TN-C-S (PNB) versus TN-C-S (PME).

My understanding is that the TN-C-S(PNB) earthing arrangement is installed by the DNO to provide a minimum of 8.000 metres separation between the transformer HV earth system and LV earthing system provided for the customer by the DNO.

www.spenergynetworks.co.uk/.../EART-01-002.pdf

Sparkingchip said:

My understanding is that the TN-C-S(PNB) earthing arrangement is installed by the DNO to provide a minimum of 8.000 metres separation between the transformer HV earth system and LV earthing system provided for the customer by the DNO.

Agreed ... but there are two variations

 - one in which the system is earthed at one point only (at or before the first customer, typically of a maximum of 4 customers). This, however, is still NOT TN-S.
 - one in which the system is earthed at or close to each customer's premises (again typically no more than 4 customers). In this case, the N-PE siplt is often at the customer's premises.

Whilst one variant of PNB is closer to TN-S than PME, I'm sure it's not correct to term it 'TN-S'.

TN-C and TN-C-S systems come in more variants than 'PME'

My understanding is that the TN-C-S(PNB) earthing arrangement is installed by the DNO to provide a minimum of 8.000 metres separation between the transformer HV earth system and LV earthing system

From my occasional gazing at rural distribution systems I think it's quite common to have the (first) LV earth "one span" away from the transformer - as with HV overheads (what we used to call "hot" sites) the LV earth needs to be kept separate from HV earth regardless. I'm sure I've seen it on what must be PME on the LV side (as it feeds >>4 customers).

My understanding was PNB was used where there really wasn't really the space to have multiple electrodes on the LV side (e.g. one span away from the transformer is the house). Of course it then gets extended when another house gets built next door... then it gets a lot messier.

   - Andy.

AJJewsbury said:

I'm sure I've seen it on what must be PME on the LV side (as it feeds >>4 customers).

Agreed, that would normally be done to full PME requirements.

AJJewsbury said:

I'm sure I've seen it on what must be PME on the LV side (as it feeds >>4 customers).

Agreed, that would normally be done to full PME requirements.