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Fire Alarm -- Description

Firefighters Phone -- Firefighters use the phones to talk to each other in a burning building
The firefighters use their radios, but, as they put out the fire in a large building, when their radios don't work, they use the firefighters phone system to communicate.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

What is a Firefighter's Telephone System?

By Douglas Krantz

In a large building, so they can talk to each other while battling fires, firefighters use their two-way radios. But when the radios just don't work, the firefighter's phone system is the next best way to communicate.


The whole firefighter's phone system is a stand-alone, Plain Old Telephone System (POTS) - just like the dialup system for the phone company. The main difference between ma-belle and the firefighter's phone system is the firefighter's phone uses no bell ringing voltage, no dial-tone, and you can't dial-up any other building.

Instead of ringing, the phone at the fire command center just detects another phone plugged in and sounds an alert tone until answered.

Building Wiring

It's a two wire system connecting the firefighter's phone jacks to the command center.

When debugging the two wires of the firefighter's phone system, you can connect:
  • A Regular Telephone
  • A Butt Set
  • The Supplied Wall Jacks and then plug the Supplied Handset into the Jack


For troubleshooting purposes, the main difference between the firefighter's phone system and the real world telephone system is the firefighter's phone wires are supervised for opens, shorts, or ground faults.

The building wiring system is a Class B wiring scheme (complete with end-of-line resistor), or a Class A wiring scheme (with the wires going back to the main panel after daisy-chaining the jacks). If a wire breaks, the supervision circuitry at the command center goes into trouble.

Resistor/Capacitor Network in the Jacks or Handsets

Here's where the fire alarm technician or installer can get into trouble.

Between the two wires of the building wiring and the microphone/earphone inside the handset, a resistor/capacitor network is required. All phone systems have this network, or the equivalent.

Depending on the manufacturer, this network may be behind the wall-plate in the jack, or in the handset itself.

Normally, as long as the manufacturer's instructions are followed, and the manufacturer's compatible parts are used, the installer doesn't have to be concerned with this. However, with the elevator cars there is an exception.

Elevator Jacks

Chances are good the firefighter's phone jack inside the elevator car is not going to be a manufacturer type-approved wall station. Instead, it's going to be an aesthetically pleasing jack that the fire alarm installer has wire into the system.

The request to the elevator installers to provide a single-gang hole so the manufactures approved firefighter's wall plate phone jack can be installed will only fall on deaf ears. This is especially true, since this is in their expensive elevator, the building owners don't want one of these.

I know that this violates every code in the book, but the fire alarm technician or installer has to use common sense here. (Tech support for the firefighter's phone system may help also.) The fire alarm system installer is going to have to be creative and wire the jack.
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To see how to wire the jack inside the car, look at the wiring on the wall jack that is used for the rest of the firefighter's phone system.
  • Is there a resistor/capacitor network on back of the wall jack for the rest of the system? Copy that wiring exactly, including the resistor and capacitor values.
  • There isn't a resistor/capacitor network behind the jack? Copy that wiring.


Overall, the firefighter's phone system is the same as the system that Ma-belle uses. With the exception of fire alarm type supervision, the system can be troubleshoot just like Ma Belle's Plain-Old-Telephone-System.


Douglas Krantz

Describing How It Works

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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