Douglas Krantz - Technical Writer - Describing How It Works
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Why is the NAC Shorted?

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Hi Douglas,

I have a question about the alarm circuit in my Zonesence (Ampac) FACP. When I install an alarm bell, I connected 1 diode at + pole of NAC (Notification Appliance Circuit), the Fire Alarm Control Panel (FACP) always shows "Alarm Short Circuit" on the FACP.

I've already checked the wiring with my ohmmeter, it's OK. I've also changed the bell (buying a new one) but the panel still shows a fault. How can I solve this problem?

Thanks so much.

Signed H V L

The short on the wires is caused by installing the bell backward. You are making your measurements when the panel isn't in alarm. At the time you're making your measurements, the bell should be standing-by.

I know this doesn't sound right, but I'll explain it. When the bell isn't supposed to sound the alarm, you're supposed to connect the wire you've measured as positive to the negative terminal of the bell, and connect the negative wire to the positive terminal.

You aren't the only one to be having this problem. I've seen quite a few instances where fire alarm installers put the horns or strobes in backward because they put the positive wire to the positive terminal and the negative wire to the negative terminal. Sometimes this reversed wiring doesn't show up on the panel and no one tests it. (Always test to see that it really works because maybe someone's life depends on it working.)

Bell Standing-by versus Bell in Alarm

When there is no alarm (the bell is just standing-by), the bell is designed so it doesn't conduct electricity. On the other hand, when there is an alarm (the bell is sounding the alarm), the bell is designed so it does conduct electricity.

Keep in mind though, it's the panel that provides the voltage on the wires in the first place, and it's the panel that controls the voltage polarity on the wires. Depending on the polarity of the voltage on the wires, the bell just does what it's told.

Normally, The Panel is Checking Continuity of the Wires

When the panel isn't in alarm, it's checking continuity of the wires. (Supervising the Wires)

It normally runs electricity through one wire, through the end-of-line resistor, and back through the other wire. That way, if a connection comes loose or a wire breaks, the fire alarm panel will see that there is trouble with the circuit. Then the wiring can be fixed before there is a fire.

Shorted Wires

Because a short between the wires would be a big problem, the panel is also checking for a short on the wires.

In essence, while just standing-by, the panel is watching to see how much electrical current is being used. If the current increases from what the panel thinks should be on the circuit, the panel considers that to be a short. This is the "Alarm Short Circuit" message you're seeing on the panel.

Basically, while the panel is standing-by, you've measured the polarity of the wires. After measuring, the measured positive wire was connected to the positive terminal of the bell, and then the bell conducted electricity. The panel saw that extra current running through the bell as a short.

Positive to Negative and Negative to Positive

When it is only standing-by - not in alarm - the positive wire should be connected to the negative terminal and the negative wire should be connected to the positive terminal.

Connecting the bell that way will also mean that when the panel is in alarm and reverses the polarity on the wires, the bell will conduct electricity and sound the alarm.

Douglas Krantz
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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