RCD Tripping when cutting cable...

[Reg_Prescott]

...with the circuit MCB switched swtiched off.

I have a ring circuit in my garage which is supplied via its own 32A breaker. Now, I have just put in a new work bench and had to move a socket (surface mounted back box) a few inches. Nice easy job. So I turn off the MCB, I also check with my multimeter that there is no power at the socket. When I cut the cable... the RCD trips. Any ideas?

Also, what resistance should I expect to see between Earth and Neutral conductors?

Thanks guys.

 

[mo2]

mcbs just cut the live, the main switch on the CU will cut the live and neutral and I am GUESSING that this is the problem.

 

[Steve]

Yeah, your MCB only cuts the live. neutral and earth are still connected to their sources.

On a PME system, Neutral and earth will have very low resistance between them, due to being connected in the cutout.

 

[RF Lighting]

As others said this is normal.

You are causing a neutral - earth short when you cut the cable, and this causes an inbalance between live and neutral of the other circuits as some of the "neutral electricity" (I'm using laymans terms) returns to the oricgin via the earth wire instead of the neutral wire.

An RCD will trip if more electric leaves through the live than returns on the neutral, as the missing live could be flowing through a person being electrocuted.

I hope this makes sense. I have kept it as simple as possible so you will hopefully understand it.

 

[plugwash]

I have a ring circuit in my garage which is supplied via its own 32A breaker. Now, I have just put in a new work bench and had to move a socket (surface mounted back box) a few inches. Nice easy job. So I turn off the MCB, I also check with my multimeter that there is no power at the socket. When I cut the cable... the RCD trips. Any ideas?

as has been said mcbs only isolate the live,

when you cut the cable you almost certainly created a brief neutral to earth connection which due to the voltage difference between the neutral and earth bars at the CU provided enough current to trip the RCD.

the only way arround this is to adopt a "work like its live" policy when working on cuircuits that are only single pole isolated.

Also, what resistance should I expect to see between Earth and Neutral conductors?

on a TN-S i'd expect the N-E resistance to be a bit lower than the EFLI, on a TN-C-S it will be lower still.

note though on a TN-S or a loaded TN-C-S there will almost certainly be a voltage between earth and neutral already and therefore you will not get a meaningfull measurement of the resistance with a resistance meter.

this of course also assumes that the cuircuit is either live or only single pole isolated (e.g. by a mcb), if its double pole isolated (e.g. by a main switch or RCD) the resistance should be very high.

 

[Adam_151]

Also, what resistance should I expect to see between Earth and Neutral conductors?

Thanks guys.

I'd expect a figure thats not too dis-similar to your Ze as long as any DP switches upstream are closed, if they are open then a reading would indicate a neutral -> cpc fault on the circuit

(though I wouldn't advise putting any instruments set on resistance range onto it unless the neutral is isolated)