In addition to the basic X and Y systems sequences there are some minor variations for particular fault conditions.

Loss of grid supplies

In the event of loss of grid supplies, all plant is cleared from the 3.3 kV and 415 V essential switch — board^-and the diesel generators will be automatically connected to the 3.3 kV switchboards. The plant is then sequenced to start in exactly the same manner as with grid available. Plant sequencing is delayed after all reactor ‘trips to allow sufficient time for the diesel generators to start and run-up to speed should they be needed, and also to provide a satisfactory loading schedule. Should the grid fail sometime after the trip, the switchboards will be cleared and the diesel will be connected at this stage, and all plant restarted in the same sequence as previously.

Since the starting and standby feed pumps are sup­plied at 11 kV, they are not available following loss of grid, and emergency boiler feed will be introduced into the main boilers for plant protection purposes.