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53Fig. 14Battery fuses with equipment for automaticcharging of the batterydeveloped a transient limiting distributionsystem. The current from the distributionrack is fed via individual cablesdirectly (i.e. without branching) to thefunctional units in the telecommunicationequipment, fig. 13. A characteristicfeature of the system is that the resistancein each of the distribution branchesis approximately 10 times the totalinternal resistance of the power supplyplant. A special two-conductor cable,with an area of twice 2.5, 6, 10 or 16mm 2 , is used in order to ensure lowinductance between the conductors.This distribution system effectively eliminatesdisturbing transients that mayarise as a result of short circuits in thedistribution network.Equipment for automaticbattery chargingUninterrupted d.c. power supplysystems usually contain storage batteriesthat serve as a standby energysource and filter. The function and lifetimeof the batteries are dependent ontheir being kept fully charged at alltimes. Thus the system contains equipmentfor automatic charging in order toprovide rapid and complete rechargingafter each discharge. The equipment includedin the standard version of BZD112 functions in accordance with a simpleprinciple based on the measurementof voltage and time. The time measurementis carried out with the aid of digitalcircuits.The function, in a 48 V plant with 23 batterycells, is as follows (fig. 15):The battery voltage falls during discharge(mains failure t, —1 3 ). When it hasfallen to 47 V (2.05 V/cell) (t 2 ) the measurementof the discharge time starts.The time measurement is interrupted (t 3 )when the mains voltage is obtainedagain. The rectifiers start to charge thebattery (t 3 -t 4 ) and the battery voltagerises. The measuring of the chargingtime does not start until the voltage hasrisen to 52 V (2.25 V/cell) (t 4 ). The charginggoes on until the measured time(t 4 -t 5 ) is 8 times the measured dischargetime, but not more than 15 hours.The charging is carried out with the currentavailable from the rectifiers until thevoltage has risen to 54 V (2.35 V/cell)and thereafter at constant voltage.It can be shown with the aid of charge/time characteristics that this type of automaticcharging gives a very highcharging efficiency regardless of theamount of charging current availableand the degree of discharge of the battery.At the same time overcharging,which reduces the life of the battery, isavoided.In addition to the charging process describedabove, periodic equalizationcharging is also carried out. The lengthof the periods can be 10, 20, 40 or 80days. The normal setting is 40 days andthe charging time 15 hours. Fifteenhourcharging can also be initiatedmanually. Functional tests of the chargingequipment can easily be carried out,and it takes only about a minute to runthrough a complete function cycle.The equipment consists of a cassetteFig. 15Charging and discharging sequences controlledwith the aid of voltage and time measurement.The upper curve shows the battery voltage andthe lower the d»»grw "' charge of the battery