Question:
While I'm familiar with burglar alarms, fire alarms are all new to me so please forgive my ignorance. One of the circuits is showing a fault condition on a two wire circuit. Can anyone explain what the three(?) states safe, alarm, fault are and what the circuit should be seeing diode/resistor or whatever.

Answer: What panel and detection are you using. The panels made by Protector alarms give a three state fault indication (short circuit - open circuit - detector removed) Move the end of line to the control panel and see if the fault clears, if not the panel could be u/s. Fault finding is the same as conventional circuits. Half the circuit and move the eol to that position, if the fault is cleared then the fault lies after that point. Repeat the halfing process until you locate the problem. Make sure there are no earth faults. One type of system I install and service is the Savwire system. The system works by reversing the polarity of the zone when it is in an alarm state, ie +ve becomes -ve. The sounders are wired reverse polarity and due to being polorised, they dont operate when the zone is not in an alarm condition. Detectors use special savwire bases made by Apollo and Hochiki, using the standard detectors. Mac points have to be polarised with a diode so if the call point is operated 24 volts is not placed accross the 470 ohm resistor urning it out. In short the sounders and detectors are wired on the same two wires and once an alarm is raised the polarity of the zone is reversed.