ectifier is provided, which is con- very light, such as tiny rural units, unless very large batteries nected to the mains supply only to reduce load to the standby engine capable of giving say 24 hours reserve can be accommodated, and set, and acts as a reserve should one of the other rectifiers fail, or be the relatively high costs are acceptable. out of service for maintenance purposes. In the full availability situation this rectifier gives an enhanced recharge capability if the battery has been discharged. Most rectifiers of this type achieve A.C. Supply regulation by static control methods comprising thyristors or trans- ductors as the main elements. Such plant can be used to supply loads Rectifier from a few amperes to hundreds of amperes, and achieve a very high Battery level of service reliability. Rectifiers Figure 24.7 Single reserve system Mains Supply 24.3.2 Dual reserve systems Mains/Standby Supply This basic system, Figure 24.8, consists of a similar plant to that of the single reserve, but to which has been added an engine generating set which acts as the long term source of reserve. This means that the d.c. plant battery can have a fairly short reserve capacity to bridge the gap between the incidence of a mains failure and the Figure 24.9 Basic d.c. plant engine being started. Typically in the BT practice this reserve period is one hour, but the value chosen is of course a matter for an operator to choose dependent on the economics of the particular case. 24.3.3.2 Small exchange plant 24.3.3 D.C. power plant design A typical example of an 'end cell switched' plant can be used to The variations in plant design are legion, but the fundamentals of illustrate a method of switching in cells to maintain supplies to some of the more common designs may be outlined. equipment within defined limits (Spurgin, 1970). The arrangement is shown in Figure 24.10. By floating the 22 cells at the optimum level of 2.27 volts per cell, the exchange load is supplied at a 24.3.3.1 Transmission equipment plant nominal -5