htingale, 1986; Woolley, 1986; Archer, 1979; Kageyama, owned either by the supply authority, or the customer. Dependent 1982). on capacity these may be of a dry winding Class C air-cooled type Whilst the foregoing cases quoted are elaborate examples, simpler for units of say 600kVA, or oil-cooled versions for capacities in the multiple engine generator plant of just a few kilowatts are entirely megawatt range. Because of safety considerations, such plant has to feasible, and available. However, there are a limited number of be installed in purpose built accommodation. In the case of oil filled examples in service in the U.K. because such excellent standards of plant, be they transformers or protective circuit breakers, oil catch- power supply reliability, and availability, are at present generally pits and comprehensive carbon dioxide automatic fire extinguishing available. systems have to be provided. Additionally, rigorous safety precau- tions and defined permit to work practices are essential operational prerequisites. 24.2.2.2 Wind, thermal, solar Medium voltage systems at 415 volts, 3-phase, 4-wire, 50Hz, or in the case of very small installations, 240 volts single phase, are Whilst not generally in service in much of Europe, or other densely widely employed for more modest sizes of communications unit. populated parts of the developed world, the potential of plant such The incoming supply, with its metering and protective gear, (usually as wind and thermal generators, and solar cell plant, has received a HRC fuses), may well be installed in a designated power room, lot of attention. Many examples are now in use in countries with the which could also contain other power plant such as a standby engine need to power remotely located communications buildings and generator system. The tolerance of the supply authority system to equipment, or where extreme climatic conditions cause problems in harmonics generated by customers' apparatus in the U.K. is becom- securing the supply of energy (Lee, 1986). Such install