Voltage across neutral to earth

[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??

 

[Des 56]

One day someone will come up with a procedure that could be named "safe isolation"

It may very well reduce the chances of people getting bumps

 

[Obbit]

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

That wasn't directed at you. Sorry if I wasn't clear. Me and the guys in the company have a laugh about the Trade Forum version of Godwin's law. "How many posts until the inevitable accusation of not being a genuine sparks/plumber/engineer etc"

 

[Obbit]

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.

Thank you mate. I'm really glad you could really understand my question. The capacitance of the disconnected neutral is the answer. I knew it was a flaw in the testing procedure but as I couldn't explain it like you I couldn't purseude the main electrical contractor.

 

[Obbit]

Thanks again everyone who posted. I really appreciate being able to get to the bottom of this.

 

[w0z]

Thank you mate. I'm really glad you could really understand my question. The capacitance of the disconnected neutral is the answer. I knew it was a flaw in the testing procedure but as I couldn't explain it like you I couldn't purseude the main electrical contractor.

Are you contending that someone got a bad shock off an isolated cable solely due to capacitive coupling to that cable?

 

[Obbit]

Are you contending that someone got a bad shock off an isolated cable solely due to capacitive coupling to that cable?

No. He just explained the faulty test procedure I was doing which gave the voltage reading on neutral even once everything was disconnected.
The original fault which caused the shock would most likely have been a loose neutral somewhere on the circuit. This was corrected even if we never found exactly which cable was loose. The isolator is 4 pole, so maybe the neutral switch didn't connect properly when first switched on. The circuit was fully tested 3-4 times and what could be changed was, so I'm comfortable that the fault is no longer on the circuit.