f STARTING UP 201 



ance of the voltmeter itself, this value usually being given inside 

 the cover of the instrument. 



Before using power from a commercial circuit for testing insu- 

 lation, tests should be made to determine if the supply circuit is 

 grounded. One side of the circuit must be free from grounds and 

 the ungrounded side should be used in series with a voltmeter in 

 taking resistance readings. 



It is impossible to give any hard-and-fast rules regarding the 

 minimum value of the insulation resistance that will cover all 

 classes and sizes of machines, and the results must be used with 

 judgment and common sense. The insulation resistance of a 

 machine indicates, as a fact, little more than the condition of the 

 insulation as regards the moisture; and the rate of change of the 

 resistance as the machine is being dried is, perhaps, the best indi- 

 cation as to when the drying has been carried far enough. 



The following approximate rules have been developed to give 

 what may be termed a fair value of what the insulation resistance 

 should be. It must be understood, however, that they are to 

 be used merely as a guide: 



For A.C. generators 



3,000,000 X Rated volts 

 Rated Kw. 



For exciters and D.C. generators 



300,OOOXRated volts 

 Rated current 



The above formula? give the insulation resistance in ohms, 

 but as a rule it is given in megohms, which is equal to the ohms 

 as obtained above divided by one million. 



