TNS system Earthing and Neutral

[wcpg12]

On testing an 3 phase supply I had various voltages between live and earth (old colours) red 263v, yellow 240v, and blue 210v but 240v between live and neutral on all colours this being a TNS system I then tried a ZE test and could not get a reading on further investigation I could not find a link between earth and neutral at source or at the LV board any opinions????

 

[Knobhead]

I beleave it is good practice to install a seperate earth conductor but please bear in mind this installation is over 30 years old and the swa would of been used as the main earth at that time, it is bonded at the tx room via the gland and at the LV board , now here comes the fun the earth taken from the gland (banjo) to the MET is 16mm, but there is still no link to be found between the neutral and earth ether at the tx room or the LV board

It looks like your going to have to fit one.


This is a post I wrote for another board, this was for a 500KVA TX

One to add to the experience pile. Got a call to a plant, all the office equipment had failed. On arrival I tested the supply to the office and all seemed OK. Called the lads from office services to come and sort their junk out. (It was junk, it's a wonder you didn't need to shovel coal in to the computers). All power supplies smoked! I'm puzzled now, how could 4 go wrong together. Got talking to the plant manager and he tells me that the lighting had "gone weird" for a while. Went back and checked the Ph -N voltages again and all OK.
It wasn't until lunchtime when I got talking to some of the other electricians that things became clear. Two of them had installed a 150mm 3½C cable in to the plant main switch board that morning and had disconnected the neutral - earth cables to make it easier to get their cable in, when finished they reconnected them. To quote one of them "I thought there was a bit of a spark!" What galled me was I'd fitted an engraved warning label to the earth bar when I installed it!

 

[wcpg12]

Depending on the protection fitted the N/E link could be at the transformer but not likely. It would be fitted where there’s easy access for testing. If REF is in use then the link will definitely be at the main board.

I’ve only ever seen a N/E link fitted in the TX chamber once, that was in India. A real bodge job! I would have been ashamed of it.

OK small TX’s the link is likely to be in the LV box, but not in a 250KVA TX.
I say 250KVA because of the cable size.

The N/E link is not at or near the tx room or in the LV board, there is no link, the incoming supply neutral is connected directly to the main neutral bar which has no facility for a link, being a tns system there should be a link close or near to the origin of supply, or at the LV board, as you say. Being a 240mm 4c supply I would still expect to see a seperat earth cable from the main earth bar in the tx room but there isnt one either, there are relying on the swa as the main earth to the LV board, the tx is a 500kva 11kv/433v it did supply another building but that has since been disconnected.

 

[Knobhead]

For 500KVA the cable is under sized for a start. 800A switch is the right size, TX FLC is 666.6A.

For all TX’s I’ve fitted I’ve run 95mm earths from the earth bar to each of the:
· 11KV termination
· Tank
· 2 X neutral link
· 2 X earth nest
This was standard up to 1000KVA in our company.

Just to spoil things, what are the earthing arrangements?

This is the last transformer I fitted

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