RCD Trip on non-live circuit

[mrscalex]

I managed to cause a neutral/earth short when the neutral cable touched the metal face-plate. The RCD tripped even though the circuit I was working on was switched off.

Is this expected behaviour?

 

[plugwash]

i guess by cuircuit off you mean mcb off

mcbs only isolate the live

its very tempting to stick some nice big isolators on rings for this reason

 

[mrscalex]

Yes, I meant the MCB was off.

Could you possibly explain in simple terms why a trip happens when to a DIY'er like myself there is 'no current' in the circuit.

 

[plugwash]

there will always be a slight voltage difference between neutral and earth

on a TN system this is due to voltage drops on the cabling and devices before neutral joins with earth

on a TT system this is due to ground potential differences and voltage drops on the neutral wiring and devices

this causes a small current to flow in the even of a neutral-earth fault and this is often enough to trip a rcd

 

[andy]

i remember finding tht out the hard way. whilst workin on a house the wirin was bein done 1 room at a time, so the old wirin was temp connected to the new CU. then it came to the kitchen so the mcb was switched off and the live taken out (just incase). every so often the RCD would trip for no reason. after a few days of this i realised in the kitchen where the old sockets had been taken off my dad had left some copper pipe shortin the neutral and earth

 

[mrscalex]

Thanks. I feel suitably informed now without feeling like a complete fool for not realising this could happen!

 

[ban-all-sheds]

imcbs only isolate the live

its very tempting to stick some nice big isolators on rings for this reason

Or just use DP MCBs....

 

[dingbat]

Thanks. I feel suitably informed now without feeling like a complete fool for not realising this could happen!

Don't feel a fool. (Unless, of course, you have the fool's permission!!) Many a spark has unwittingly tripped the RCD when working on a circuit he thought he'd isolated. Even when you know why, it somehow seems unfair!