0.6 volts. The two 'end cells' (which are not connected to the load in the normal operating condition) are floated by a Where the equipment can accept a fairly wide spread of voltages, it separate rectifier to give a voltage of -4.6 volts. The rectifiers are is possible to use the basic circuit set out in Figure 24.9. A typical rated to cope with the full load of the exchange equipment, and plant used in association with transmission equipment at a nominal recharge the battery as necessary. 24/10 Telecommunication power systems Rectifier No.1 A.C. Supply Two Cells 4.6 Volts A.C. Supply 22 Cells 50.6 Volts Rectifier No.2 Figure 24.10 Small exchange plant If the plant voltage should fall for any reason, for instance the a.c. volume per unit of switching capacity, leaving a disproportionately mains fails, then at about -46.5 volts the full 24 cell battery is large power plant (O'Connor, 1990). The solution being developed switched across the exchange load by the contactors. The plant for the future, is for the application of an a.c. to d.c. converter using voltage is therefore boosted to -48 volts. It should be noted that the switched mode techniques as later described in Section 24.3.3.6, contactors are interlocked to prevent their simultaneous operation and an associated operational battery. Instead of an engine-gener- which would short circuit the end cells. When the mains restores, ator back up for any long duration a.c. supply failures however, an the 24 cell battery recharges to -52 volts, and the contactor sequence innovative design of fuel cell is proposed. With this system, if the is then reversed, and the two sections of cells continue to recharge associated battery approaches its designed point of discharge, a to their full capacity, with the load again being fed at -50.6 volts. mechanically rechargeable aluminium-air fuel cell battery (O'Cal- This sort of plant has a limited rating, about 100 amperes maximum. laghan, 1989) is brought into use, and via suitable d.c. to d.c. More modern installations may w