There was a fashion a few years ago for the detector side of fire alarm systems to be wired in ordinary cables, on the basis that all the signals failed-safe - i.e. if a cable shorted, or went open circuit, an alarm would be triggered. I think that approach has gone out of fashion again now, but the logic would suggest that if the fans were allowed to run by a 'no fire' signal from the alarm panel, safety wouldn't necessarily be compromised by using a non fire rated cable. Standards might take a more conservative view though.

   - Andy.

Those systems have a well defined termination resistance at the far end of the cable which allows the integrity of the wiring to be confirmed, the nicer systems have a loop through at each detector so that if a detector becomes disconnected for some reason., then that loop is broken and a system fault (not a fire alarm) is raised. This allows cables like FP200 which do not last for ever in a fire, (unlike pyrotenax/ MICC) but do last much longer than needed to raise an alarm.

M

Exactly, this is what I was thinking about....fail safe relay. If we take any random low fire resistance cable to bring that normally close signal from the fire interface to the ventilation plc (during normal operation) and for some reason the continuity is lost due to cable melts, disconnection etc, The plc will cut power to the fans off and we are ok. If we use the FP cable and it survives the fire event, then the fire alarm signal will probably be triggered by one of the smoke or heat sensors and the signal will be passed to the fire interchange causing vents stopping so the same reaction.

It all depends on where the fire starts and what happens first during the fire, but I can't see any point in using the FP cable in this scenario. My understanding is that the connection between the fire alarm system and some other internal panel/installation can be made using any suitable cable that is not particularly fire resistant as long as it's done in "fail safe" way.
It is the same process on tram/train. Due to the event of power loss when the vehicle is downhill its brakes automatically start braking as the no power situation trigger some fail safe relay to open or close the circuit automatically in situations like this. I hope that makes sense :)

do you expect your cable to short, or open circuit during a fire, and has this been verified ? there is a reason for the well defined termination impedance.
Mike.

do you expect your cable to short, or open circuit during a fire, and has this been verified ? there is a reason for the well defined termination impedance.
Mike.

there is a reason for the well defined termination impedance.

For inputs yes. For outputs however it feels a bit simpler (although I might be overlooking something obvious). Either the cable remains intact (in which case the relay remains under the control of the fire panel) or it fails - and if it fails (either by short or open) the relay should drop out in the same way as if the panel had removed the 'no fire - all OK' signal. Other faults might be more interesting - fault to earth should be OK if conventionally wired (but perhaps not if the panel switched N for example), faults to a line conductor of another circuit might be problematic (if hopefully rather unlikely).

   - Andy.