n be used to produce plants of many Figure 24.13. Once again a form of end cell switching is employed thousands of amperes capacity, by the simple expedient of adding for the d.c. plant (Humphreys, 1971) with associated enclosed lead additional rectifier units, but it suffers from the disadvantage of acid Plante' cells being maintained at their optimum constant poten- having a common control and distribution system. The latter factor tial float voltage of 2.27 volts. can be a major problem because in practical terms it has to be determined at the outset of any particular plant installation, and accurate long term forecasting is notoriously difficult. Standby Rectifier Supply 24.3.3.4 Booster converter plant Voltage Monitor A Booster converter system (Wolpert, 1978) may be employed as an alternative to the arrangement previously described, and has signi- ficant advantages. This system may use a fixed number of cells, say 23, to achieve the nominal -50 volt system. See Figure 24.12. If, in Supply 2 Cells periods of a.c. power failure, the battery discharges to an unaccept- able value, then an a.c. to d.c. converter connected to the battery Auxiliary Load Diode boosts the voltage output, and hence keeps the distribution voltage Rectifier constant. The battery can also be automatically recharged. Supply 23 Cells Rectifier Battery D.C./D.C. converter A.C. Supply Rectifier Other Modules Distribution Figure 24.12 Booster converter plant Figure 24.13 Modular plant This fundamental plant arrangement has the merit of being rela- 24.3.3.5 Modular plant tively straightforward, and can be arranged to cater for virtually any configuration of communications equipment design. The system The diversity and complexity of larger communications systems, can be adapted to cater for all but the smallest installations, and which require various voltages to power them, is such that the single or multiple engine standby plant can be associated dependent concept of centralised power plant with a tree and branch busbar on the overall degree