Phase Neutral Reverse

[sherman0251]

Im stumped! Been to a call out today saying they were getting shocks from metal work in house.
There are 2 DB'S.
32A fuse supplying kitchen ring off one board, and a 32A fuse supplying upstairs ring off the other board. When both fuses are in the kitchen sockets and upstairs sockets are phase neutral reverse. When I pull the fuse for the upstairs sockets and just leave the kitchen sockets on the polarity is correct.
Im thinking there is a fault on the upstairs ring!!
I pulled out the fuse for the kitchen sockets and left the upstairs on, I still somehow get a supply to kitchen sockets but its phase earth reverse. A bit more serious!!!
Going back on sun to dissconnect everything from circuits and test!
Only way I can think to solve!
The 2 Circuits are linked somehow I think!
Any advice would be appreciated!

 

[Spark123]

Smells of a TT system without an RCD, or an open circuit earthing conductor on a TN system. What is the Ze like? Could be relying on the MEBs for its earth return path.
Then there is another possible fault on one the upstairs socket circuit to find.
Think it was Securespark who had a similar fault once before on a lighting circuit?

 

[Click-Sure]

You measured any voltages in the course of your investigations?

I wouldn't have re-energised that installation, refused and let householder do it at their own risk.

 

[RF Lighting]

Yep, sounds like a missing earth to me.

This is very dangerous and should be sorted ASAP.

Someone might have been killed by Sunday. (And I'm not joking)

 

[Lectrician]

Martindales......good aint they

 

[Spark123]

Yep, good old socket n see type testers!

Found the post I was thinking of here: //www.diynot.com/forums/viewtopic.php?t=69140
(Almost 2 years old )

 

[sherman0251]

I did Isolate the circuit! They have no boiler or upstairs sockets till sun!
I was getting 240v between neutral and earth in the kitchen. I dont understand how it can go from phase neutral reverse then to phase earth reverse just by energising another circuit!

 

[Spark123]

With the problem type as I described above, the CPC is becoming live to 230v via a fault. There is a problem causing it not to automatically disconnect i.e. a bad Ze.
As the CPC is at 230v and the neutral at 0v you see 230v N-E. The phase is at 230v, the CPC at 230v, result P-E = 0v. You are reading this as a reversal as opposed to the CPC becoming live via a fault.
Energizing the upstairs socket circuit is causing the CPC to become live to 230v, so there appears to be a fault on this circuit.
There is a more serious fault as the CPC should never become live to 230v, which is why I asked about Ze (external loop impedance).
A majority of TT systems have a too high Ze to allow an MCB or fuse to disconnect hence RCDs are required. What is the supply type?

 

[sherman0251]

i think i get what your saying!
So if the phase is at 230v and the earth is at 230v and I test between the two I will get ov?

The DB is an old rewirable type there are no RCD's on the system at all!
So the fault maybe a nail through a cable shorting phase and earth?

 

[sherman0251]

sorry spark just got the end of your message! Didnt see the bit askin about the supply type!
I dont think it was a TT system Apparently the supply comes into next door then into a meter in there house!

The meter tails to DB are in a bit of 2.5mm t+e!!!!!!!
Ill be replacing them to!

I just thought that the easiest way would be to disconect all socket fronts upstairs and bell each cable out and also do an insulation resistance test on them to!

 

[Spark123]

That is about the size of it, however the fuse for the upstairs sockets should have popped which leads me to think there is an issue with the means of earthing.
The issue with the earthing needs resolved as a matter of urgency.
The first thing that needs confirmed is the external earth fault loop impedance. Can you confirm the supply type i.e. TN-S, TN-CS, TT?
The upstairs sockets problem is a secondary issue - it is a fault which is showing a problem with the first fault. Once the earthing is fixed then start looking for the fault on the upstairs sockets.

 

[sherman0251]

will try and find out what type of system it is and will take a measurement when im there of the ze.
Thanks for your help! Will keep you up to date with my progression and maybe some more help off you!
Im only 23 and just taken my 2391 test and inspect 1 month ago! still waiting for results! lol! You might have guessed!

 

[Lectrician]

Yep, good old socket n see type testers!

When I said "Good old martindales", I was very tongue in cheek. This is the very type of fault that can be masked by someone using a martindale.

As a quick means of identifying the correct MCB they are great - for fault finding, they are useless!

 

[Spark123]

Tell me about it! (I was tongue in cheek too )
We used to use martindale isolation testers, they are not too bad - had to think twice what device you were on about!

 

[TicklyT]

If there are two CUs, it sounds like it might be a borrowed or crossed neutral to me. Isolate one board, and the borrowed/crossed neutral could drag the isolated neutral bar up to 240 Volts. Any loads left connected to the isolated CU could then drag the live up to the same potential as the neutral by back-feeding.

Pull the fuse/breaker on the offending circuit on the other board, and the live and neutral would behave normally again.


Plug one of those socket testers in, and it could easily get confused. It expects two conductors (N&E) to be at about the same potential, and one (Live) at a different potential. Those testers don't care about what the voltage is with respect to a true earth, just what differences it sees between it's own pins. Seeing live and neutral at the same potential, and earth at a different potential, it could erroneously conclude that live and earth were reversed.