Replacing security light

[Seanchadwick2k3]

Hi

I want to replace an old security light, the twin and earth goes into a bedroom and is connected to a small fuse box which is wired into a wall socket right next to it. I worked out which of the rcds I needed to switch off so that there was no power and cut off the excess cable but this caused the house alarm to go off as the main breaker tripped.

Why if I had disconnected power would something trip? Is this some sort of earth fault protection? Am I safe to work on this and is there anyway I can prevent the tripping main switch?

On a side note I noticed that if I switch the RCD for the central.heating this also causes the main switch to trip and the alarm to go off!

Thanks
Sean

 

[ericmark]

There is a difference between switching off, and isolating, you only switched it off, i.e. turned off one live wire call the line, you did not turn off both the live wires, the line and neutral.

With a TN-C-S supply, there is no danger in leaving the neutral connected, but it can trip the RCD.

 

[AndyPRK]

Are you sure you have your terms correct.
Post a picture of fusebox

Maybe you switched off MCBs
And RCD tripped

Your alarm neeeds a new battery

 

[Seanchadwick2k3]

Thanks both for the replies.

Here is a picture , I hit the switch on one of those on the left which I assumed were RCD's and confirmed the plug that the light is wired to was no longer working. When I cut the cable (or hit the switch on the left labelled central heating) the black switch on the right trips which causes the alarm to go off which I understand is to do with
Alarm battery being dead which I'll look to replace.

Im guessing if af suggested I haven't switched off both live wires is this unsafe to do? All I have left to do now is strip back some cable to wire up the new light.

Cheers

 

[AndyPRK]

The coloured buttons are MCBs.

When you cut the wire and touched the neutral and earth wires together, that can cause the black RCD to trip.

Yeah, I would say you are safe to continue.

Avoid touching the neutral and bare earth together, to prevent your alarm going off!

 

[Seanchadwick2k3]

Thank you! That was my thinking but thought I'd check before frying myself! Much appreciated.