176 COMMERCIAL ELECTRICAL TESTING 



lation will result in a drop in voltage indicated by the 

 electrostatic voltmeter. An excessive charging current 

 will flow and the insulation will burn if the discharge 

 is continued for any length of time. 



For any test above 10,000 volts always use a spark 

 gap, setting it according to the sine wave curve of arcing 

 distances (Fig. 52). Connect both ends of the primary 

 winding to one terminal of the high potential transformer 

 and ground both ends of the secondary to the core a"nd 

 frame, connecting the other terminal of the high potential 

 transformer to the frame. Set the spark gap for the 

 voltage to be applied and connect in the proper electro- 

 static voltmeter. Be sure that everything is clear, then 

 apply the voltage, bringing it up gradually until the gap 

 arcs over. Then decrease the voltage until the arcing 

 ceases and again bring it up just to the arcing point, 

 holding this voltage for one minute before gradually 

 taking it off. A note of the charging current should be 

 made on the record sheet. 



When a transformer breaks down, the defective coil 

 should be located by making it "smoke up." In doing 

 this, burn only enough to show the coil. If much damage 

 is done by smoking it may be impossible to discover 

 the cause of the breakdown. 



