The main auxiliary supply required is electrical power to drive gas circulators, feed pumps (where not steam turbine driven), CW pumps ventilating fans and other motor driven systems, electric actuators (including control rod drives), instrumentation control systems and main and emergency lighting. As with conven­tional stations, an auxiliary electrical system uses sup­plies from the station busbars and the generator (unit) busbars which are fed via transformers to 11 kV (or

6.6 kV), 3.3 kV and 415 V busbars. Large loads are fed from high voltage busbars and smaller loads from lower voltage, as this gives a more economical motor and cable design. Some magnox stations used separate small turbine-generators to supply the gas circulators, as turbine governor control could be used to vary the gas circulator speed and gas flow for low load opera­tion, but in practice the reactors have been operated at maximum output.

Some electrical supplies are required under fault and post-trip conditions to ensure safety, to remove reactor shutdown heat and to maintain control, in­strumentation and lighting services. These essential supply systems are described in the next section.

Other auxiliary supplies are also required. Cooling water is required on certain plant and this is supplied by electric pumps from a reservoir (or from the sea). Air supplies may also be required, e. g., pneumatic operation of valves or actuators, for which diesel or electric compressors may be used. In all cases, ade­quate redundancy must be designed into the systems in a similar way to that required on electrical sys­tems. The supply systems should be operated in sepa­rate sections with an adequate number of pumps or compressors to cover maintenance and failures. Causes of common mode failure must be removed. It may

be practicable to provide local storage and so reduce the urgency of restoring supplies after a fault.