In the offshore sector there is tradition to supply 400V sub switchboards by two transformers each able to carry the full load. Often a "2 of 3" scheme is selected where either the bus coupler or one of the feeders are normally open (NO) which will allow lower rating of the switchboard.

If the bus coupler is NO and a transformer has to be taken out for maintenance the bus coupler is closed and immediately after the transformer feeder is opened which secures uninterrupted supply. The period with parallel operation of transformers shall be limited to a few seconds as the short circuit rating of the switchboard is exceeded! Transfer can be made automatically or manual with an instruction of only a few seconds of parallel operation.

Above operation principle is applied on a number of installations, but I am not able to find any permission for short time exceeding short circuit rating in any IEC/EN standards?!
Can anyone give me a reference to an existing or superseded IEC or EN standard?

Best Regards
Preben Jakobsen
Denmark

I don't believe you will find any such permission. It is possible to build switchgear which is capable of withstanding fault levels approaching 200kA, and the only reason for installing gear with inadequate rating for duty is likely to be financial. The IEC and EN standards don't worry about details like cost.

That said, some operators do decide to take the chance, presumably weighing up the low probability of a fault occuring while the sources are paralleled against the capital saving to be made. Personally I wouldn't specify equipment knowing its capability would be exceeded, but I'm an end-user as well as a specifier so my view is perhaps different to EPC contractors who walk away and don't have to take the operational risk.

Hi ScottyUK
As you predicted I haven't found any permissions in IEC / EN standards (untill now :) )

I have however found Offshore S
tandard DNV-OS-D201 Electrical installations which in sec.2 §202 item i) states:

When a switchboard has two incoming feeders, necessary interlocks shall be provided against
simultaneously closing of both feeders when the parallel connected short circuit power exceeds the
switchboards' short circuit strength. A short time parallel feeding as a “make before break” arrangement is
accepted when arranged with automatic disconnection of one of the parallel feeders within 30 s.

Best Regards
Preben Jakobsen