The following loads must be supplied under all cre­dible fault conditions:

• Plant identified as essential to maintain adequate reactor cooling post-trip. Whilst the full power of gas circulators is not required, some circulation is needed and pony motors are usually provided for this duty. Emergency boiler feedwater and some CW will also be required.

• Information systems required by the operator in­cluding instrumentation, computer systems, etc.

• Essential controls of the above plant and equipment.

• Emergency lighting.

4.1 Essential electrical supplies systems

Normal auxiliary supplies from the grid and station busbars may be lost in the event of a fault, and sup­plies from the turbine-generator will be lost whenever a trip occurs. Electrical equipment which must remain available post-fault is therefore supplied from an es­sential supplies system which normally forms part of
the auxiliary system, but following a loss of normal supplies is separated from it. Battery systems are used to supply loads which must be maintained in continuous operation (no break), but are very expen­sive to supply large loads. Loads such as gas circu­lation and feedwater flow which can be lost for a few minutes without endangering safety are supplied from diesel generators. In the event of loss of normal

3.3 kV supplies, the busbars are cleared and diesel generators automatically started: after a ‘short break’ of no more than two or three minutes, supplies are restored and the short-break loads are reconnected as required by the operator. These short-break sup­plies are also used to feed the battery chargers and prevent the batteries being completely discharged. Small gas turbines are used at Oldbury and Wylfa instead of diesels, as they provide a more economical way of meeting the essential loads.