Generator earthing....Again!

Hi All,

We've just installed a 400A manual changeover switch for a client that requires a backup generator (Not permanently installed - as It's being supplied by a generator hire company as and when there are power outages) It's a semi rural location and they seem to get power cuts several times a year.

The generator company aren't being particularly forthcoming with information regarding the sets they will be supplying (Other than saying they all have in built earth fault Leakage protection) 

The existing supply is TNCS & I know we can't rely on the DNO earthing during a power cut. With this in mind and little more information to go on from the generator company,  Should we be installing a Rod(s) and just ensuring we have a resistance lower than 20 Ohms.  Is there anything else I need to consider, Obviously my concern is ensuring that any existing protective devices will still operate under fault conditions whilst supplied by the generator.

Given that  411.4.2 now recommends an electrode at the point of supply, I assume we have no real issue with a combined  TT / TNCS arrangement!

All thoughts are more than welcome,



  • Yes, you'll need your own local electrode. That won't make it TT though - in island mode you'd normally have a TN-S arrangement (check the generator has a N-PE link, introduce one if not) and in grid mode the electrode is just another bonded extraneous-conductive-part. Obviously the change-over switch should switch all live conductors including N, so that the generator's N-PE link isn't in-circuit when running off the grid.

    And yes, you'll have to check overcurrent protective devices given that the generator will likely have a much lower ability to supply fault currents than the grid (effectively a might higher Ze, although loop testers won't usually give you a sensible reading as the generator's AVR etc can fool the meter's mechanism). At worst (and if poor discrimination can be tolerated) a suitable RCD close to the generator might do.

       - Andy.

  • If it helps, a couple of diagrams we've used before:

    basic layout:

    Swicthed alternative generator, TN

    and more specifically for TN-C-S grid supply (you may or may not need the RCD):
    Generator, switched alternative, TN-C-S

       - Andy.

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