res of the supply authority a.c. mains bility with the normal mains supply, low impedance machines are supply will occur from time to time, and since long term standby desirable. using batteries would be prohibitively expensive in terms of capital cost and utilisation of accommodation, some form of prime mover standby generating plant provision becomes prudent in all but the 24.2.6 Control systems smallest installations (Nightingale, 1971). Control systems (Mealing, 1984) for modern installations can give 24.2.4 Prime movers standby engine generator plant which is fully automatic in oper- ation. Built in monitors are typically set to detect that the mains has 24.2.4.1 fallen below say 10%, (or gone above 6%), and signal the engine Diesel engines plant to start. Once the plant has reached stable and optimum Diesel engine driven generators (alternators) are the normal source operating conditions, the load would be switched to the generator output. The integrity of the plant's fuel, lubricating oil, water and air of standby a.c. power (Jones, 1989). They tend to be used because they are capable of being made such that they are reliable in starting cooling circuits would be continuously monitored, and pre-set alarms given in the event of any malfunctions. Plant can be auto- and operation, capable of being started automatically; they use readily available fuel which presents few problems in storage and matically restored to the normal quiescent condition once the mains have returned to within acceptable limits for a predetermined time, handling; and there are a wide range of types and ratings available or of course, the system can be restored manually as determined by world-wide. Set speeds tend to be kept down to a maximum of 1500 revolutions per minute (rev/min); with larger sets operating at 1000 the operator. Suitable hand operated by-pass switch-gear should be or even 750 rev/min. Such speeds, of course, give a synchronous provided to allow the plant to be totally isolated for maintenance purposes, (without putti