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Fire Alarm -- Description

A Notification Appliance Circuit (NAC) that has a short will never have power applied because that short could otherwise shut down the fire alarm system.
A shorted Notification Circuit, whether it's a wire-to-wire short, a shorted device, or a shorted end-of-line resistor, will never have 24 volts applied from the fire alarm panel.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

What will a Shorted Fire Strobe Do?

By Douglas Krantz

Fire Alarm NAC Circuit Shorted Trouble

Some time ago, at a high rise business and condominium building, I inspected a fire alarm system. On arrival, the fire alarm control panel showed trouble with the strobe NAC circuit on one floor.

For several months, this trouble had been on the panel, but because no one there thought the trouble was important, the owners had not called for service. (I'll address the failure to take care of life safety troubles some other time.)

Now, on a fire alarm NAC circuit, there are three major causes of trouble: a broken wire or open connection, a ground fault, a wire-to-wire short.

Supervision of the NAC Circuit

When the NAC circuit is not active (24 volt power is not applied to power the devices on the loop), the loop is being supervised. During supervision, inside the panel, the circuitry detects opens (no supervision current passing through the end-of-line resistor), and wire-to-wire shorts.

If an open is detected, a yellow light shows up on the panel. Some of the devices may still work because, in alarm, 24 volts is still applied.

The detection of a wire-to-wire short is a different matter. The detected short still turns on up the yellow light, but there's also a protective circuit that also gets turned on. The protective circuit makes this a larger problem.

Seeing a shorted strobe circuit on the fire alarm panel's display, I knew this was the case.

NAC Power Protection - - Keeping the Rest of the Fire Alarm System Working

Short Detection is a protective circuit; when there's a short detected during supervision, the automatic protective circuit in the fire alarm system won't let the NAC circuit activate. In this case, if there was an alarm, the NAC module would fail to apply 24 volt power to the strobes and none of the strobes on the whole floor would turn on.

It also protects the rest of the fire alarm system by preventing a shorted NAC circuit from shorting out the rest of the 24 volt power supply. On smaller fire alarm systems, connecting a shorted NAC circuit to the power supply might cause a catastrophic failure of the entire fire alarm system.

In other words, this protection of the rest of the fire alarm system means the NAC circuit will never work; the strobe circuit would not turn on.

Fixing the Short

Months earlier, a pipe in the ceiling had leaked and a ceiling strobe had been flooded. Later, during the fire alarm inspection, the problem could was found visually because water from the broken pipe had left a dirty film inside the clear plastic dome of the strobe.

Right then and there, using an owner supplied strobe, the shorted strobe was replaced. Once the strobe was replaced, the short was corrected, and all the strobes on the floor passed inspection.

The fix was easy.

Watch Out for Shorted NAC Circuits

Remember, if there's a short on the NAC circuit, none of the devices on the whole circuit will receive power to activate.

There's also a moral to this story. Owners and Building management - - when there's a problem with the fire panel, call for help right away. That way, in case of unscheduled fire, the fire alarm system will work.

 Get your free diagram showing supervision for Class B wiring

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