o a minimum the need for distribu- tion cables or busbars. Most communications d.c. power systems rely heavily on the lead This concept carries with it the merits of flexibility in the way that acid secondary cell (Feder, 1987; Halliwell, 1979). it is associated with the equipment it serves. Demands on purpose A high proportion use cells of the Plante' form of construction. built accommodation are reduced, and there is an enhanced oppor- Such cells have negative plates which have open grids which sup- tunity for factory assembly and testing of the power unit with the port a paste of active materials, and positive plates which are pure switching equipment concerned, together with reduced on site in- lead and have a ribbed form of construction to give a large surface stallation time (Yotsumoto, 1988; Luigi, 1989). Figure 24.14 shows area. A 'forming' procedure, which is achieved by the passage of a the basic configuration of the power system, which has at its heart carefully controlled current in a special forming process results in the switched mode power unit (Fry, 1982) and the valve regulated the active material of the negative plate becoming pure lead, whilst gas recombination design of secondary cell (Harrison, 1982) which the positive plates are converted to lead peroxide, and which has a is fully sealed under normal operational conditions. characteristic brownish colour. Power supply distribution 24/13 Early power plant tended to use various forms of automatic charge able, and from a commissioning point of view, there is a need to take discharge systems, but these have largely disappeared in favour of greater care with interbar joints. the SO called float trickle charge methods using the rectifier plant As an alternative to busbar, hybrid arrangements using a combi- previously described. Given that batteries of this type are charged at nation of bar and cable distribution are feasible, or even all cable about 2.27 volts per cell within the limits of plus or minus one systems. This is especially the case wit