Discussion on “economy in the operation of 55,000-volt insulators” (Crawford), Spokane, Wash., September 11, 1914. (see proceedings for August, 1914)

J. Harisberger: What brought about the development of testing insulators as mentioned in the paper, was a severe storm in the Puget Sound district last winter, causing a number of breaks in our transmission line where it goes through heavily timbered country, these breaks causing arcing grounds and breaking down insulators all over the system. We had been very proud of our record of uninterrupted service, and this trouble set me to thinking about aging of porcelain that has been so much discussed recently. I have been unwilling to admit to myself that there is such a thing as the aging of porcelain, and I do not believe it now. Last spring I made a trip down the Coast to visit operating companies that have been operating lines of 50,000 volts or over for some time, to get what information I could as to their experience. I visited eight companies and found but one man who was positive that porcelain deteriorated and gradually lost its dielectric strength. I found that the experiences of most of the companies visited were quite similar to our own. The porcelain of some of the insulators in service five to six years, on close examination, showed a mottled surface similar to that of Dedham china ware. On breaking the insulator, discoloration of the cracks to a depth of one-quarter of an inch was occasionally found. There is no doubt that this condition cuts down the insulating quality of the porcelain and these insulators should be weeded out before they are broken down by some disturbance on the system. During my absence, Mr. Crawford developed the method as outlined in this paper for locating such insulators. I believe that everyone having had experience in the operating of high-voltage transmission lines will agree with me that high-frequency disturbances such as are caused by arcing grounds, etc., are most to be dreaded. While in California, I spent half a day with Professor Ryan, of Stanford University, who showed me a practical demonstration as to what high-frequ- ncy currents will do. An insulator that would show practically no distress at 150,000 volts at 60 cycles, would be punctured in a few seconds when subjected to over 100,000 cycles and not over 25,000 volts.