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Neutral-Earth switching in transformerless UPS's

Former Community Member
Former Community Member

This question likes to rear its head within my company (consulting) every now and then and has come up a handful of times recently. The query is in regard to neutral-earth switching with transformerless UPS's and how to maintain the neutral link in place to prevent the UPS from floating during changeover.

When there are two different sources (i.e. mains as TN-S and generator standby as essential backup), regulation 444.4.6 requires the system to be ‘earthed at one point’ and clause 444.4.7 requires ‘a multipole switching device which switches the line conductors and the neutral conductor’. So the way to achieve both requirements  would be to have a N-E link within the main LV switchboard and another within the essential board, and complete the changeover using a 4 pole device to ensure the neutral is switched. 

The problem then comes of how to switch the UPS between the two sources of supply (and their neutral-earth links) and maintain a NE link in place during the changeover period. The other problem comes downstream  when the UPS is transformerless and cannot derive its own neutral for its outgoing connections, so during changeover none of the circuits downstream of the UPS have an NE link in place. 

There are a few solutions that can be implemented, but most of them contravene BS7671 is some way. 

So the question is, what is the best way to do this?

 

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    AJJewsbury: 
     

    Just to clarify - are we talking “big” UPSs here - not the little under-desk ones (typically <3kVA single phase) that will typically operate in separated mode during mains fail events?

      - Andy.

    Just because the little “under-desk” UPS can float doesn't mean it's a good idea though. The issues for using this type of arrangement for a computer include accidental or fortuitous earthing (e.g. via connected peripherals that aren't UPS backed), and static charging of wired LAN connections.

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  • 0

    AJJewsbury: 
     

    Just to clarify - are we talking “big” UPSs here - not the little under-desk ones (typically <3kVA single phase) that will typically operate in separated mode during mains fail events?

      - Andy.

    Just because the little “under-desk” UPS can float doesn't mean it's a good idea though. The issues for using this type of arrangement for a computer include accidental or fortuitous earthing (e.g. via connected peripherals that aren't UPS backed), and static charging of wired LAN connections.

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