N - E Fault on TNS Supply?

[LeckyLad1990]

Hi All,

Right, I have a very strange fault occurring so I'll start from the beginning.

I have recently carried out a consumer unit change from an old rewireable to a new split load RCD protected consumer unit. So I had changed the board and put new meter tails in.

I have tested all the installation cabling and it would be classed as satisfactory (it's not perfect considering it's over 35 years old).

When it came to energising the board, I switched on the main switch and the two RCD's (63A 30mA) first then started energising the circuits. However when I energised the first circuit ( in this case a shower ) the RCD tripped. So knowing that there could be an issue with that particular circuit I tested another circuit (in this second instance it was a feed to a bell transformer with no earth as it was double insulated) and disconnected all other circuits including neutrals and earths and bonding leaving only the main earth. So now it was just a bell transformer on a 6A MCB connected to a 63A 30mA RCD connected to the 100A main switch and off to the cut-out. Once again the RCD tripped instantly.

I carried out a Ze which came back as 0.30 ohms which would likely be classed as a ‘pass’ from the DNO even though it's quite close to the limit.

I swapped the RCD’s around in case one was faulty, but it still tripped.

I checked the supply polarity and that too was correct.

I am genuinely stumped by the fault, all I can assume is that maybe there is a neutral - earth fault on the incoming supply?

If this is the case whats the solution? Would the DNO have to repair the cable?

I’d appreciate any help you all could give.

 

[ProDave]

You have got the neutrals in the correct busbars?

 

[Sharpend]

Shared neutral? 

 

[Andy™]

I carried out a Ze which came back as 0.30 ohms which would likely be classed as a ‘pass’ from the DNO even though it's quite close to the limit.

0.30 is nowhere near the limit of 0.8...

N-E fault on DNO network cant cause this. almsot all DNO networks that are TNS are really TNCS with multiple N-E connections. most likely you have a fault with the installation. either a N-E fault or a shared neutral

 

[kerching]

Crossed neutral!

how can an upstream fault cause the RCD to trip?....but I know little of these things being a bit of a newbie

on a side note....why a dual RCD aboard?

 

[SPECIAL LOCATION]

I swapped the RCD’s around in case one was faulty, but it still tripped.

What.. why... WTGrape..??

why not just test the RCD's as per the guidance of BS7671..

(e.g. no loads connected.. )

Random swapping stuff really is amateur DIY concept..

Sounds like you have configured your outdated split load board wrong..

or you have undetected fault(s) on your wiring..

Very, very, very simple to test RCD's with no loads direct at the CU..

:C    

 

[Sharpend]

Very simple to Test per se but correct equipment is needed to do such. 

 

[SPECIAL LOCATION]

Very simple to Test per se but correct equipment is needed to do such. 

Who's Percy?

But you would expect someone who has carried out a board change to have the correct equipment..?  

and know how to do the simple testing to suss out this sort of problem?

:C

 

[kerching]

️  

 

[Bruspark]

Are we in the DIY forum again ?