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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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  • 0

    How is the ‘interlinking’ section achieved? Would this not contravene the BS7671 requirement of only one NE link being in place at any time?

    Perhaps. 

    However,  it is what happens in practice when transformers are arranged so that the HV sides are in parallel, and LV sides may end up in parallel or supporting each other's loads if for some reason one TX is to be taken out of service, or an external generator fed in. 

    It is vastly preferable to have two bonds some of the time than the alternative of a system where the wrong sequence of operation may result in a transformer  without any NE bond at all and HV still applied. After a fashion it behaves like  a TNC, but it should be arranged so that the bits that might need to are bosky enough to take the full neutral current in either neutral or CPC, and as they are, or should be, before any circuit breakers for final circuits, it is not really part of the current consuming installation, more of a private distribution network.   This is not prohibited by BS7671. (The ECSQR however, maybe - but it happens, as does the practice of having an NE bond at the first breaker or fuse panel, rather than at the transformer - what do you call the cable from the TX to that panel - neutral, earth or PEN, and what does the transformer earth to ?)

    Some thought is needed with the location of earth fault relays and so on to ensure they are only on the load side of all NE bond (s), and that ADS covers all cases. (get that wrong and the fault relay won't stay on…. )

     

    Mike

     

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  • 0

    How is the ‘interlinking’ section achieved? Would this not contravene the BS7671 requirement of only one NE link being in place at any time?

    Perhaps. 

    However,  it is what happens in practice when transformers are arranged so that the HV sides are in parallel, and LV sides may end up in parallel or supporting each other's loads if for some reason one TX is to be taken out of service, or an external generator fed in. 

    It is vastly preferable to have two bonds some of the time than the alternative of a system where the wrong sequence of operation may result in a transformer  without any NE bond at all and HV still applied. After a fashion it behaves like  a TNC, but it should be arranged so that the bits that might need to are bosky enough to take the full neutral current in either neutral or CPC, and as they are, or should be, before any circuit breakers for final circuits, it is not really part of the current consuming installation, more of a private distribution network.   This is not prohibited by BS7671. (The ECSQR however, maybe - but it happens, as does the practice of having an NE bond at the first breaker or fuse panel, rather than at the transformer - what do you call the cable from the TX to that panel - neutral, earth or PEN, and what does the transformer earth to ?)

    Some thought is needed with the location of earth fault relays and so on to ensure they are only on the load side of all NE bond (s), and that ADS covers all cases. (get that wrong and the fault relay won't stay on…. )

     

    Mike

     

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