N - E Fault on TNS Supply?

[LeccyLad1990]

His point was that a TNCS links earth and neutral, so the earth to neutral 'fault' scenario you suggested exists all the time.

Sorry, I knew what he meant I'm just getting flustered with this issue, I have the customer ringing me on my personal number on a daily basis asking for updates XD

 

[Pretty Mouth]

Sorry Pretty Mouth I think we have some crossed wires here it's a TNS supply not TNCS.

What @timhoward said.

Also like I said to Tim, I had all of the circuits disconnected and had just 1 circuit connected at a time and the rcd was still tripping. There couldn't have been crossed neutrals because nothing else was connected.

If you had the neutral of that one lonely circuit in the wrong neutral bar, it would trip.

 

[LeccyLad1990]

What @timhoward said.


If you had the neutral of that one lonely circuit in the wrong neutral bar, it would trip.

Oh sorry yeah I had it in the right neutral bar at the time

 

[buzzlightyear]

before i read this post fully either N-E FAULT OR NUETRALS IN THE WRONG BAR ,NOW IM SHOUTING .

 

[Pretty Mouth]

Oh sorry yeah I had it in the right neutral bar at the time

If you have absolutely 100% ruled out mis-wiring at the consumer unit (this is still #1 suspect in my book), then there are limited possibilities:

1. All circuits are faulty/interconnected, or
2. Neutral to earth fault somewhere between outgoing neutral terminal of RCD, and neutral bar inclusive, for both RCDs, or
3. Both RCDs defective

 

[James]

It can be easy to get flustered and start going in circles chasing different ideas and possible faults.

go back to site

remove ALL outgoing cables from the board (actually out if you can, not just disconnected)

connect your bell transformer circuit
rcd will not trip

check each circuit before adding it back into the board, especially careful to check that it is not connected in any way to another cable that is in your “to be connected bunch”

connect all the rcd circuits up individually before moving on to any that are not rcd protected.

 

[Zerax]

Can the OP post a pic of the board with just the bell tranny circuit wired in such that it's causing the RCD to trip ? It needs to be a good pic (or several pics) that shows the wires connected and all other stuff disconnected.

My money is on a neutral wiring issue in the board.

 

[timhoward]

It can be easy to get flustered and start going in circles chasing different ideas and possible faults.

100%.
After a very difficult job arguing with rusty SWA, I managed to mess up the easy bit of changing three B6 MCBs to RCBOs once, they all tripped once fitted.
The CU was high at the back of a kitchen cupboard, the wires were all short and I mixed up the Neutrals.

In the gloom when I tried each N in turn in one circuit it still tripped. As I was now imagining borrowed neutrals or serious earth leakage on all three circuits I stuck the MCBs back in and went home.
The next morning, calm and collected, I tested each circuit for inter-connections and IR, all fine, tried each circuit in a wago box plugged into an RCD protected socket, all fine, and fitted the RCBOs, all fine. Whatever I'd done the night before it couldn't have been to try each N in turn.

A new day, coffee, and a clear mind helps enormously.
I'm sure if the OP goes right back to first principles as suggested by @James he will solve this at the next visit.

 

[Zerax]

It can be easy to get flustered and start going in circles chasing different ideas and possible faults.

I had something similar ages ago... end of a long day, rushing to finish off... ended up trying to chase a fault for about 2hrs... at about 10pm I gave up and decided to go home ready for a fresh look first thing in the morning. Within 5 mins of getting in the van and setting off... Bazinga !! I knew exactly what I'd done... took 2 mins to fix the next morning.

 

[Wing_na_Prayer]

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.

I had the same problem a couple of years ago and found that I had the circuit neutrals on the wrong neutral bars. I re-terminated the circuit neutrals in the correct neutral bars and cured the problem.