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SPD connection on LV switchboard with two supplies


We are working on a project where the main LV panel had two incomers, one from a transformer and one from a generator. The supplies are interlocked so only one will be used at a time (not in parallel operation).

The LV switchgear manufacturer advised that they would install the SPD on the load side in order to cover both supplies (if they installed it prior to the incomer of the TX then we wouldnt have surge protection when the generator would be operating).

Can anyone confirm if this is correct? Or maybe I need two SPDs (one for each incomer)?

I am not sure whether connecting the SPD on the load side would be protecting the system.

Thank you

  • It depends a bit.

    Are you anticipating the spikes coming in from the public supply, or from noisy loads in the main installation ?

    Is there anything on the supply side of the change over switch that needs protection, or just DNO metering  ?

    Is the genset so far away (an a shed at the end of hundreds of metres of overhead line perhaps) that it too is a source that might bring in large induced voltages ?

    The answers to these will inform the thinking.

    Along with other things like where you earth the SPD to, (as during genset operation you have to assume the external mains earth may be cut...)  and if the building has its own lightning conductors and if so where they meet the main earth...