[Trade Journal]
Publication: The Journal of Electricity, Power and Gas
San Francisco, CA, United States
vol. 40, no. 7, p. 333-337, col. 1-2
STANDARDIZATION OF PIN TYPE INSULATORS
BY L. M. KLAUBER AND R. E. CUNNINGHAM
(With the idea of reducing duplication in manufacturers' stocks, now necessary owing to the variety of practice in the use of insulators by the various distributing companies, and in consequence reducing costs and improving service, this committee was appointed to consider possibilities of the standardization of such supplies. The result of their investigations is hire presented as a portion of the Engineering Committee's report to the coming Pacific Coast Section N. E. L. A. convention. The recommendations offered cover the ground most comprehensively, and form the basis for important action on the part of the association. The authors hold high positions with the engineering staffs of the San Diego Consolidated Gas & Electric Company and the Southern California Edison Company, respectively.—The Editor.)
Scope
This report covers the standardization of pin type insulators and pins for distribution circuits. This subcommittee was appointed with the idea that if standards could be agreed upon not only would manufacturers' stocks on the Pacific Coast be reduced but the standard types adopted would be Chore readily obtainable and at reasonable prices.
Necessarily, for commercial reasons, our recommendations must have to do with types, specifications and ratings rather than with brands and particular style numbers. However, we propose to be sufficiently specific so that if our recommendations be approved each manufacturer need provide but a single style for each voltage and class of service, and as we have further handled voltages by groups, the total number of styles required is materially limited. In addition, we have divided classes of service into two divisions based on severity of operating conditions and for each of the voltage groups under moderate conditions, we have recommended the same styles as those suggested for the next lower voltage group under severe conditions. We have been able to make our recommendations as to insulator types fairly restrictive without favoring any particular manufacturer, since all appear to put out essentially parallel lines; pins, however, are not nearly so well standardized, as several manufacturers fabricate unique types.
The term distribution is an elastic one, especially on the Pacific Coast, where our sparsely settled agricultural and mining districts require distribution lines of great extent, which in some cases assume the characteristics of transmission lines. But while some of our member companies operate purely distribution lines at 33 kv. and even higher pressures; it has been deemed advisable to limit this report to lines of 22 kv. or less, since at the higher voltages the special conditions of each case limit the possibilities of standardization.
Data Accumulated
Early in October, 1917, your committee sent out to our various member companies a questionnaire covering the investigation at hand. Complete responses have been received from thirteen companies and partial data from several more. The replies detailed first, second and third choices of insulators and pins for each voltage used, with remarks as to special operating conditions tending to affect the choice of type.
Operating Conditions
As may be expected, practice varies widely amongst the various companies. Some of this variation is due to climatic and other differences in operating conditions and these must of course be allowed for in any scheme of standardization. Other differences, however, are merely due to lack of knowledge of what other companies are doing, and these variations it may be possible to eliminate with the co-operation of our engineering committee.
It is well known that the peculiar climatic conditions on the Pacific Coast and the wide variations in temperature, rainfall, fog, etc., met with in closely contiguous territories, require different standards for lines of similar voltage in different territories. In general, it may be said that the coast or fog belt conditions are particularly severe. Here the long rainless summer, during which the insulator becomes coated with salt and dust, and is simultaneously subjected to the moisture of fog and spray, produces a condition far more serious than the heat and dry dust of the interior valleys or the snow and sleet to which the mountain lines are subjected. So we find that nearly all companies whose lines cover both coast and interior valleys have at least two sets of specifications, one for the severe coast and the other for interior service. In line with this necessary practice, your committee has recommended separate standards for coast and interior service on lines of similar voltage.
No sharp line can be drawn in any territory to indicate where the use of the coast insulator should begin. Some companies segregate their districts by arbitrary boundaries drawn a certain distance from the coast; others learn from experience just how far inland the heavy insulators are required. In general, three to five miles appears to be the limit. High cliffs along the ocean shore which prevent spray from being blown inland bring the limit closer to the shore; on the other hand, salt marshes and sloughs carry the line further inland.
Voltage Classes
Within the voltage limits above prescribed, we find that standardization of voltages has so far progressed in our district that grouping is comparatively easy. The great bulk of the mileage operated falls within one of the following classes: (1) 2300-4000 Y, (2) 11,000, or (3) 22,000.
In addition, there are sonic lines operating at 6600, 13,200 and 15,000 volts. For the purposes of this report, it has appeared advisable to assign a separate 6600 volt class and to group the 13,200 and 15,000 volt lines with the 11 kv. class. This appears particularly advisable since the largest 15 kv. net work is operated by a company having an extensive 11 kv. system.
2300 Volts Delta-4000 Y-Interior Districts
Although a number of companies operating in the interior use porcelain insulators for this service, the bulk of the insulators sold in this territory for 2300 volt service appear to be D G D P glass. Porcelain unquestionably has advantages in strength and durability, yet the tow price and ease in obtaining the glass insulator render its adoption advisable. Wires of 1/0 or larger are not well adapted to the D G D P insulator, and it is recommended that one of the standard cable glass insulators be adopted for this service.
