Automatically trip the main RCD deliberately in a fire?

11 Feb 2011
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United Kingdom
I am idly contemplating how to go about automatically isolating the house power in the event of fire.

I have a TT earthing system, a 100ms 100mA 100A incomer into the house and a 16th edition split board Hager CU. All circuits in the house are protected by the split board 30ms 30mA RCD or individual 30ms 30mA RCBOs.

There is a CU in a detached garage and another in a shed and all circuits within these outbuildings have 30ms 30mA RCD or RCBO protection. The cables to these outbuildings are wired into the house CU in the non split section and are overload protected by MCBs, and from earth faults by the main incomer RCD. There is a pump in the shed that runs during the day for a pond, and associated other equipment. There is a solar PV system on the garage. All of this increases the risk of causing a fire if it malfunctioned, or exacerbate a fire if one started.

I have 12V smoke alarms in various places in the house, in the shed and in the garage wired into a Texecom Premier 48.

My thought is that in the event of a smoke alarm being activated and not reset within 30 seconds then I can set an output to toggle on the alarm panel which I could then use to "somehow" trip the incomer.

Any thoughts on how to trip the incomer safely? Is there an official way to do it? What are the issues with, for example, using a relay and resistor into one of the MCB protected circuits to pass a small current between phase and earth?

Ready for the flak!
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There is no need to do anything like this in a simple domestic.

But the idea of delibaratly casuing a leakage current to trip an RCD is dog rough. There are two ways to acheive this kind of system.

i) A contactor wired up to latch in the on position using the aux contact and having the fire panel cause it to drop out.

ii) A shunt trip device on the side of a circuit breaker
Why on earth would you want to?

Is it so that while you're trying escape from the fire all the lights go out and you have to try and escape in the dark?
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I was thinking more for when there is no one in the house - ie. when the alarm is armed.
Why do you think you need to isolate the power in a fire anyway?
Generally services which are tied in with fire panel activation are normally things like:

Air handling plant (dropped to prevent airflow)
Main building gas value (dropped out for obvious reasons)
Magnetic door locks (dropped out if of the type which unlock without power)
Automatic doors (can normally be triggered to open and hold open)
Supplies to catering equipment (dropped for obvious reasons)

Its not normal to drop the power to general purpose parts of the installation
Scenario I had in mind is that the pump runs dry or has a bearing failure and overheats/catches fire.

Or, having seen some of the pics posted on here recently, a connector block, or screw terminal works loose causing a fire if there is sufficient load on that circuit.
Once the fire has started it will make no difference whether the circuit is live or dead.
I'd be more worried that you've got your solar PV feeding onto the load side of your RCD incommer :evil:
RF, fair point on a fire not going out by itself.

Adam, thanks for the explanations. What's your concern with how the PV is connected?
Generally the fire service will try to find the incoming supply point and switch it off themselves anyway, even if told it has auto switched they will still do so.

a) The rcd is de-sensitised because a percentage of the leakage current is being sourced by the grid tie inverter and not passing through the RCD

b) When it does trip, the disconnection time can be a lot longer than 40ms the rcd trips in for faults of 5x, its even a lot longer than the 300ms for 1x faults. Granted with a double pole RCD, it looses its reference to eath for this extended shutdown time (unless there is also a N-E fault on the installation)

c) A distorted waveform from a faulty grid tie inverter can in theory staturate the coil of a type AC or even A RCD and prevent it from tripping.

I do not believe any gneration should be fed onto the load side of any RCD which feeds other equipment. It would appear that many installers do not have any such reservations!
Adam, interesting points - thanks. Is that still the case given the following?

The PV is fed by it's own RCBO in the garage, and all other circuits in the garage CU are independently protected by their own RCBOs.

The garage is connected up to the house via an MCB in the house CU. This MCB is located before the board split. It is not an RCBO as this would not give discrimination. So the only earth leakage protection on this cable is the main incomer. All other circuits in the house CU are protected either by their own RCBO (located before the split), or the dual pole RCD at the split.
I am confused by what you are saying you have

One minute you are saying you have an RCD incommer and the next that you have RCBOs on everything.

If both are true then uts likely you have no discrimination!

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