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Three supplies

lyledunn

Consider a relatively large steel portal frame building in a remote rural location which is used for metal fabrication. The building has had several extensions over the years. Unfortunately, the DNO supply could not be enhanced to meet the growing needs of the enterprise. An upgrade was an eyewatering sum.

There is an original DNO 100A TPN supply which feeds several fuse boards around the building, a 500KVA generator supplies a large piece of machinery and a 100Kw inverter with solar PV and batteries supplying two TPN MCB boards to various items of plant. There is no grid supply to the inverter. The inverter has a NE bond and uses the steel frame as an electrode as does the NE connection on the gen set. The DNO earth is PNB and also bonded to the steel frame. 

There is plenty of equipment and opportunity for simultaneous contact with different disconnection time requirements. I haven't done any detailed analysis of touch voltages, but is this all I need concern myself about in terms of fault protection?

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    If it's all one common earthing system and the steel frame is adequately in contact with the soil to form a decent electrode if the DNO's goes off for any reason, it all sounds pretty healthy to me. You still have adequate Zs and for the local supplies enough ommph to open protective devices to worry about - but that would be the same whether the other two supplies were there or not.

       - Andy.

  • 0 in reply to AJJewsbury

    The other things that come to mind is how clear is it what is supplied from what, and where to isolate which bits of the system, and if a partial power-down of some machinery and not others could cause an issue - could a welder  keep heating but the machine loses cooling for example ?

    Obviously one has to treat all the lives as having an arbitrary phase relative to those on any other supply but apart from lamp flicker on older style florry or gas discharge lights there are almost no cases where that might matter unless there are synchronous motors on the same part of the plant supplied independently - but then the what happens if A goes off and B does not applies.

    M.

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  • 0 in reply to AJJewsbury

    The other things that come to mind is how clear is it what is supplied from what, and where to isolate which bits of the system, and if a partial power-down of some machinery and not others could cause an issue - could a welder  keep heating but the machine loses cooling for example ?

    Obviously one has to treat all the lives as having an arbitrary phase relative to those on any other supply but apart from lamp flicker on older style florry or gas discharge lights there are almost no cases where that might matter unless there are synchronous motors on the same part of the plant supplied independently - but then the what happens if A goes off and B does not applies.

    M.

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