Will you have private transformers  or two LV supplies? There are others on here who do this for a living who will probably join in at some point but some thoughts.

The interconnection of earths on the LV load side, if not by design then by things like plumbing is almost unavoidable. What must be avoided is circulating currents between the 2 systems.  In cases where for example there is a separated fire supply, then one supply is often made 'TT' so that the earth loop potential fault current is limited. Multiple TNCS/PME supplies on one site, are especially problematic.

Independent earths for the HV sides of substations is the norm and is a case of keeping the electrodes (or rather the 'nest' or mesh of electrodes) out of each others near field zones - usually a separation of 10m or more. The exception is when 2 transformers are side by side and in effect in parallel, but it sounds like that is not what is proposed.

Really for a complex pre-existing site perhaps with other factors like lightning earths and building steels, you need to engage someone who does this for a living, as there is potential for significant pitfalls - and be prepared to engage with the DNO about exactly what they expect.

Mike.

I agree - first question is whether the supply is LV or HV. HV in a way would be a lot easier since separating LV and HV earthing is pretty standard and you could then presumably have a common LV earthing system.

Otherwise, does the layout of the site lend itself to being divided up into distinct and separated equipotential zones matching the two supplies?

   - Andy.

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

I agree with the sentiment above that you really need to speak to the DNO. Before you can consider the "How" you need to get clarity on precisely "What" and indeed "Why". Not meaning to be flippant but if it's boilerplate text (intended to be) saying your LV needs to be separated from their HV, that's a different kettle of fish to separating the HV earths, and if the latter understanding why will help determine how to go about achieving it. It could be something bespoke to your site relating to the existing supply.

I wonder if the issue is transfer of ROEP from 33kV faults, or indeed 11kV faults if the earthing arrangements don't match, onto their network extending the hot zone to other customers who might otherwise have been cold, in which case I would guess the solution would look like an HV version of separating HV and LV earths, as others have said, though I don't know how you'd deal with the 11kV motor.

It's certainly one for an earthing consultant though.

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.