3 phase problem

[a1colly]

Hi all

This is probably very easy but for the life of me I haven't been able to discover the answer and solution.

Three phase taken to a tpn fused isolator, then taken to a sub meter and split to three spn consumer units (each being sub metered again) The neutral has been taken to a Henley block and split to the three boards and meters.

Phase sequencing checked and correct at tpn meter, and also correct at the spn meters. however upon testing incorrect polarity at all consumer units, despite when testing polarity the MFT says its correct.

I have 210V. L1,L2 & L3 to N. across phases I have 410V and to earth I have 245V. Which seems odd as would expect 245V and my instant thought is a loose neutral connection.

Continuity and insulation checked and all okay. It a TNS system with a reading of 0.18Mohms however when trying to complete a Zs or RCD my MFT gives an incorrect polarity result.

I have checked that I haven't wired up the supply incorrectly for the tenth time but all looks correct. I have left isolated for the night and returning in the morning after a good nights sleep to look at again.

Any help appreciated as my brain is not engaged at the moment.

Thanks and sorry if it is so obvious

 

[SirKit Breaker]

Have you got your leads on the correct terminals when you do the Zs and RCD tests? Its probably late in the day, and you have had enough, easy mistake to make, and miss the obvious. I can't see that you have done anything wrong other than not connecting your meter properly. Or have you wired the meters up wrong?

Cheers……………Howard

 

[Lucien Nunes]

It does sound like your neutral is nearly floating, if your tester connections are correct and the tester working correctly. For %$£'s sake don't connect any loads yet! What do you get when you test Zpn with a 2-wire connection? Zpp? The difference between these two should show it up.

The incorrect polarity indication can be misleading, it does not always mean that the line and neutral are reversed, on some testers it can occur whenever the neutral is not near earth even if the line is OK.

BTW what is measuring 0.18 megohms?

 

[a1colly]

Cheers Howard nah triple checked the leads, made that mistake before in poor light conditions. Even stuck them on the terminals the other way just incase

 

[a1colly]

Hi Lucien Zpp is 0.18 zpn is greater than 2000 which pushes me more to the neutral connection issue

 

[Darkwood]

Yep after reading OP what stands out is the difference between Ph - N and the Ph - E value especially because you have a better Ph to E than you do N.

What is the nature of the supply out of interest?