The RCD trips on a board that is dead?

[mattmoo1974]

Right, working in a shop adding a few extra sockets..
All old rewireable ccu some 1 had added a RCD garage unit to cover the whole installation(tailed into RCD feeds 1st ccu you no the drill)
Right im working on the 2nd ccu fed from the first..
I lock off the circuit im working on, test dead away i go, i cut in and trip the RCD??
Words of wisdom please

 

[DaveyD]

Unless I'm reading this wrong moo, an RCD will pop when you cut a circuit even if that CB is off. Disconnect the neutral you are working on as well as turn off the cb.

 

[SirKit Breaker]

Is the RCD live at its incoming terminals? If so then this is normal, some RCD's trip when you cut into a circuit that is dead, i think they are known as passive, someone will correct me if i am wrong.

Cheers............Howard

 

[topquark]

What happens when you lock of the circuit mate? .... it's only the line (unless it's a multipole breaker) that's isolated. So when you are cutting in (assuming you mean a circuit cable) you are prob shorting that cable out.

Better to isolate the circuit completely (remove line, neutral and earth if possible to do safely) if you are intending to work on it.

 

[mattmoo1974]

Why tho? In stupid terms plz iv had a few stella' s I say cut iv had it when you touch cables together...

 

[DaveyD]

The RCD is looking for a balance of the line and neutral. When the line is switched off all is ok unless you introduce a current into the neutral. When you cut the wire you cut accross the earth and neutral. The RCD senses that imbalance and shuts off.

 

[mattmoo1974]

Ah is that basicly because im only isolating the line and the netural in affect is still part of the over all circuit?

 

[topquark]

If it's an RCBO, then less of an issue. When it's an RCD you are making a route from the neutral to earth when cutting the cable!

 

[topquark]

Ah is that basicly because im only isolating the line and the netural in affect is still part of the over all circuit?

Yep, you got it! Remember the RCD is protecting the whole of the neutral bar

 

[mattmoo1974]

Thanks lads that was really doing my head in(and the shop owners)
So touching cpc to netural will make another path and make a imbalance the RCD!!