2300 Volts Delta-4000 Y-Coast Districts
Some companies apparently experience no difficulty with D G D P glass insulators under coast conditions; others, where fogs are more severe, find these inadequate. It is recommended that where this is found to be the case the one-piece porcelain insulator recommended for 6600 volt interior service be used.
6600 Volts-Interior Districts
Few companies are now operating at this voltage in our territory and reports were received from only two. We recommend for this service a one-piece two-petticoat porcelain top-groove insulator with one-inch pin hole. The diameter should be about 34 inches, the height 3 inches, and the leakage approximately 5% inches. Such insulators are ordinarily rated by the manufacturers at from 6600 to 8000 volts. Insulators which fulfill this specification, selected from catalogs of four prominent manufacturers, are detailed as follows:
6600 Volts-Coast Districts
We recommend that in these districts the irsulators specified for 11 kv. interior districts be used.
11,000, 13,200, 15,000 Volts-Interior Districts
Many of the companies in this territory operate 11 kv. lines as their high voltage distribution circuits and but few at 13 or 15 kv. It would seem advisable to choose one type of insulator for these several voltages. If these he found insufficient for 15,000 volt service, it is recommended that the type suggested for 11 kv. coast service be used.
Nowhere is the desirability of specifying different types of insulators for service on the coast and in the interior better evidenced than in the reports received from companies operating at 11 kv. In almost all cases those which operate only in the interior specify styles which experience has proved to be entirely inadequate to coast service. and those which operate in both classes of territory, with one exception, specify separate types for each condition. We recommend for interior districts a one-piece two-petticoat porcelain top groove insulator with one-inch pin hole. The diameter should be about 5-1/2 inches, the height 3-1/2 inches, and the leakage distance at least 7 inches. This is a small and relatively cheap insulator with the desirable strength and durability of a one-piece unit, yet with adequate electric qualities.
A few companies •have used with great success on 6600 volt lines in moderately severe districts, and on 11 kv. lines in especially favorable districts, an insulator somewhat smaller than that above recommended. Owing to the great savings effected by the employment of this design, it is recommended that its use be continued (although it is not here incorporated as a standard) in districts where it is of proved adequacy. This insulator has the following characteristics:
11,000, 13,200, 15,000 Volts-Coast Service
All companies operating 11 kv. lines along the coast, and this includes all of the largest operating companies in this territory, report the use of insulators rated by the manufacturers at 27 kv. for this service. Five companies report the use of what may be termed a "standard" type ; two companies report the use of a "higher efficiency" type, with the standard type second choice. This is the lowest voltage at which the modern higher efficiency type is met. In view of the strong recommendation which these insulators are given by the manufacturers, their obvious mechanical advantages and the fact that but two out of seven companies have adopted them as standards, it would appear advisable to present at this point a general comparison between the "standard" and "higher efficiency" types and to examine into the relative merits of these two types. We have tabulated first the standard types for 27 kv. service of four manufacturers; then the higher efficiency type. From each table we have then taken the salient points of. the two types and compared them in a third table; a diagram is likewise presented to show the general appearance of the two types.
From the above tables we deduce a comparison using composite rather than average values for each type.
Of course, as operators, we are interested not so much in design as in the practical results of that de-sign. It is not within the province of this report to discuss the technical features of the design of these so-called higher efficiency units; our purpose in presenting the above tables is to show, first, that several of the more important manufacturers are now putting out insulators of this type and, secondly, to make clear to our members by the last table the structural differ¬ences between composite examples of these types. In general it can be said that mechanically the modern small, heavy insulator is to be preferred in every way. The thick shell gives it greater mechanical strength, less liability to breakage in transit, greater resistance to cyclic temperature changes, affords a smaller target to incipient sharpshooters, and, as it requires a shorter pin, results in a cheaper and stronger pin installation. As to the comparative electrical properties of these types, a discussion of these is beyond this report. Suffice it to say that the newer designs were originally made particularly to solve the problems of our severe coast conditions, and the success of these designs has been proved in the experience of several of our member companies operating along the coast. For further discussion of the relative values of these types our members are referred to the article by Mr. A. 0. Austin, entitled "Factors Affecting Selection of Insulators," Electrical World, Volume 70, No. 19, Page 905, and the paper by the same author read before the Toronto Section of the A. I. E. E., and obtainable through the courtesy of the Ohio Brass Company.
Your committee recommends, therefore, for use on 11 kv lines, under coast conditions, a 27 kv. insulator of the higher efficiency type following closely the design outlined in the right-hand column of Table No. 5.
22 Kv.—Interior Districts
The 27 kv. insulator adopted above for 11 kv. coast districts is recommended.
