 switching equipment has tradition- A.C. Supply ally been operated from a nominal -50 volts, and this continues to be the case world-wide. For main network or private business systems, the source of back- Battery up reserve has almost invariably been the ubiquitous lead acid secondary battery. Until comparatively recently, most users relied on a central battery type of plant to supply the equipment load, and Standby Engine for the most part modern systems derived the charging power from Generator various configurations of rectifier (Wolpert, 1987) although the very earliest designs did, of course, use motor generator charging plant. Figure 24.8 Dual reserve system Electro-mechanical switching systems such as the Strowger, and the later Cross-bar systems, have power load patterns which to a greater or lesser extent, are traffic dependent. Electronic equipment 28 volts can comprise one or more rectifiers floating a single battery however, tends to have a high level of standing load, and place (Spurgin, 1970). Such a plant will typically float twelve lead acid different demands on power plant. Plante' type secondary cells at a constant potential of just under 2.3 volts per cell. Regulation of plus or minus 2% is common. 24.3.1 Single reserve systems Such a plant will give a voltage variation of 28.2 volts under float conditions, down to 22.8 volts when the battery is discharged. At the In simple terms such a plant comprises a rectifier to convert a.c. normal float voltage of just under 2.3 volts per cell, gassing of the mains to low voltage d.c., and a battery which gives a reserve of battery is limited, whilst maintaining the battery in a healthy energy, and the period of reserve and the power loading determine charged condition. To give a high degree of plant security and hence the capacity of the battery needed. Figure 24.7 shows the elemental reliability, it is usual to have two rectifiers capable of coping with circuit. Such a system is normally only viable for loads which are the full load condition. A third r