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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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  • Former Community Member
    0 Former Community Member

    Of course a private substation, or a large UPS are not really consuming equipment, more supply equipment, and a bond is expected and indeed required for correct operation of the ADS though it may be some distance from the transformer, depending how things are wired. Indeed no NE bond at all is very bad on a substation type transformer as the LV side can float up to a dangerous fraction of the HV via the capacitance between adjacent windings, and must be solidly earthed.

    Apologies for not being clear here, the question is in regard to sites with private substations where the NE link is solidly bonded through the LV switchboard NE links. Interesting points on DNO derived networks though.

     

    It is common to have multiple NE bonds on one site if for example your installation is large enough to have two or more HV to LV transformers, but the N and E would normally be interlinked in such a cross-section of conductor that a diverted current equal to the full load would not be an overheating issue.

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

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  • Former Community Member
    0 Former Community Member

    Of course a private substation, or a large UPS are not really consuming equipment, more supply equipment, and a bond is expected and indeed required for correct operation of the ADS though it may be some distance from the transformer, depending how things are wired. Indeed no NE bond at all is very bad on a substation type transformer as the LV side can float up to a dangerous fraction of the HV via the capacitance between adjacent windings, and must be solidly earthed.

    Apologies for not being clear here, the question is in regard to sites with private substations where the NE link is solidly bonded through the LV switchboard NE links. Interesting points on DNO derived networks though.

     

    It is common to have multiple NE bonds on one site if for example your installation is large enough to have two or more HV to LV transformers, but the N and E would normally be interlinked in such a cross-section of conductor that a diverted current equal to the full load would not be an overheating issue.

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

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