694 



ELECTRICAL MEASUREMENTS 



prolonged application of voltage, and are factors which cannot be 

 taken into account in the analysis. 



Paragraph 684 of the Standardization Rules of the American 

 Institute of Electrical Engineers, 1916, is as follows: 



"The following test voltages shall apply unless a departure is con- 

 sidered necessary, in view of the above circumstances. Rubber-covered 

 wires or cable for voltages up to 7 kv. shall be tested in accordance with 

 the National Electric Code. Standardization for higher voltages for 

 rubber-insulated cables is not considered possible at the present time. 



"Varnished cambric and impregnated paper insulated wires or cables 

 shall be tested at the place of manufacture for five (5) minutes in 

 accordance with the Table XIV below. 



TABLE XIV. RECOMMENDED TEST KILOVOLTS CORRESPONDING ; TO OPERAT- 

 ING KILOVOLTS 



"Different engineers specify different thickness of insulation for the 

 same working voltages. Therefore, at the present time the test kv. 

 corresponding to working kv. given in Table XIV are based on the 

 minimum thickness of insulation specified by engineers and operating 

 companies.''! 



*The minimum thickness of insulation shall be He in. (1.6 mm.). 



f The Standards Committee does not commit itself to the principle of bas- 

 ing test voltages on working voltages, but it is not yet in possession of suffi- 

 cient data to base them upon the dimensions and physical properties of the 

 insulation. 



When testing insulations for dielectric strength, it is essential 

 to employ a generator which under all conditions gives practically 

 a sinusoidal voltage at the specimen, since the maximum stress 

 to which the insulation is subjected should be known. For 

 example, altering the length of cable under test, that is, changing 

 the electrostatic capacity placed across the secondary terminals 

 of the testing transformer must not deform the wave. The wave 



