nergy demand is very small, tens to a few hundred watts, the used where a manually transportable unit is needed. thermoelectric generator (Grant, 1986) or Closed Cycle Vapour To ensure the ability of any engine to operate over long periods of Generators (CCVT) (Gropper, 1985) may be considered, but gas or time, it is necessary to ensure that adequate fuel supplies are main- liquid fuels are, of course, still necessary. tained. The usual arrangement is for a 'day' service tank to be located fairly close to the engine set; this being automatically 24.2.3 Standby supplies replenished by pumping from bulk storage tanks, which can be buried or surface mounted outside the main building (Nightingale, The level of standby plant provision is obviously related to the 1986). probability and duration of mains failures, but any studies inevitably give results which are very subjective, and like any generalisation 24.2.5 Alternators cannot be specifically applied to any given location or point in time. Nevertheless, such assessments do give a yard stick to assess the Alternators used to supply communications equipment loads tend to type and nature of AC Standby plant necessary to meet given Mean be specified as having a good quality on-load wave form; a maxi- Time Between Failure (MTBF) targets. If an assumption is made mum total harmonic distortion of 5% is not unusual. Associated that the majority of a given communications system equipment is regulators capable of maintaining close limits of voltage, and con- backed up by, say, a one hour battery, and the supply MTBF is of trol to within plus or minus 1% or 11/2% of the nominal voltage are those orders shown in Table 24.1, then a.c. system MTBF rates of readily obtainable under steady state load conditions. Built in over- at least 75 years can be achieved, even in the worst case considered load protection and discrimination can be obtained, and to limit (Nightingale, 1985). transient output variations, and maintain the best possible compati- Since it is inevitable that failu