RCCB tripped when neutral - earth touching

[winayetun]

When replacing a light housing, I found the RCCB was somehow tripped. I first used a pen tester to check the wires and found them normal (no voltage). I then used a multimeter to test again. Here are the results:

L-N 7.4 v
L-E 6.8 v
N-E 3 V

After the RCCB was tripped, the readings were zero. So, I think the readings were real.
Only N-E would trip the RCCB, other combinations wouldn't.

Any guess what's going on here?

 

[Mikegh]

.

 

[ipf]

Recommend an electrician with proper testing gear.
On here, we're just guessing and it could cause you more problems.

 

[Risteard]

Shorting neutral and the cpc would be expected to cause an RCCB to operate where there are other circuits protected by it. I'm not seeing the issue here.

 

[pc1966]

The RCCB detects an imbalance in current between the L & N. It does not matter if that is due to a L-E fault or a N-E fault.

The former is usually more dangerous as it could be someone getting an electric shock, the latter is not as common (less chance of poor insulation breaking down due to low voltages) but is often responsible for difficult to find spurious trips of RCD as systems with a normally very low N-E difference might only see enough stray imbalance current on the occasional heavy load.

 

[Mike Johnson]

To make it absolutely clear, taking the title of the post " RCCB tripped when neutral - earth touching" it's supposed to, that is what it's there for to detect an imbalance between the earth and live conductor.

 

[Rockingit]

I think the OP is probably asking why there were some voltages measured on a 'dead' cable and how come that same 'dead' cable could trip a RCCB?

There's a few things about why you got some readings, but firstly you need to know that you NEVER rely on a pen stick for safe isolation!

The most likely reason that there were a few odd volts here and there is because there were - either through what we call induced voltage (commonly seen on two way lighting circuits and where multiple cables run along each other) and/or the switch itself is not fully isolating/internal arcing. MCB's tend to have a larger internal gap space than a 50p faceplate switch.

There is normally always something floating around on what you perceive as a 'dead' neutral unless it's actually been fully disconnected (or is via a two-pole isolator which is unusual in a domestic situation) and so as others have said, the RCCB has enough to compare the two and will disconnect.

 

[Marvo]

The thing to remember is that 'isolating' a circuit using the circuit breaker only disconnects the live. The neutral is still connected and the RCD or RCCB is monitoring the neutral, hence the tripping you experienced. I'm not sure of the definitions written in the UK regs but I wouldn't normally consider a circuit to be fully isolated if it was just switched off by an MCB.