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????

 

[ezzzekiel]

what link were you looking for an a tns?

 

[truckster]

Voltages fluctuate all the time but the lack of an earth reading is especially worrying. Where did you make the tests?

 

[wcpg12]

what link were you looking for an a tns?

On a TNS system there should be a link between the earth and neutral at source or at the LV board

 

[wcpg12]

Voltages fluctuate all the time but the lack of an earth reading is especially worrying. Where did you make the tests?

The tests were made on the incoming supply cable at the Lv board the supply cable is a 4c 240mm using the swa as the earth as there is no seperate earth conductor installed from the tx room

 

[NMc]

Why are you expecting a link between earth and neutral on a TN-S supply?

 

[wcpg12]

A TNS system should have the neutral of the source of supply connected with the earth at one point only,or as near as possible to the source.

 

[pushrod]

Think you are confusing a TN-C-S.

 

[wcpg12]

Even on a TNS the neutral is still connected to the earth at the source of supply (only at one point ) where as on a TNC-S PME) system the supply neutral conductor is connected to earth at intervals along its run

 

[Knobhead]

Is the incoming LV cable direct from the TX? If it is there should be a neutral / earth link in the LV main board.

What voltage are you getting between neutral / earth at the main board? If the link is fitted there should be non what so ever.

There’s no confusion with TN-C-S. Even a TN-C-S system will have a link fitted at the main board.

 

[truckster]

I may be wrong and the BRB is not to hand but if you have a 240mm swa you should have a seperate eathing conductor as the armour is not big enough. If it's a TN-S system the earth will be bonded to the sheath at the origin. You can't have a PEN conductor in a consumers premises ESQC Regs

 

[pushrod]

on further investigation I could not find a link between earth and neutral at source or at the LV board any opinions????

Didn't realise that your further investigation took you back to the transformer.

 

[wcpg12]

The LV cable comes direct from the transformer via a 800amp main switch (which is somewhat old) then to the LV board there is no link at the tx room between neutral and earth or at the LV board, there is a voltage of 29v between neutral and earth

 

[wcpg12]

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

 

[Knobhead]

I may be wrong and the BRB is not to hand but if you have a 240mm swa you should have a seperate eathing conductor as the armour is not big enough. If it's a TN-S system the earth will be bonded to the sheath at the origin. You can't have a PEN conductor in a consumers premises ESQC Regs

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.