Neutral Bar in Consumer Unit

[newby10]

Hi! all,
I have worked as an electrician's mate for years. I recently did a PAT course at a local college. The tutor told us that in a domestic installation, if you were to touch the neutral bar, you would get a shock. This confused me, as although I'm not a qualified electrician, I always thought that my understanding of electrical theory was quite good.
I understand that there has to be a difference in potential for current to flow, so how can there be a voltage on the neutral bar.
Hope someone can help me out.

 

[telectrix]

only time you'll get a shock off the neutral bar is if the supply neutral is missing, or the polarity is reversed.

 

[newby10]

So I was right. Many thanks

 

[telectrix]

some tutors live in cloud cuckoo land. mine once told us that the 110v used for on site tools was derived from a 230v 3phase transformer using 1 phase.

 

[Kamikaze]

Telectrix is correct, but all neutrals should be treated as live conductors. Under certain conditions you can get a shock if you happend to be touching the neutral bar and the earth at the same time. You might need some hair straighteners after that.

 

[newby10]

Thanks Kamikaze. If I'm right in my understanding that if you were to measure between live and earth, you should get a Ov reading.
The way my Tutor (ex tutor now) explained it was that because AC voltage goes both ways, the neutral was live for one half of the cycle.
That to me means that if there are (in theory) two phases of 230v, there is no potential difference, and therefore no current flow.
As I said, I am not a qualified electrician, but am desparately trying to get my head around this AC Theory.

 

[gutterball]

i was told to assume all conductors were live unless proved otherwise.

 

[newby10]

Yes me too gutterball,
but my point really was that if there are no fault conditions, the neutral bar should have no voltage on it.
Surely the Earth and the Neutral are connected at the power station, so there should be not voltage there?

 

[Kamikaze]

Your understanding is incorrect. Potential difference between live to neutral= 230V, Live to earth= 230V, Neutral to earth = depends on their supply connection, most of the time 0V.

 

[telectrix]

Yes me too gutterball,
but my point really was that if there are no fault conditions, the neutral bar should have no voltage on it.
Surely the Earth and the Neutral are connected at the power station, so there should be not voltage there?

you are right there. your ex tutor is talking through his other end when saying that. obviously as kamikazi and gutterball have said, for safety, always treat neutral as a phase conductor. normally it should have no potential diff. wrt. earth but in fault conditions, it may become "live"

 

[newby10]

Thanks, that's what I meant. I understand L to N 230v L to E 230v.
So how will the supply connection affect the voltage between Neutral to Earth.
I'm not questioning you by the way, I just need to know.
Hard to express yourself on here.

 

[Kamikaze]

If the earth is connected to the neutral at the supply then they should both be at the same potential. However, If the earth is not connected to the neutral at the supply then they could have a different potential under fault conditions. Just be careful!

 

[telectrix]

imagine a circuit, 230v on the live. goes through a load to neutral. all the 230v is dropped across the load leaving the neutral at 0v. volt drop across load is I x R = 230. now, if the neutral is not connected to the supply for any reason, then current flow is 0Amps, therefore ther is no volt drop across the load, so there is then 230v on the neutral. if you were to get hold of that neutral, then you would become the load, down to earth. PAINFUL

 

[Colin Integrale]

Got to love a backfeed off a borrowed neutral.