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Separation of Electrical Supplies

John Squires

We are installing an air source heat pump into one of our buildings at work, a bivalent system where we are still maintaining use of the existing gas boiler. As a result of the additional electrical load, the building will require an electrical upgrade and in this case it warrants a 500 kVa substation, which is based on feedback from our DNO.

We have liaised with the DNO and they have agreed that a secondary supply can be installed just for the heat pump which will mean that the electrical upgrade will require a feeder pillar for the new supply supply and therefore remove the current requirement for a substation and therefore driving down costs.

However, there are certain proviso's for having a secondary electrical supply and one of these is confirmation of electrical separation between the second (new) supply and the building's existing electrical supply. For this, we can incorporate engineering measures into the design for the heat pump's electrical installation, its controls (use of fibre optics) and the physical pipework (include non conductive break points) as this pipework will travel between both the heat pump out-house and the main building. Although in theory we can confirm electrical separation, one area of concern has been flagged and that is the pipework will be carrying flow and return water from the heating distribution systems (bivalent use: heat pump and existing boiler), and we may not be able to provide a statement of separation to the DNO. 

I was wondering if any members have encountered a similar situation and I would be interested in how they have approached this. The main driver for myself is based on cost and the difference between having a feeder pillar for a second supply to the heat pump installation only as opposed to a substation for the entire building (and heat pump) means that we can potentially save IRO £50,000. 

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    confirmation of electrical separation between the second (new) supply and the building's existing electrical supply.

    My first question is what do the DNO mean - separation of the live conductors (Ls&N) or separation of everything, including PE? The former is usual in many situations (e.g. terrace of houses sharing metallic water/gas supply pipework). The latter will be much more difficult to achieve, in many situations (e.g. steel framed buildings) impossible. Trying to have two separate equipotential zones in the same single occupation building might be 'interesting'.

    PME often confuses matters - with some DNOs occasionally only providing an earth terminal for the first supply and insisting subsequent ones were TT'd - but there are alternatives - e.g. having one common earthing point outside the building and separate N & PE conductors from there.

       - Andy.

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  • 0

    confirmation of electrical separation between the second (new) supply and the building's existing electrical supply.

    My first question is what do the DNO mean - separation of the live conductors (Ls&N) or separation of everything, including PE? The former is usual in many situations (e.g. terrace of houses sharing metallic water/gas supply pipework). The latter will be much more difficult to achieve, in many situations (e.g. steel framed buildings) impossible. Trying to have two separate equipotential zones in the same single occupation building might be 'interesting'.

    PME often confuses matters - with some DNOs occasionally only providing an earth terminal for the first supply and insisting subsequent ones were TT'd - but there are alternatives - e.g. having one common earthing point outside the building and separate N & PE conductors from there.

       - Andy.

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