Douglas Krantz - Technical Writer - Describing How It Works
Go to the Fire Alarm Home Page of Douglas Krantz -- Describing How It Works
Go to the Fire Alarm Operation Map Page of Douglas Krantz - Technical Writer
Go to the General Electrical Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Description Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Installing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Maintaining Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Testing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Suppression Map Page of Douglas Krantz - Technical Writer
Go to the Science Article Map Page of Douglas Krantz - Technical Writer
Go to the Writer Home Page of Douglas Krantz -- Describing How It Works

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.

PDF Book PDF of Make It Work - Conventional Fire Alarm Systems

Douglas Krantz

Describing How It Works

View Douglas Krantz's profile on LinkedIn

Short Circuit
Free Subscription
I'll Send You the
Fire Alarm


How Does Class A Fire Alarm Wiring Work?-- Fire alarm systems save lives and protect property. Fire alarm systems also break down because... Read More

Just What Is a Signaling Line Circuit (SLC)? -- The SLC (Signaling Line Circuit) is another way of saying Data and Power Circuit. Along with added power to run the sub-computers and their input and output circuits, it's a computer data-buss ... Read More

How is a Buffer Relay Wired Into a Door Holder Circuit? -- Like a door stop, a door holder keeps a fire door open. When smoke is detected, the door holder releases, allowing the door to shut. The door holder looks simple and innocuous enough... Read More

How Does One Find a Soft Ground Fault? -- Normally, we think of resistance like that of a resistor. The amount of resistance is built-in; no matter what voltage is used to drive the electrical... Read More

Can a Magnet Really be Used to Test a Smoke Detector? -- Smoke detectors usually have two ways of being tested. Smoke (smoke particles in the air, or some sort of canned smoke), and magnets (the activation of an internal magnetic... Read More

Electrical Flow

On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at