Publication: The Journal of Electricity
San Francisco, CA, United States
PROPERTIES OF HIGH POTENTIAL INSULATORS.
BY ALBERT T. BELL.
I HAVE read the recent article in the Journal of Electricity, entitled "Insulators for Transmission Lines," and the articles in the January number entitled "A Multiglaze Insulator" and "The Making of Porcelain Insulators." The articles are all of great interest to those who have had experience with the difficulties of insulation OE long distance transmission lines, but they are rather surprising in one respect, namely, that they all indicate a belief on the part of the writers that no "successful" insulator has yet been found and used in the Pacific Coast market. This is particularly so as there has been no trouble with insulators on the part of the Eastern companies at Niagara Falls, Chambly or Lachine, in all of which localities the climate conditions are supposed to be much more severe than those existing on the Pacific Coast.
In the first mentioned article I notice a statement that a glass insulator is better than a porcelain insulator in that "it will not puncture". The experiments which have been made in testing glass and porcelain insulators at voltages of 40,000 and upward indicate that while it is not feasible to puncture a small glass insulator yet the reson for the failure to puncture it is not so much because of its "puncture resisting qualities" as it is because the glass avoids the puncture test by permitting and assisting the current to go around rather than through it. Porcelain, on the other hand, meets the test squarely, and is either punctured by the current or holds it surely, not allowing it to pass around the surface readily.
I notice the further statement of the superiority of glass in that defects existing in it "are readily discernible with the eye". Experience again with high voltage testing of both glass and porcelain indicates that tests with the eye are of value only as against mechanical defects, and that nothing less than a very high voltage "break down test" with salt water and a transformer of great strength and pressure will determine the electrical resistance of an insulator.
It is further stated, that "by the proper annealing of glass the objection of extreme brittleness is removed." It is a well-known fact that the additional strength which is imparted to glass by annealing lies in some peculiar manner on the surface only and that it pertains exclusively to the object so annealed as a whole and not to its individual molecules. In other words, annealing is what might be called a centripetal force or protection, and further when any part of the object is broken, this strength and protection departs at once from the remaining portion of the object. In the case of insulators, as is well known by telegraph linemen, a glass insulator when struck by shot or stone "flies" and allows the conductor to fall to the cross-arm. The experience of the same men with porcelain insulators is, that in the first place, shot will not break a porcelain insulator and further that when struck by a stone or heavy missile only that part of the insulator which is in immediate contact is broken and the remained of it stays on the pin and holds up the conductor, showing that the strength of the porcelain is evenly distributed throughout its whole body.
It is also stated in the article in question, that "the difference between the hygroscopic properties of porcelain and glass cannot be measured; and there seems to be good ground for this assertion." It is not clear whether the writer means that the hygroscopic difference is so great or of such a nature that no means have been devised for measuring it, or whether, on the other hand, he intends to convey the impression that there is no hygroscopic difference. The fact of the case, however, is, that the hygroscopic difference is very great and very material. Laboratory and working tests by electricians in the East have conclusively determined that there was such a difference, some of the tests being over parallel lines of glass and porcelain approximately one hundred miles long; the most conclusive and absolute proof, however, is the fact that porcelain insulators are in exclusive use in England and ether foreign countries by reason of the failure of glass to satisfactorily insulate their lines in the almost continuous moist and foggy condition of the atmosphere.
The reasons which are given in favor of "a multi-glaze insulator" appear to indicate the cause of the trouble which transmission lines on the Pacific Coast have had with insulators. The assertion that "the puncture resisting properties of a porcelain insulator for use on high potential lines depends largely, if not entirely, upon the quality of the glaze with which it is covered and the thoroughness with which it is applied" is certainly based on a misapprehension. Table-ware potters are well acquainted with the fact that glaze is absorbent to a certain degree and that it is only a question of time and proper exposure when it will allow itself to be penetrated by acids, alkalies or moisture. So well recognized is this fact that the best class of table ware is made from a porcelain body which in itself and without any reference to the glaze is absolutely and entirely acid, alkali and moisture proof. In other words, the glaze is the weakest part of a good piece of table china.
