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Thoughts on Temporarily Paralleling Two 1600A ACBs with a Bypass Castell Key?

AMK

Hi everyone,

Looking for some insights on a proposed electrical setup and potential risks. We have a single LV switchboard with two 1600 Amp ACBs, each fed by separate transformers but connected to the same HV ring main. Both ACBs are equipped with Castell keys for interlocking. Normally, if one transformer fails, the procedure is to switch off its ACB, remove the Castell key, and close a bus coupler to maintain supply.

We need to perform some HV maintenance, which requires isolating each transformer one at a time. To avoid any supply interruption, there’s a suggestion to use a bypass Castell key to temporarily parallel the two supplies for a few seconds. The idea is to parallel the transformers, then switch off one ACB, keeping the load live throughout.

Has anyone done something similar or seen issues with this approach? Keen to hear your thoughts and experiences!

 Thanks -Andy

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  • 0

    Subject to a detailed check that both supplies have the same phase rotation, phase sequence, and are reasonably close in voltage. then it would probably be fine. But is "probably" good enough ? perhaps not if one remembers just how destructive and even fatal the consequences could be.

    I have done as described, more than once, without ill effect, and I am still alive. But the fact that I have got away with it, does not prove that so doing is sensible.

    The main risk is the near doubling of fault current, when the two transformers are briefly paralleled. It is very unlikely that a fault would occur during this brief interval. But is the risk "low enough" ?

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  • 0

    Subject to a detailed check that both supplies have the same phase rotation, phase sequence, and are reasonably close in voltage. then it would probably be fine. But is "probably" good enough ? perhaps not if one remembers just how destructive and even fatal the consequences could be.

    I have done as described, more than once, without ill effect, and I am still alive. But the fact that I have got away with it, does not prove that so doing is sensible.

    The main risk is the near doubling of fault current, when the two transformers are briefly paralleled. It is very unlikely that a fault would occur during this brief interval. But is the risk "low enough" ?

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  • 0 in reply to broadgage

    I endorse everything that has been said. The interlock is there for a reason.  Having said that I cant think of a reason why you couldnt check whether the equipment could be safely used in this way.  This isnt a comprehensive list but you would need to calculate the worst case fault levels and check installation details such as cable sizes and lengths.  I would also want to talk to the switchboard manufacturers re fault levels etc and check that appropriate terminations etc had been made. There is also the matter of the fault capability of downstream equipment. If there are two transformers fed from a single HV source or ring (with open point) the fault level wont be as high as if there are two independent supplies. 

    This might all be worthwhile if the disruption caused by a switch off is severe but if that were the case you have to ask why it wasnt designed in in the first place.

  • 0 in reply to broadgage

    There is always the extra risk, not well covered in the safety literature, that for any temporary works and unusual arrangements, that there are many layers of protection from the Swiss cheese model that have been implicitly removed by the "it'll be alright" assumptions we all make (esp those that say they don't ;-). Most Swiss cheese approaches cover normal running and common failures (until the unusual happens).

    It'll be a "Normal Accident" (Perrow) once it's happened. Reason's "Human Error" is also a good read for how these slips, lapses, and mistakes occur.

    Also beware of the 'false' "What's the worst that could happen" being discussed here which are local technical issues (nasty big bangs), rather than also considering the consequential 'worst that could happen' - e.g. Heathrow shut down et.al. vs small(ish) transformer fire. If it's a Hospital supply the minor slip-up becomes big news

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