ng a break in the supply), or to allow alternator output at 50Hz. At these speeds the sets can be kept reliably in operation for long periods, weeks on end, and acoustic by-passing under emergency conditions. A typical basic circuit configuration is shown in Figure 24.1. noise breakout, (which can be a major environmental nuisance to neighbours), can be contained within reasonable bounds at tolerable cost. 24.2.7 Standby engine generator plant The fundamental features of a typical plant comprise the main 24.2.4.2 Gas turbines engine generator set, which more often than not in modern installa- tions, (up to say 200kW), will be in the form of an unit package Gas turbines are occasionally used, and offer substantial advantages (Henderson, 1988). This set can be mounted on a rigid steel base over a diesel engine of comparable horse power in terms of both frame which can be secured direct to the power room floor. In turn, physical size and weight. However, the limited availability of suit- this main frame can support a unit construction engine generator, able units in an appropriate range of outputs has rather limited their fitted direct with anti-vibration mountings which prevent the trans- use hitherto, and in any event they tend to burn far more fuel than mission of vibration to the building structure. Alternatively, the set the diesel, perhaps as much as double the consumption of the latter. can be mounted on a sub-frame which supports the individual These factors then, rather restrict their use to special applications electrically started engine and generator, both the latter being inter- such as mobile plant. connected by a flexible coupling. Vibration isolation is then A.C. systems 24/5 Emergency Bypass Switch A.C. Supply Mains Mains Monitors Isolate & Bypass Section Switches A & B Interlocked with Engine Start System Engine Control Cubicle Logic System Mains Contactor Standby Generator Contactor Monitors Battery Charger Engine Starter Battery Starter Motor Water-Lubricating Voltage Engine Oil-Fuel Tanks Regula