Normally (in dual loop) both loops will have to detect fire in order to trigger a fire warning. If one loop shorts or loses power, the system will automatically revert to single loop operation.
There will be no indication for this.
Fire Fail caution light can come on for two reasons:
- One loop sensing fire/one loo not sensing fire.
- Both loops have failed.
During Fire Warning test, if the applicable FIRE WARNING light + FIRE FAIL caution light comes on - this indicates single loop operation due to a shorted loop, or a loop with no power.
During Fire Warning test - if the applicable FIRE WARNING light and the FIRE FAIL caution light DO NOT come on - this indicates an open loop. The system is still in dual loop and will not be able to detect fire, unless both loops (including the open one) detects fire. By manually swithing to the “normal” loop, the pilot can make a successful Fire Warning test and dispatch with an open loop.
Pretty complicated system.