Douglas Krantz - Technical Writer - Describing How It Works
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What is a Firefighter's Telephone System?

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.


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.

POTS

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:

Supervision

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.

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.

POTS

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.
Having serviced fire alarm systems for nearly 20, Douglas Krantz has compiled his knowledge of the causes of Ground Faults and how to reliably detect them into the book Make It Work - Hunting Ground Faults. The book shows the three types of ground fault, what equipment should be used with each type of ground fault, and how to locate those hard-to-find ground faults.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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