AC control via fire alarm

[holmslaw]

..

 

[mattylad]

so in this case the relay coil would be continuosly monitored.

This is not necessarily so - it does not have to be on a monitored circuit. And AFAIKR not required by BS5859 (or whatever the reg is).


Often a key switch will also be put into a panel in order to override the aux output that supplies a relay used for the AC to turn it off in the event of a fire, the keyswitch used to override the panel to turn it back on (as well as the aux output being reset)
The panel may also be in need of servicing or repair and the AC still in need of power hence the use of a keyswitch.

Often a small relay is used to turn on a larger relay to control mains equipment in fir alarm panels.


If the panel is powered the aux relay will close, so the COM/NO contacts will be shorted. This is then fed to another relay powered from a 24v battery backed supply (usually) which then controls the AC which is also powered normally and hence the AC should be across the COM/NO contacts as they would be closed when powered.

Thus providing power to the AC via the normal control method, when a fire is detected the panel aux relay can open, removing power from the remote relay and hence the power from the AC.

A fire does not need AC to provide more oxygen to feed it.

Even with modern addressable fire alarms customers ask for the humble keyswitch to be fitted. Often with a common key that the fire brigade have a copy of. I had to make the point on every new panel designed that a space for a keyswitch was required to be left.

 

[sparkticus]

I suspect that when there is a "call for AC" the relay is energised and the NO contacts close causing the AC to switch on.

I think, you have misunderstood.

The relay is a fire alarm interposing relay, and its purpose is to switch off the ventilation when the fire alarm sounds. It will do this by interupting the ventilation control circuit via a n/c contact that opens when the relay is energised.

I would also expext that somewhere in the system there is manual override switch to restart the ventilation, it would normally be part of or beside the fire alarm panel.

You may be right. I read/thought the OPs message indicated that the fire alarm system is now separated from the air conditioning system (AC system) so that is why I assumed the relay (in question) was independent and not the standard interposing relay used to disable the AC during a fire alarm event. On reading it again I am not quite sure which relay we are all discussing.