I am assuming you mean a "conventional" Fire alarm system.

Lets talk about resistors...

It's all about supervising the circuits. (i.e. monitoring the integrity of each of the circuits.)

I am mostly familiar with what is done in the United States.

Older systems used to have a redundancy built into the wiring, 2 wires would go in and out of all of the devices and then return to the panel, this used to be called class "A" wiring (NFPA now refers to this as Style D or Style Z wiring).


Most current alarm systems use a resistor after or at the last device on a 2-wire circuit. (Class "B" wiring, or Style B and Style Y wiring under the newer NFPA 72.) The control panel or control device on an addressable system is constantly looking for that resistance as a "normal" condition. Conventional Smoke detectors, for example, will lower this resistance when activated to a point below the alarm threshold and place the panel in alarm. Pull stations and most heat detectors will short the circuit (in the U.S.) also creating an alarm (a short is basically zero resistance, although the wire itself offers a small amount).

If a wire connection is loose or is cut, the panel or device stops "seeing" the resistor and enters a fault or trouble condition. Normally, this is then looked into by a maintenance man or authorized service company.

I am continually amazed by the number of schools, hospitals, hotels, and often city and county owned properties that I enter and see the panel in a fault condition! Scary.

Diodes -

Diodes control the flow of current. Kind of like a "check valve" but for electricity.

Anyway, I don't see them as commonly as resistors on fire alarm systems. Where I have used them, it's to assist with maintaining supervision as described under resistors, but on a polarity-reversing circuit. An example would be if I had to maintain supervision of a circuit that controls a solenoid type device (like a sprinkler solenoid on a pre-action system). Since the solenoid's coil would act like a "short" I might install a diode/resistor combination just before the solenoid to maintain supervision when the circuit is not in alarm.

For more information on the styles of circuits used in the U.S., check out NFPA 72 (the National Fire Alarm Code.)

Source :
http://uk.answers.yahoo.com/question/index?qid=20060914095613AAeBlsu