, connect the wire that has been measured negative to the "+" terminal and the wire that has been measured positive to the "-" terminal of the horn or strobe.

Of course, there could be other things causing the problem, but I would check voltage polarity on the horn or strobe, first.

Douglas Krantz
Life Safety
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