help-tripped rcd

[Marco1976]

Hi,

I was changing a socket faceplate on a socket on then upstairs circuit.
I switched off the relevant mcb that covers the upstairs circuit.
I disconnected all the wires from the faceplate and at that point I realized that the washing machine downstairs was quiet.....all the downstairs circuits are on a different mcb but same rcd. The rcd had tripped.
Is this normal or is there a fault with the wiring? I thought that maybe when i disconnected the earths at the socket the rcd may have picked this up. But I don't understand why this would have happened since i have switched off the mcb serving the upstairs ring.

Everything is back on now but i am curious to find out as to what might have happened.

Many thanks

 

[ericmark]

The MCB often only switches off the supply to one live wire the other live wire is still connected. The Line wire is switched of the Neutral wire is not. The neutral is nearly the same voltage as earth so far less current will flow but very likely the 30ma required to trip an RCD will.

A RCD measures power out and power back and if the same all is OK. If any power does not complete the circuit it assumes this mush have gone to earth and trips out.

 

[Marco1976]

thanks but when you say one live wire is still connected does this pose a risk whilst doing the work?
I thought that by disconnecting the mcb there would be no power. I checked the power with a meter before i started the work and it wasn't there.

 

[mikhailfaradayski]

i imagine you touched the neutral and earth wire together briefly whilst changing the socket. If there was any load on the rcd (washing m/c) then that would allow the rcd to trip.

If it resets and all is ok now, then no worries and you also now know your RCD is operating.

 

[electronicsuk]

thanks but when you say one live wire is still connected does this pose a risk whilst doing the work?
I thought that by disconnecting the mcb there would be no power. I checked the power with a meter before i started the work and it wasn't there.

While Eric is correct, as is his terminology, I can see how it would be confusing for a DIYer. To clarify, what you would call 'Live' (brown or red) and Neutral (blue or black) are both termed 'live' conductors. What you would call 'live' should actually be referred to as 'Line'.

So, the MCB disconnects the 'line' conductor, which is at 230v with respect to earth, but it does not disconnect the neutral. If neutral and earth touch, often enough current will flow to trip a 30mA RCD, but it is no cause for concern.

 

[Marco1976]

thanks to all. yes everything working fine now.
I did touch the two wires together. I guess i should have wrapped them in insulation tape and this would have avoided the tripping.