In the "multi-glaze" insulator there are "six skins of glaze and two layers of glass." An experienced table-ware manufacturer would at once say that there were four times as many chances of trouble with such an insulator as with one having only the inner and outer coat of glaze to protect it from dirt and to give it its hygroscopic qualities.
The claim that "should the outer shell absorb moisture the two layers of glass, together with the remaining five layers of glaze, will unfailingly prevent the passage of current or moisture beyond the outer shell" is a confession of the weakness of the glaze and that the manufacturers put no dependence on it. Insulators of this style have been tested at a high voltage and found to stand a good pressure, but after having their outer petticoats broken and being soaked in salt water, they have failed utterly to withstand the same test; whereas one piece insulators made of a superior quality of porcelain have undergone a parallel test at the same time and stood up just as well after the breakage and soaking as before. This fact is of great importance in view of the necessity of having insulators that can be depended on in long stretches of wet or foggy weather, and also to support the wire and insulate, even though damaged by stones thrown by small boys or rifle balls shot by gunners.
In thus advocating and presenting the advantages of a porcelain insulator I fully agree with Dr. Perrine's statement that "only that amount of glass should be present which is necessary to fill up the porosity of the rehydrated silicate of alumina" for the introduction of any greater amount of glass than this will at once give to the porcelain that brittleness which is the characteristic and one of the chief disadvantages of glass. The temptation to do this is very great. A reference again to the condition of the table-ware trade will show that it is a very common thing for manufacturers of cheap goods to use the glassy pastes. The difference, however, is well recognized in the trade, and a broad distinction is made in price between the true porcelain of Limoges and the glassy product of Carlsbad. The differences between the bodies can 'be detected practically by simple tests of shock as well as by a visual comparison of the fractures of the two grades of ware.
I must take exception, however, to his statement that "it is at present unnecessary to attempt any re-designing of the insulators in use until we have thoroughly tested the capabilities of our porcelain manufacturers .to produce insulators of the present form strong enough to carry the wire and sufficiently solid to prevent the escape of electricity through the body of the porcelain without reference to the character or existence of the glazing."
All the insulators made of the best grade of insulator porcelain are tested electrically before they are glazed in order to determine the existence of any cracks or flaws in the body itself. They are then glazed and re-tested in order to determine whether they have met with any accident in the subsequent handling or the second fire in the glost kiln. An insulator which passes these tests of from 25,000 to 100,000 volts according to its size, can be depended on for service. The cracking in the body of many of the insulators which have been on the market, and the porosity as well, have been due to the use of formulas which were not calculated to produce a satisfactory article, to carelessness in the process. of manufacture, to difficulty of moulding, and to the inability of the moulded insulators to withstand or the failure of the manufacturer to give them the proper heat.
In substantiation of my assertion that insulators have been made capable of doing the work, I need only to refer to the previously published letter of Mr. Paul M. Lincoln, Electrical Superintendent of the Niagara Falls Power Co., dated February 4th, 1898, which was written soon after the recent heavy storm which so interrupted travel and electrical communication in the East. The lines therein referred to are approximately 26 miles long and are transmitting about 3500 horse power over six 350.000 c. in. cables at about 11,000 volts.
The recent test for three days of the Pioneer Electric Power Company's lines, as described in the editorial columns of the November, 1897, Journal of Electricity, indicated that they had used one-part porcelain insulator, which fact has since been established by correspondence with Mr. F. O. Blackwell, who conducted the test.
Surely little more can be asked from the porcelain manufacturer who is able to supply insulators which will stand such tests as these. We must therefore look for further improvement in insulation to come from the adoption of improved mechanical and electrical forms. Many people are considering such matters very carefully, and the experience of the last two or three years with high voltage transmission ought to afford a basis for determining what changes can be made to advantage. The principal trouble at present seems to be in connection with the pin, and I hope to see that weak feature eliminated from the best construction within a short time.