Automatically trip the main RCD deliberately in a fire?

Westie, I was trying to avoid having to use the fire service.

However, from the responses, it sounds like waiting for the smoke alarm is too late, by the time that has triggered, the fire has already started.

I could have got my hands on an old Vesda sniffer from work, Guess that's overkill for the shed! :)
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Lay out is roughly as follows. May just be my terminology.

House CU
100mA 100ms 100A Incomer/breaker
30mA 30ms 32A RCBO sockets1
30mA 30ms 32A RCBO sockets2
20A MCB Garage
30mA 30ms 63A RCD (Board is split here)
various MCBs

Garage CU
100A breaker
30mA 30ms RCBO PV
30mA 30ms RCBO Shed
30mA 30ms RCBO Lights
30mA 30ms RCBO Sockets
Is the incommer of a time delayed type?
Is this a TT installation?

Single pole RCBOs will not discrimnate with the incommer on a neutral-> earth fault!

The incommer of the garage board is likely to be a switch diconnector. 100A DP MCBs, while available, are rare.

Why the RCBO to the PV system?

What size is the submain cable to the garage?
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Hi Adam,

Yes incomer is time delayed. Does saying 100ms instead of 30ms not mean time delayed, or have I got that wrong?

Yes TT

Yes garage board main switch is a disconnector only.

RCBO on PV to provide discrimination if earth fault on inverter or cabling.

Yes I know that N->E faults are only protected by the double pole devices.

Submain to garage is either 4mm or 6mm (would need to check the paperwork to confirm) and is between 20m and 30m long. It used to have a 32A MCB but the electrician said this was too high and changed it down to 20A.
Once the fire has started it will make no difference whether the circuit is live or dead.
Not completely true - it depends very heavily on the type of fire, how long it's been going, and how long it takes to detect it (the latter two going somewhat hand in hand).

Bear in mind that all electric cables and enclosures should be self extinguishing, so cutting the power will cause then to go out. The question is really whether you can detect the fire and cut the supply before there is enough heat to ignite self sustaining materials - and that depends very much on where the fire starts.

Unless you put fire detectors in voids (not really practical for joisted floors) then it will take a long time to detect an overheating cable or joint - so the joist and/or the floor boards above it are likely to be well alight before an alarm triggers.
Conversely, a faulty socket in a block/brick wall won't have anything close by to ignite other than the plug and cable (though wall paper changes that). So there is a chance of detecting the fault before the carpet goes up.

But, for many of these faults, you'd need such a sensitive detection system to detect them in time that false alarms would be a real nuisance.

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