phase to Neutral and Neutral to Earth voltages.

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davidk65 | 10:35 Sun 27th Feb 2011 | DIY
7 Answers
I have been following a thread on another forum in which a guy reported that the neutral bar on a fused board was at times "glowing hot" At other times, when a neutral was disconnected there was a spark. This was on a 240 volt single phase AC supply. He also reported that when measuring voltage across neutral to earth he got a reading of 130 volts.
This intrigues as I have never come across this before. I have had three phase motors and battery chargers burn out due to a phase having gone down on a three phase supply, but never this.
With regards to disconnecting the neutral, I thought the rule was never disconnect whilst it is carrying current. Could that be the reason for the sparking i.e. it was in use?
Would a loose/bad connection on the earth or the neutral be the cause of the 130 volts on neutral to earth. On average would one always expect a zero voltage reading?


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I hope BrightSpark or one of the other electricians sees this. I'd love to know the answer.
The trouble is, you and I only have second-hand details. We don't know the full story.
As you know, a neutral bar that hot ........... means it's carrying a huge current load. It surely must be a fault current, and the protective device(s) i.e. fuse/MCB has been bypassed or is mega faulty. Maybe it's the old 4-inch nail for a fuse syndrome!
It may depend on what the earthing arrangement is. It could be combined earth and neutral......... so there would be no potential difference between them.
It may have earth and neutral combined at the incomer, but separated through the system.
Maybe ................ it's an old TT system where there is no earth incomer...... just a good old earth rod in the ground. If the earth rod has become disconnected, a fault current might be finding its way to earth through......... anything!
Could go on guessing forever. Either way, any decent Spark would surely not take very long to suss out the system and/or faults.
Need to know more to determine why the N bar is "glowing"...not good..could be as TB suggests a v high res joint with poor protection. As for N conductor arcing on disconnection...this is either due to the circuit being energised...a "borrowed" N from another circuit..(the cause of a lot of nasty suprises)or circulating N currents ...usually due to energised inductive loads elswhere on the system.As for the 130v N-E... without more info is not easy to diagnose.
Are you sure this is on a single phase supply as it would make more sense if it were an unballanced three phase and N.
Question Author
Hi Bright Spark.
As far as I know it is a 240v single phase. I thought your comment on a borrowed N was interesting.

I thought I would do a quick check at my home. I found this:
L -N 240 volts. N- E 30 volts. L - E 30 volts.
The above readings taken with the normal household stuff connected and drawing current.
I was surprised by the L - E reading as I expected mains voltage. On N - E. I expected something just above zero volts.
Looking at the incoming supply at my home" (100amp Cut Out) there is a sticker that reads "PME Earthing System Terminal".
Sorry I'm no help here David. I'm just interested.
You might know that, in PME systems ......... all that comes into the property is a phase live and a phase neutral. They strap an earth to the neutral to give you an earth. N and E are entirely separate around the installation,
With the N and E effectively connected in parallel around the house, it follows that each can mimic each other. I'm not explaining this very well. I'll leave it to BrightSpark.
Apart from all the "funny" things that can go on with borrowed Neutrals etc, it has been known for neighbouring faults and "idiosynchrasies" from next door to show up with PME ......... in extreme cases........... because of the shared N/E
I'm out of my depth with 3-phase, but I'll bet it has something to do with it, as BSpark says.
In a street supplied by PME, any of the houses could be at different phases, so if a connection, or an N is "borrowed" from next door............. all kinds of silly things can happen. I just remember my tutor telling me all this .......... we left it at that though lol
Question Author
Thanks for that comment.
I have had another look at the installation in my home and I have found the problem. I dismantled a few bits and pieces, reconnected them and bingo it cleared. By that I mean I now get the following reading.
L- N. 240 volts. L - E 240 volts. N - E zero volts.
Thanks again.
I would have thought 3-phase was involved here somewhere.
Would not think you could get this happening with 1-ph.

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phase to Neutral and Neutral to Earth voltages.

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