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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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    The water is unlikely to be live unless something has gone badly wrong, it is however likely to be at or near earth potential - and it is as above the separation of the two earthing systems that is tricky, but even before you start  the disparate systems will be connected by adventitious earthing to the order of tens to hundreds of ohms anyway via their substation electrodes and anything metal that sits on the ground.

    Pure water is actually a good insulator (many megohms per cubic cm), and where you have to that can be maintained with propylene glycol antifreeze and ion exchange resins, but the addition of typically required  corrosion inhibitors and brass pipe fittings tends to spoil that ideal somewhat. Even so,  if a few feet of 22mm PEX pipe can be tolerated, the water column resistance of tap water mixed to the recommended dose with some Fernox F1 or similar can be kept to tens of K ohms or higher, so negligible coupling.

    (The corrosion inhibitor is an alkali so makes the water more conductive than straight out of the taps, to the tune of TDS of 300-800 ppm equivalent or perhaps a few hundred to a k or two ohms per cm cube .)

    Mike.

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    The water is unlikely to be live unless something has gone badly wrong, it is however likely to be at or near earth potential - and it is as above the separation of the two earthing systems that is tricky, but even before you start  the disparate systems will be connected by adventitious earthing to the order of tens to hundreds of ohms anyway via their substation electrodes and anything metal that sits on the ground.

    Pure water is actually a good insulator (many megohms per cubic cm), and where you have to that can be maintained with propylene glycol antifreeze and ion exchange resins, but the addition of typically required  corrosion inhibitors and brass pipe fittings tends to spoil that ideal somewhat. Even so,  if a few feet of 22mm PEX pipe can be tolerated, the water column resistance of tap water mixed to the recommended dose with some Fernox F1 or similar can be kept to tens of K ohms or higher, so negligible coupling.

    (The corrosion inhibitor is an alkali so makes the water more conductive than straight out of the taps, to the tune of TDS of 300-800 ppm equivalent or perhaps a few hundred to a k or two ohms per cm cube .)

    Mike.

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