re, was conceived on the basis of a readily available, cheap, fuel source: SO successfully employed in major organisations such as BT in the natural gas, and a high electrical base load. The latter gave oppor- tunities for the utilisation of waste heat from the engines to power absorption chiller plant used for air conditioning systems, and provide hot water for the space heating radiators in office accom- A.C. systems modation. To give some idea of the scale of such an installation it comprised five 1300kW engine-generators, and associated gear. The plant has been highly successful, giving an acceptable level of 24.2.1 Public mains supplies reliability. However, the prime factors effecting such an installation; those of a higher initial capital cost; the need for a dedicated and Where a public a.c. mains supply exists, it is normal to use this as highly skilled maintenance team; the necessity for an accurate the primary source of energy for a communications system, al- forecast of load build up; and of key importance, the need of stable though rarely in an unmodified form. Indeed, the vast majority of fuel costs, and their guaranteed availability; all need to be carefully energy consumed by equipment is d.c. and largely at low voltages. evaluated (White, 1979; Johnson, 1980; Nightingale, 1983). For big centres with large power demands of the order of hundreds There is also the case of a BT installation at Goonhilly Downs. of kilowatts, electricity supply authorities often prefer to use high- The installed engine-generator plant there has been operated on a voltage feeders. In the U.K. such supplies are commonly 11,000 site generation basis for long periods to overcome incoming a.c. volts, three phase, 50Hz, three wire. Such supplies would normally supply instability and transients, arising from lightning ionisation be transformed to a nominal voltage of 415 volts at a sub-station, and switching conditions on long and exposed overhead power lines which may be co-sited with the communications building, and be (Nig