Connected as both tns and tncs

Brad_88 · Sep 29, 2012

Done a board change yesterday and was renewing the meter tails when i noticed that the incomings sheath was bonded like on a tns system but there was also a 4mm earth link from the neutral side of the fuse carrier to the earth henley block which would normally mean tncs. i upgraded this 4mm to a 16mm earth but i was wondering if i should have disconnected this. not really came across this before. could this cause problems if in fact it is a tns system and connected up as both basically. what are you thoughts on this

Richard Burns · Sep 30, 2012

This type of arrangement has been mentioned on the forum before.
Some consider that have a TNCS system and also earthing the sheath of the supply cable is not on.
Others that if the sheath of the cable joins to a PME system in the street then there is little difference.

I note that you state that you have modified the connection on the DNOs property, which is slightly worrying.

Overall if your PEFC and PSC are very similar then it is likely (but not certain) that the supply is probably TNCS and I feel you should be OK with this arrangement so long as your Ze is <0.35.

However this is my opinion only.

needasparks · Sep 30, 2012

Need to be carefull as mentioned here before some look like a PME being on the side of the neutral terminal but don't always have a link and are just a separate earth terminal.

drew35 · Sep 30, 2012

This seems to be a new thing that the DNO are doing here also, they do slap a PME sticker on so I go TNCS.

dagrat · Sep 30, 2012

Maybe a phone call to the DNO to confirm whether it is in fact a TNCS

Brad_88 · Sep 30, 2012

Thanks for your replies. There was no pms label to be seen. I tested the Ze and with the link between earth and neutral and the sheath earthed the ze was 0.13 and with the link taken out and just testing with the earth on the sheath it was 0.14. The PEFC was 1.69 and the PSC was 2.09. In your opinion would you disconnect the link or leave it as is. Im just worried that if it in fact tns then would this affect the system if the link was kept in place

sparks1973 · Sep 30, 2012

might be that the TN-S was high....so someone has just `PME`d` it...

dagrat · Sep 30, 2012

If it is a true TNCS then you would expect both PEFC and PSSC results to be substantially the same

Brad_88 · Sep 30, 2012

what would you guys do disconnect or just leave it in.Im just worried that if it is not tncs but is connected as a tncs and the sheath earthed then what damage can this do. every other board i have changed has been clearly tns or tncs so this is new to me.

dagrat · Sep 30, 2012

I would definately contact the DNO and ask them, they should tell you what system it is,
and then connect as appropriate.

Brad_88 · Sep 30, 2012

ill give them a phone tomorow. considering it was a 4mm earth that linked the neutral and the earth henley block it could well be someone has added that in themselves. just for reference is it dangerous to have a tns connected as a tncs