CABLE TESTING 



the test repeated, A frequency of 25 cycles per second is com- 

 inoiily employed in these tests. 



In the practical execution of breakdown tests, mu.KTous 

 questions arise, such as: 



1. Given the size of wire, thickness of insulation and the 

 character of the insulating material, what test voltage should 

 be applied? 



2. For how long a time should the test voltage be maintained? 



3. How can the wave form of the test voltage be assured? 



4. How shall the voltage be measured so that the maximum 

 stress to which the insulation is being subjected may be 

 known? 



5. What precautions must be taken in order that high-frequency 

 disturbances set up by spark discharges from the testing circuits 

 may be eliminated? The cable breakdown may be due to the 

 high-frequency disturbance rather than to the regular test 

 voltage, so the observer is misled. 



6. Is it possible to determine whether or not a cable, which 

 has not been actually broken down, has been overstressed by the 

 high- voltage test .so that it is permanently injured? 



Though breakdown tests are of the highest importance to 

 operating companies, up to the present time no generally ac- 

 cepted procedure has been developed. 



A high test voltage is advisable since it promotes care on the 

 part of the manufacturer; on the other hand, it is possible to so 

 overstress a cable, without actually breaking it down, that it is 

 permanently injured and may in consequence fail at some 

 future time when conditions are not abnormal. 



In the past, many companies have specified that the cable 

 must stand 2% times the normal working voltage for 5 minutes, 

 but there is no rational basis for this particular requirement. 



At the present time, on account of the lack of necessary data, 

 it is not possible to specify the proper test voltage from the 

 dimensions and properties of the insulation. 



A great difficulty in applying mathematical analysis to this 

 problem arises from the non-uniformity of the insulating materials. 

 Air pockets in the dielectric and lack of perfect adherence of the 

 dielectric to the conductor, especially in stranded cables, product 

 local overstressing and deterioration of the insulation under 