22 Kv.—Coast Service
A 45 kv. higher efficiency insulator is recommended. Typical products of various manufacturers are shown in Table No. 6.
Selection of Pins
Having selected insulator standards, the selection of pins becomes much simplified, since pin dimensions are largely determined by the insulator characteristics. The types of pins to be considered are as follows:
Discussion of Types
Cement is the bugbear in all insulator work. It is under serious suspicion in suspension insulator construction, and it has been proven that the less used in pin type insulators the better. To cement in an insulator thimble is adding questionable material to the device, and furthermore a standardized screw socket which will fit a variety of pins is at once converted into a special socket which will usually fit but one brand. Your committee advises against the use of any pin engaging a cemented-in thimble. This eliminates types 4, 9, 13 and 15.
The all wood pin especially for insulators having one-inch pill holes has the advantages of low first cost, reasonable strength, and is unaffected by cyclic temperature changes. With few exceptions, all of our member companies use this pin in 2300-4000 volt service, although several of those which employ porcelain insulators in such work likewise use combination pins. We recommend that the standard 9-inch N. E. L. A. pin be used with class 1 insulators. N. E. L. A. specifications should be used in making purchases, except that as locust is becoming increasingly difficult to obtain, and eucalyptus is produced on this coast and has been used with success by many of our members, it is recommended as a substitute. Sufficient data is not on hand with respect to ohia, birch, oak and other woods proposed, and we withhold recommendation of these.
All dependence for insulation should be placed on insulators, not on pins. A pin is primarily a mechanical support and to design it to act as a subsidiary insulator must invariably weaken its mechanical strength. While wood thimbles are satisfactory from the point of view of relieving insulators of stresses due to cyclic temperature changes, their performance is distinctly unsatisfactory whenever they are subjected to appreciable charging or leakage currents. This inevitably results in a digesting or charring of the wood and failure of the pin to properly hold the insulator. This condition is exaggerated in pins having steel bolts and wood thimbles, owing probably to the concentration of the electrostatic stress on a comparatively thin shell of wood. This experience has been universal amongst our member companies and need not be further commented upon. In view of the above, we recommend against the use of all wood pins, except with class 1 insulators, and against the use of combination pins containing wood for any service. Should pins of this type be returned from the lines, and it is desired to re-install them, we recommend that wood thimbles be replaced with lead. Several of our members now follow this procedure with success. The elimination of wood thimbles cuts out types 2, 5, 7 and 11.
The proposition that dependence for insulation should be placed entirely on the insulators above eliminates pins having porcelain parts. These have a high percentage of breakage and lack strength. They may be used with lead thimbles on light lines when second hand, but should no longer be purchased. This eliminates type 6.
Types 3, 8, 10, 12 and 14, being all metal devices, will give adequate service. Of these types, 8 and 14 lack -the strength of the others and are not recommended for insulators having 1-3/8-inch pin holes, although their low first cost is distinctively attractive for class 2 and 3 insulators. Type 12, while more expensive than the other two, has the great advantage of a separable stud bolt which permits the use of the same head on wood or steel cross arms of any dimensions. After due consideration of these facts, we recommend for class 2 and 3 insulators a pin of types 8, 12 or 14, and of the following general dimensions:
All metal parts should be galvanized according to N. E. L. A. specification. For pins with split thimbles felt insertion is preferred.
For class 4 and 5 insulators, type 8 and 14 pins are too light; type 3, 10 and 12 pins are to be preferred, and of these three, the best standardized types are numbers 10 and 12. However, type 3 is a style which any company can easily fabricate locally, and for this reason must be given consideration. Of the two former types, number 10 has the advantage of strength, while number 12 is to be preferred on the score of first cost and adaptability of separable bolt. Type 10 is usually standardized with a 4-inch bolt. We recommend against this size and in favor of a 3/4-inch bolt, as this has ample strength when used with these sizes of insulators and wood crossarms and will permit the use of arms with drilling similar to those used with smaller insulators. For instance, by standardizing on the 5/8-inch bolt, companies operating 11 kv. lines in interior districts and along the coast will require but one standard crossarm for lines of this voltage, where otherwise two would be necessary. As to the height of pin above the shoulder, class 4 insulators require at least 53/4-inch, while class 5 requires at least 7%-inch. It may also be noted that the older "standard" insulators, corresponding with class 4 service, required pins 71/2-inch above the shoulder. We therefore suggest that companies operating 11-15 kv. lines use pins 5-3/4-inch to 6-inch above the shoulder, while those operating 22 kv. lines or expecting to replace large quantities of pins on lines insulated with old standard class 4 insulators, use pins 7-1/2-inch to 8-inch above the shoulder. We recommend that bolts be 6 to 6-1/2 inches long. The diameter of the base should be not less than 3 inches to afford a good bearing area to resist bending. As in the smaller class, all metal parts should be galvanized according to N. E. L. A. specifications, and insulators with split thimbles should be furnished with felt insertion.
