Voltage across neutral to earth

[Obbit]

Hi
I recently had a PV system we installed switched off after the Electrical Contractor got a shock off the Neutral. He was wiring lighting but after a processes of elimination found the fault was on our PV circuit.
We did dead tests on the circuit
Insulation resistance (1000v)
Continuity
Polarity

But no faults came up. He would not let us re connect it until the fault was found. Now the question:
He would let us connect the live but not the neutral (in case it livens up all the neutrals and earths in the school), so live and earth was connected to their DB). We disconnected the PV system so we just had a 20m 3 core flex from plant room to roof. When the live was connected to he live DB we would get 104v across neutral (not connected to DB and earth (connected to the DB)
When the micro-inverters were connected we would get 240v across earth to neutral.
When we finally connected the neutral to the DB the voltage between earth and neutral dropped to 1v.
Anyone shed any light on this? Happy to give more info if it's not clear but it's a long post already.
Thanks people

 

[streamer]

I very much doubt there was a fault on your installation which was causing the electrician working on lighting circuits to get a shock from a neutral.
How did he decide the fault was on the PV installation?
Telling you to connect the live and cpc of your circuit and not the neutral is dangerous and absolutely pointless as an energised circuit with the neutral disconnected will give you a voltage on the neutral if there is something on the circuit switched on, as you found out!

 

[Marvo]

I was going to make a comment but I'm completely confused about several things to be honest.

What's the earthing arrangement of the installation?

 

[PEG]

Yep,there is a host of details required,here...not least of all,what instrument was used to measure the voltage.

 

[dmxtothemax]

My guess would be there is a small leakage current flowing in the earth line, because when he took the first measurement this system was NOT grounded properly, so this current had no where to go. So it took the path thru the main bond and up the neutral line.Once the system was grounded properly, this small leakage current will bleed off thru the ground.

 

[Obbit]

Cheers for the comments so far. Happy to give more detail:
Earthing: PME
Testers: Fluke Multi-function and Ethos Multi-meter

How he decided it was a fault on my circuit:He spent awhile disconnecting each lighting circuit, then switched off each MCB, my circuit was one of the last.

When you say the system was not grounded properly, do you mean my circuit or all circuits. My first fault finding was to go over all my connections and visually inspect the cables and all was perfect. If you mean the whole installation, he did the same on his DB.

 

[Octopus]

But no faults came up. He would not let us re connect it (the neutral) until the fault was found.

"he" is not an electrical contractor then ................. and why didn't he just switch off the PV system?

 

[suffolkspark]

Yea either we dont have every detail here or the "electrician" didnt know what he was doing

 

[Obbit]

Focus people. I'm asking a specific electrical question not asking for judgment. If there are gaps in the explainstion then show your experience and ask specific questions.
Murdoch- re read the OP. He switched off the circuit
Suffolkspark: Re read the last line of the OP. Happy to give more info. What would you like to know?

 

[Wilko]

I'm thinking perhaps a wiring polarity fault in that circuit or problem with floating N?
Related thought - TNCS system has N and E solidly connected at the cutout. Verify. Then I'd be using wander lead to see what's actually connected to what, as large N-E voltages shouldn't happen.

 

[Obbit]

Wilko, yes we went through all that. The Electrical contractor thought it might be to do with the LPS.
But floating Neutral seems to be the key word here. I found a great article explaining it
IET Forums - Floating neutral? - https://www.------.org/forums/forum/messageview.cfm?catid=205&threadid=103200
I'm thinking
1) The initial Neutral fault on the system wasn't from our circuit after all
2) As Streamer mentions, our testing procedure was flawed by dis-connecting the neutral (and creating a floating neutral). I didn't think that, by dis-connecting the neutral of a cable (even when not connected to a load) would mean I would get a voltage reading from neutral to earth when the live of that cable is connected.

I think I've been nicely schooled here, but still open to other comments/opinions on this.

 

[telectrix]

how do you know there wasn't a load connected? any load however small will make the N of the circuit "live" if N is not reliably connected back to source.

 

[Obbit]

We can't do an insulation resistance test on the micro-inverters so the cable was dis-connected and terminated in a connector block.

 

[Octopus]

Are you a spark obbit?

 

[Obbit]

Indeed I am. Does this mean you will now give a more technical response than your last 2 posts?

 

[davesparks]

The neutral conductor will only be neutral whilst connected to the neutral point of the supply, once it is disconnected from the neutral point it becomes just another conductor and will behave as such.
So if it is connected (via a load or fault) to a live conductor is will be at that potential with reference to earth or the neutral point. If it is not connected to anything but is in close proximity to live conductors it will have a capacitively coupled voltage on it which could be anything up to the full potential of the live conductors.

 

[davesparks]

For the neutral of the lighting circuit to be live and give the electrician a shock it must have been disconnected from the neutral point of the supply allowing it to rise to a live potential.
This suggests either a borrowed neutral or incorrect isolation of the circuit.
If a light is switched off via the switch and not the MCB then all other lights on the circuit will still be using that neutral, so if the neutral is broken at the light being worked on one of the ends will become live. As an apprentice I did this exact thing on a lighting circuit in a conduit system.

 

[streamer]

The neutral conductor will only be neutral whilst connected to the neutral point of the supply, once it is disconnected from the neutral point it becomes just another conductor and will behave as such.
So if it is connected (via a load or fault) to a live conductor is will be at that potential with reference to earth or the neutral point. If it is not connected to anything but is in close proximity to live conductors it will have a capacitively coupled voltage on it which could be anything up to the full potential of the live conductors.

That is a far more eloquent way of saying what I was trying to say and makes far more sense!

 

[PEG]

Focus people. I'm asking a specific electrical question not asking for judgment. If there are gaps in the explainstion then show your experience and ask specific questions.
Murdoch- re read the OP. He switched off the circuit
Suffolkspark: Re read the last line of the OP. Happy to give more info. What would you like to know?

Hi bud,what i am focusing on,is that you may not have used the correct instrument,to determine the nature of those voltages.

 

[bigspark17]

So you disconnected the neutral into a choc block then tested from the N in choc block to earth and wondering why you got 240v??