Tricky Tripping Problem

[N1G3L]

We installed a new split load consumer unit in a large country house about 2 years ago. No Leakage protection existed before and there were a few faults that had to be sorted. A Neutral/Earth short in one socket and very rat chewed cables replaced. The system worked fine until recently. The RCD trips for no apparent reason. Another (MK) unit was substituted but the fault remains. Trouble is no particular circuit seems responsible for the tripping. All MCBs have to be turned off before the RCD will reset. Whilst turning on the circuits one by one even in different order any one with an apreciable load attached will cause RCD to trip.
This fault is very strange and has got me completely stumped. I do not have a megger tester but do have a very good quality DVM.

1. What is the correct way to test all these circuits?

2. Should Neutral be tied to Earth inside supply termination box, ie. before the main company fuse?

 

[JohnD]

. Whilst turning on the circuits one by one even in different order any one with an apreciable load attached will cause RCD to trip.

this is typical of an RCD installed with the wrong neutral used

I notice you say the RCD was recently changed.

 

[N1G3L]

The RCD was changed to try & eliminate the fault.
It was installed exactly in place of the old one, same type etc.
So no wiring change has been made.

 

[securespark]

Is your supply type TN-C-S?

 

[davelx]

Have you ruled out a N-E fault on each the circuits?

A N-E fault on any one circuit will cause current imbalance on the RCD as soon as any other circuit is energised, since even with the MCB turned off, the neutrals are all connected together, as are the CPCs.

This will also apply to N-E faults in any appliances that are not fully isolated, ie if they only have single pole switching.

 

[N1G3L]

Many thanks for all responses!

I am at customers house again on fri & will
check out all the above...

watch this space!!!

 

[Spark123]

It sounds like a N-E fault to me.
Anything before the meter is DNO property, not a great concern of yours. The only thing you can do is visual check and ensure it is within parameters. If it isn't then it's up to the DNO to fix it.
As you are charging for the job you need to do it properly i.e. you need the correct type of testers and ensure it is done properly. A DVM isn't a suitable tool for checking the insulation resistance is sound. A DVM measures volts.

 

[Lectrician]

Have you ruled out a N-E fault on each the circuits?

A N-E fault on any one circuit will cause current imbalance on the RCD as soon as any other circuit is energised, since even with the MCB turned off, the neutrals are all connected together, as are the CPCs.

This will also apply to N-E faults in any appliances that are not fully isolated, ie if they only have single pole switching.

I concur.

You must remove all neutrals from the neutral bar and test each circuit with your meggar seperately. You will then be able to find the fault.

Simply turning on MCB's is not the way forward!