Reversed polarity

[Garryash]

Can anyone explain to me why reversed polarity at the consumer unit can potentionally make all exposed metal work live (on a PME system), even though Earth and Neutral are seperate in the consumer unit? How can this happen?

[tedward]

"Earth" for this type of installation comes from Neutral at the consumer's intake position, therefore if neutral is "Live" then all earth connections will be live - how's that?

[Garryash]

Yes I understand that, but the Earth and Neutral are only joined at the cut-out position not at the consumer side and if the polarity was correct at the cut-out but incorrect at the consumer unit how can this make exposed metalwork live?

[Ohmslaw]

If Line and Neutral are crossed at a CU, then obviously you are sending your 230V through all the neutral conductors of your circuits aswell as back down your supply neutral which as you say is connected to earth at the suppliers cut-out. Ergo 230V travells from neutral to earth and through all the CPC's and extraneous conductive parts of the installation.

[JohnC]

If Line and Neutral are crossed at a CU, then obviously you are sending your 230V through all the neutral conductors of your circuits aswell as back down your supply neutral which as you say is connected to earth at the suppliers cut-out. Ergo 230V travells from neutral to earth and through all the CPC's and extraneous conductive parts of the installation.

I've read that statement a few times and im struggling to see how reversing L - N at the Consumer unit can send 230V back down to the suppliers neutral.. If you reverse L - N at the consumer unit you will end up with all your fuses / MCB's and switches in all your neutrals, but it wont send 230V back down to the suppliers neutral. Unless im missing something blindingly obvious that is..

[ICE]

Hi, are you a sparky?

[Garryash]

I agree with JohnC, how can this send 230v back down to the suppliers neutral? (if Neutrals and Earths are seperate at the consumer side). And if that was the case wouldn't you create a 'short' at the cut-out with both live and neutral being live?

[Ohmslaw]

Earth fault, think about it.

[JohnC]

Quote Originally Posted by Ohmslaw

"If Line and Neutral are crossed at a CU, then obviously you are sending your 230V through all the neutral conductors of your circuits aswell as back down your supply neutral which as you say is connected to earth at the suppliers cut-out. Ergo 230V travells from neutral to earth and through all the CPC's and extraneous conductive parts of the installation.Reversing L and N at the fuseboard will not lead to 230v being sent back down the "


simply reversing L and N at the CU will not send 230v straight back down the supply neutral and around all the earthing as you suggest. Yes you will be sending 230V around all the circuit neutrals and all the mcb's would be in what would become the neutral conductor. which would mean that all the circuits are infact protected by the main 60 / 80 / 100A main fuse. If an Earth - Neutral fault were to occur and the Neutral was in fact Live the subsequent fault current may or maynot depending on the characteristics of each circuit, cause the main fuse to disconnect in the required time. which would then lead to all the earthing and metal / pipe work connected to become "live".

In short simply reversing polarity will not cause the supply neutral and earthing to become "live". If a N-E fault occured in one of the circuits and neutral was "live" the resulting fault current maynot be enough to blow the main supply fuse, which would then cause the earthing to become live.