OVER-VOLTAGE PROTECTION 593 



dynamo which is still connected across 60 cells, is raised until the 

 desired charging current is obtained. When the end cell group is 

 fully charged, as indicated by free gassing and maximum specific 

 gravity, switch A is thrown up, cutting out the end cell group 

 and the charging of the main battery is completed. 



An overload circuit breaker is provided in the positive lead 

 from the generator. In some cases a reverse-current trip has been 

 provided for this circuit breaker; but this is usually omitted, 

 owing to the fact that a momentary variation of frequency on the 

 system might lower the speed of the motor generator set and reverse 

 the current, thus tripping the circuit breaker unnecessarily. A 

 momentary reversal of current through the generator would 

 usually be quite harmless. 



In the battery leads fuses are inserted rather than circuit 

 breakers, as it is not desired to have the battery circuit open 

 except under extreme conditions, such as short-circuit in the con- 

 trol system. 



When the battery is kept continually floating at practically 

 constant voltage across the D.C. operating bus, and another 

 source of current, such as a motor generator set, is provided to 

 supply the steady load of signal lamps, etc., so that the battery 

 work is limited to occasional momentary discharges when the oil 

 switches are operated or to such sustained discharges as may be 

 called for in case the normal source of current should fail in other 

 words, where the conditions call for strictly emergency stand-by 

 service from the battery the Exide or similar type of battery in 

 glass jars is recommended, this being the same type that is now 

 generally used for stand-by service in the large central station 

 lighting systems. Where a method of operation is adopted in 

 which the battery is discharged continuously on the bus until 

 nearly exhausted and then recharged, thus involving repeated 

 cycles of charge and discharge, the Manchester type of plate or 

 similar is recommended, the Exide plate being only recommended 

 for use on floating batteries at approximately constant voltage 

 and discharging only under temporary emergency conditions. 



9. OVER-VOLTAGE PROTECTION 



Classification of Over-voltages. High-voltage disturbances 

 may be divided into two broad classes. First, that covering 

 actual high voltages in which the excess voltage exists between the 



