Rule for (N) neutral wire & earth

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I had to fit a timer on the immertion heater. I went in the distribution box, I put down the breaker in question MCB/B20 I checked for any power with my tester at the end of the wires in the HP, no power so I thought that I was safe, what happened was by two times when I was working with the wires, the 63A RCD breaker went down? I didn't get any shock.
Now I am very confused because I thought that with the MCB down I was safe.
after investigation in the DB I realised that my MCB/B20 has it Neutral in common via the neutral busbar and it earth in common too via the earth busbar so If my N & earth comes into contact ( even if the Live is not powered) because they are on a common busbar and the other circuits are powered and on the top of that an appliance is working this could closed the circuit an make it goes down? I started to think that it could be the reason why my 63A RCD went down.
Could somebody tell me if this makes sense to them?
Also is there a reason why in the DB my N (MCB/B20) is twisted with an other N and connected together on the neutral busbar because there is a lot of empty connectors, I don't know why this has been done like this? is that a rule or what?
thanks
 
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Now I am very confused because I thought that with the MCB down I was safe.
Barring borrowed neutrals and polarity reversals you were.


Could somebody tell me if this makes sense to them?
It makes perfect sense and it's exactly what happened - you created a N-E fault and the RCD did its job.


Also is there a reason why in the DB my N (MCB/B20) is twisted with an other N and connected together on the neutral busbar because there is a lot of empty connectors, I don't know why this has been done like this? is that a rule or what?
thanks
It's done to annoy electricians in the future.
 
It is perfectly normal for the RCD to trip if you short N>E on a circuit isolated with a SP device.

It causes an imbalance on the RCD as the addition of the earth to the neutral allows some of the neutral current from the other circuits to flow to earth, which the RCD detects as an imbalance, and trips.

Dont know why the neutrals are twisted. Only the person who installed the CU knows the answer to this question.
 
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thanks lads! :D
Now wen I have a job to do I will put down the main fuse so I won't get into this situation.
you know you save me a lot of troubles, i thought that i had a bad wire's connection somewhere behind the partition wall.
good job :LOL:
 
Most immersion heaters will (should) have a local double pole isolator local to the heater. Turning this isolator off (as well as the MCB like you did) will prevent the RCD from tripping in future.
 
in fact was I was doing is to fit a immersion time and a double pole isolator with a 13A fuse so now job done and I know a new thing (Neutral & earth)
 
Twisting neutrals together in the CU, I love that.

I also love finding 2, sometimes 3 CPC's twisted together and one peice of sleeving used over the lot.

Makes ring CPC continuity testing a real breeze!
 

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