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Private network and grid connection earthing

Mark Swinn

Hi All 

I'm currently working on a project where a site is supplied via a private wire from a nearby power station unfortunately this connection has no spare capacity for the project as such we have approached the DNO for a a second connection to site one mandatory requirement they have made is that the earthing systems should be segregated, I struggle to understand how this can be achieved has anyone else been in this situation and how was it demonstrated that the earthing systems are segregated?

I have worked on sites that have two separate supplies (one from a docks ring and one from the DNO one at 11kV and one at 33kV) however we did have to  demonstrate any segregation of earthing .

Mark 

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    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.

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  • 0

    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.

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