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Switchgear rating HSFT (closed transition) IEC rules?

Switchgear rating HSFT (closed transition) IEC rules?

Switchgear rating HSFT (closed transition) IEC rules?

(OP)
I have an Australian site which will have duplicated identical supplies through out the HV and LV distribution networks. A single HV feed to the site feeds identical transformers which in turn feeds identical switchboards feeding multiple transformers (which always have duplicates) feeding MCC's with duplicated incomers and a bus tie. Normal operation is with open bus tie with each transformer carrying approx 40% of rated load. Across the site approx 80% of the connected loads are VSD's. Most of the loads are motors (usually connected to pumps).

Current planning in response to a fault (or abnormal condition) is for the CB's to isolate the faulted radial feed and close the bus tie automatically and as fast as possible. To me this is an open transition and little chance of the fault levels exceeding the nornmal operation condition. The only real concern is the time it takes to put supply onto the dead bus and consequential issues associated with the motors (eg: out of phase reconnection).

In return to normal operation the isolated incomer CB will close and the bus tie will open as fast as possible. To me this is closed transition and favourable in terms of the reaction of the loads. The issue is the fault levels significantly change.

Is there guidelines from the IEC standards in terms of the closed transition operation and the fault current rating of the switchgear?

In IEC 61936.1  7.1.1.2 Installations shall be capable of withstanding the thermal and dynamic stresses resulting from short-circuit current in accordance with clause 4. The circuit arrangement may, however, be configured in such a way that sections of the installation which are normally operated separately are interconnected for short periods during switching operations, even when, as a result of such connection, the short-circuit current exceeds the design rating for the installation. In such cases, suitable protective measures shall be taken to prevent danger to personnel. Defined operating procedures may be required for this purpose.
NOTE 1 This situation may be unavoidable in operation if, for example, feeders are switched from one busbar to another.

For me a fault occurring during the transfers would be extremely unlikely except if the operated CB caused the fault (3P or Earth fault). In this case I believe the fault current either side of the fault location is the same as it would be under the normal radial feed case and therefore less than the rated breaking fault rating of the current. The CB causing the fault could have fault currents supplied by both radial feeds (ie: double). The make fault current rating of the CB's is more than double the break fault current rating.
 Once the fault is initiated I'm assuming that both incomers will work to clear the fault. The nromally operating CB would be able to safely break the fault current contribution from it side (being less than the break fault current rating of the CB). If this was to beat the faulted CB then the faulted CB would be left to break the fault current from it's respective transformer (being again less than the breaking fault current rating of the CB).

My apologies for such a long email.

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