Mark Swinn said:

how was it demonstrated that the earthing systems are segregated

Between HV systems, and between HV and LV systems, this is achieved by separation below ground of any earth electrodes, and any other connected buried metallic parts. The DNO will have guidance on the separation distances (depending on the circumstances) that are necessary for separation of earthing systems.

You don't say what the earthing arrangements for the LV supplies are. The DNO's position on separating LV earths is supported by Regulation 542.1.3.3. The issue comes with how you use the supplies according to BS 7671, and whether you are going to use the distributor's earthing arrangements, because Regulation 411.3.1.1 requires simultaneously-accessible exposed-conductive-parts to be connected to the same earthing system.

If the existing supply is taken at HV, OR takes an LV supply with a TT earthing arrangement, and the site has its own earth electrode system that's separated from the incoming HV, and the same is applied to the new supply, then provided the physical separation below ground from both HV supplies (or LV supplies as appropriate) meets the DNO requirements, there is no issue.

If the supplies are received at LV from the DNO, and the exisitng supply is a TN earthing arrangement (TN-S or TN-C-S), things start to get a little more tricky if you need to co-locate equipment fed from each of the two supplies.

The site is currently supplied at 33kV on the private wire and as far as i'm aware is solidly earthed back to the supply point (I am struggling to find any drawing showing this), the 33kV is then distributed down to 11kV, 3.3kV, 415/400V  with each substation and area of plant having its own earthing arrangement  linked together and as such a global earthing system is claimed.

And what is the proposed new supply going to be? 33kV, 11kV, 0.4kV?

  - Andy.

11kV

My first thought then would be to use the DNO's earth up to and including the primary of the 1st transformer on the new 11kV supply - and then use the site's earthing arrangement for the secondary and onwards. Much like is traditionally done for a "hot" site separating HV & LV earths.

   - Andy.

ok I will look into that, the only complication maybe that we will have and 11kV Motor as well as the distribution down to 3.3kV and 400V but I would assume as long as the motor has a seperate earth connection back to the HV earth then this shouldn't be a issue

the only complication maybe that we will have and 11kV Motor as well as the distribution down to 3.3kV and 400V but I would assume as long as the motor has a seperate earth connection back to the HV earth then this shouldn't be a issue

In that case you'd have to keep the two earthing systems out of reach of each other - if the motor is own its own in a field with no connection to anything else that might be OK, but if it's part of a larger plant with metal pipes or structural steelwork interconnecting with your other earthing system (or just within reach of it) then it's not so simple.

   - Andy.

the only complication maybe that we will have and 11kV Motor as well as the distribution down to 3.3kV and 400V but I would assume as long as the motor has a seperate earth connection back to the HV earth then this shouldn't be a issue

In that case you'd have to keep the two earthing systems out of reach of each other - if the motor is own its own in a field with no connection to anything else that might be OK, but if it's part of a larger plant with metal pipes or structural steelwork interconnecting with your other earthing system (or just within reach of it) then it's not so simple.

   - Andy.