1 84 ELEMENTS OF ELECTRICAL ENGINEERING. 



and when there is a ground on one main. A workman should, 

 therefore, always stand on a dry wooden box or platform when 

 working with live wires. Rubber overshoes, if they are whole, 

 insulate a person effectively from the ground, and the use of 

 rubber gloves and of tools with insulated handles is sometimes 

 advisable. It is better, however, not to use such devices on 

 account of the habit of carelessness they develop. 



The idea that voltages above a certain minimum, only, are 

 dangerous is erroneous. Everything depends upon the nature 

 of the two contacts with the body and their proximity. Voltages 

 as low as 1 1 o volts are dangerous if the contacts with the body 

 are broad metal plates pressed against the moist skin. 



An electrical shock often produces death indirectly by stopping 

 the action of the respiratory organs, and in every case a prompt 

 attempt should be made to restore respiration by the method of 

 artificial breathing employed as a first aid to a person who is taken 

 from the water in a drowning condition. 



86. The operation of generators in parallel. A generator ope- 

 rates at its maximum efficiency at or near full load. Therefore 

 when the total output of a station varies from hour to hour, as it 

 always does in electric lighting and street railway service, it is 

 desirable to use several generators which may be put into ser- 

 vice one after the other as the station output increases, and dis- 

 connected one after the other as the station output decreases ; 

 the object being to maintain nearly full load at all times upon the 

 generators which are in service. This is especially advisable 

 when each generator is driven by a separate engine, inasmuch as 

 the engines also may then be operated under the most economical 

 conditions. This use of a number of generating units in a station 

 is also advantageous in that a spare unit (engine and generator) 

 may be installed at a moderate cost to serve in case of a break- 

 down, whereas a single-unit station would have to be completely 

 duplicated in order to provide for such an emergency. 



When a number of shunt or compound generators are installed 

 in a station as above explained they are always operated in 



