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Two 11Kv/0.4 TX's on a PWN from a single DNO POC on an industrial site. What demarcation should be considered for earthing, EMC and circulating neutral currents where phase in-balance or harmonics?

Rich Mc

We have two 11Kv/0.4 Transformers configured on a PWN from a single DNO POC on an industrial site. The neutral point of each TX are commonly bonded at the MV side. Each TX has its own earth mat consisting of multiple electrodes. Both mats are electrically connected via buried bare  copper conductor between (100m apart). 

Originally the site had one TX feeding one building. Due to a recent building addition there are now two supplies effectively feeding one larger building. Both buildings are steel framed and are joined at the structures. Couldn't upgrade original TX due to increased load required.

What demarcation if any, is needed for earthing within the steel framed buildings? Will the two buildings (into one) be considered separate earthing arrangements or due to the MV side of both being bonded single earthing arrangement - 542.1.3.3 appears to contradict? 

Several other issues also spring to mind including adequacy of protective devices, circulating neutral currents and EMC. 

Many thanks in advance for any thoughts

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  •  , You haven't said for either transformer how the earthing of the neutral on the LV sides are arranged (only that they are bonded on the HV side).

    The risk that Reg 542.1.3.3 is trying to alleviate is that of currents from the HV side being diverted through the installation.

    Are the HV and LV earths interconnected at each transformer? If so, when you connect them back together in the building, you get HV fault currents as a possibility in your building, which is undesirable, and for that reason Reg 542.1.3.3 ... in this case, a DNO would usually provide an earthing terminal for one of the supplies, but not both (meaning you need to TT one or both - which is an option regardless, except for additional cost of protective devices, of course).

    However, if the HV and LV earths are not interconnected, and the buildings/installation are kept a suitable distance from the transformer earthing system influence (e.g. > 10 m from the earth nests and other buried/accessible conductive parts that are affected by transfer potential, although this could be less depending on the soil resistivity profile and HV fault calculations) then you make a new LV earth nest for the LV supplies building, and then the systems DO share a common earthing arrangement and the two supplies can co-exist as TN-S in the same building. This would be what GN8 refers to as a private 'PNB earthing arrangement'.

    Relevant experience with BS EN 50522 and BS EN 61936-1 are required to make the decision on this one.

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  •  , You haven't said for either transformer how the earthing of the neutral on the LV sides are arranged (only that they are bonded on the HV side).

    The risk that Reg 542.1.3.3 is trying to alleviate is that of currents from the HV side being diverted through the installation.

    Are the HV and LV earths interconnected at each transformer? If so, when you connect them back together in the building, you get HV fault currents as a possibility in your building, which is undesirable, and for that reason Reg 542.1.3.3 ... in this case, a DNO would usually provide an earthing terminal for one of the supplies, but not both (meaning you need to TT one or both - which is an option regardless, except for additional cost of protective devices, of course).

    However, if the HV and LV earths are not interconnected, and the buildings/installation are kept a suitable distance from the transformer earthing system influence (e.g. > 10 m from the earth nests and other buried/accessible conductive parts that are affected by transfer potential, although this could be less depending on the soil resistivity profile and HV fault calculations) then you make a new LV earth nest for the LV supplies building, and then the systems DO share a common earthing arrangement and the two supplies can co-exist as TN-S in the same building. This would be what GN8 refers to as a private 'PNB earthing arrangement'.

    Relevant experience with BS EN 50522 and BS EN 61936-1 are required to make the decision on this one.

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