[Trade Journal]
Publication: American Institute Of Electrical Engineers
New York, NY, United States
p. 513-530, col. 1
PRESENT PRACTISE IN THE DESIGN AND MANUFACTURE OF HIGH-TENSION INSULATORS
BY A. O. AUSTIN
ABSTRACT OF PAPER
As considerable time or a severe condition is necessary to show up serious insulator defects, the favorable line conditions on the earlier lines permitted the use of inferior material and designs. The apparently satisfactory operation of inferior insulators together with the large production necessary did much to retard improvement in the insulators, for improvement was practically impossible unless same could be accomplished without materially increasing the cost.
The desire to increase production and improve the material necessitated radical changes in the manufacture and equipment, so that the well equipped plant today is far different from the ordinary pottery which it resembled a few years ago.
The rapid development in the transmission field has materially changed conditions, and rendered much apparatus obsolete, the early insulator being no exception.
As causes of losses have become evident, means have been found to eliminate the serious effects of same. The recognition of the increased value of reliability together with the study of operating conditions has materially changed the insulator situation, so that the material going to the scrap pile today is more suitable for line work than the best product a few years ago. The loss from porosity has been reduced to a negligible quantity by improved firing methods and a closer selection.
To prevent the serious cracking loss noticeable on old insulators, has been the most difficult problem. To prevent trouble on old lines, it may be necessary to give the insulators a temperature, as well as an electrical test.
Trouble from this source on modem insulators is prevented by careful attention to the temperature gradient, increased mechanical strength to resist internal stresses, and a lowering of the internal stresses by means of an elastic joint.
The performance of the modern insulator is very gratifying, and its performance must not be judged by insulators which are really obsolete.
THERE IS often an impression that it is possible to eliminate all insulator troubles by the use of some new design, test or manufacturing method. In order to eliminate trouble, however, it is absolutely necessary that all depreciation be eliminated. The very nature of the dielectric, the large number, necessitating low cost, and the many hazards to which the insulators are subjected in operation gives us no right to expect that we can hope to equip a line with insulators which have absolutely no depreciation.
Although the depreciation for some one particular cause may be made negligible in the insulator, it is highly important that this desirable result is not obtained at the expense of a greater hazard in some other direction. The best line practise will be that which reduces the principal hazards as far as possible, but takes into account the hazards produced by depreciation, so that a system may be planned and operated accordingly.
From the operating standpoint the insulator problem is one of maintenance, for the state of the art is such that the hazards can be reduced to a negligible quantity where the fundamental factors governing reliability are given due consideration and the line is maintained in a good state of repair.
The length of time required to show up the effects of depreciation and the rapid development of the art together with the low cost of the insulator is largely responsible for the greatest losses.
Material improvements have been made in the design and manufacture so that the performance of the modern insulator cannot be judged by the performance of insulators made even a few years ago.
The production of a satisfactory insulator is no small problem, if the cost is to be comparatively low. Designing necessitates a very thorough knowledge of manufacturing conditions or their possibilities, for it must be remembered that an insulator part usually goes through from 20 to 30 operations and passes through the hands of over 20 operators. To produce the best results, it has been necessary to eliminate heavy labor wherever possible, so improved machinery has been adopted and the modern insulator plant has departed very much in appearance from the pottery and the methods which it followed only a few years ago.
Fig. 1 shows one of the mold conveyors where the ware is cured before taking it out of the molds. Equipment of this kind saves much of the heavy labor, permits of easy inspection and tends to produce uniformity in the insulator and at the same time results in a considerable saving both as to labor and losses in manufacture.
Similar work has been carried out wherever possible in the manufacture, Fig. 2 showing the ware as it is ready for drying. With equipment of this kind, five or six hundred insulators are subjected to practically uniform drying conditions on all sides and at the same time cracking and the handling cost is cut to a small fraction of what it was formerly.
The inspection of ware is very rigid at all stages in the manufacture and a large portion of the ware rejected today is as good, if not better, than the first quality material of several years ago.
It was only a few years ago, that it was difficult to obtain a test of 40 or 50 kv. on a single part without having losses of from two to forty per cent. The state of the art is such at this time that it is possible to test single parts at much higher voltages than large four-part insulators of a few years ago.
Fig. 3 shows some large insulator parts on test under test conditions equivalent to 140 kv. The average loss even under severe conditions of this kind is seldom over one per cent. This has only been possible by careful systematic work. Many of the older lines are equipped with three- or four-part insulators which would have had an assembly loss of 40 or 50 per cent, if they had been given the above test which can be easily carried by a single part today.
In Fig. 4 is shown one of the assembly cars used for suspension insulators. A car of this kind handles 700 insulators and permits of their being cured under the best possible conditions and with a minimum amount of handling. The increased cost, owing to a very much higher standard, has necessitated improved handling methods at every point in order to keep down the cost which would otherwise be considered prohibitive.
Much has been written about the principal electrical characteristics of the insulator, so the discussion of design and manufacture will be largely confined to the two most important elements producing depreciation, cracking and absorption.
CRACKING
The cracking of insulators is by far the most serious cause of depreciation on most lines, and has been an important factor in the design of pin-type insulators for some years. In order that the method adopted to reduce this loss may be better understood, it is necessary to consider conditions as found on the line.
For the past seven or eight years, much cracking has been noticeable on pin-type insulators, some lines being entirely reinsulated while on others, it was necessary to locate the faulty material by visual inspection or by ringing out with a stick. To avoid this loss, pin-type insulators were replaced by suspension type in at least one instance.
The more recent cracking of suspension insulators has attracted far more attention than a similar loss in the pin type. This is largely due to the fact that they could be easily located on the line and to the higher standard of operation on most of the lines where they were used.
A curve which is characteristic of the depreciation for many insulators is shown by A Fig. 5. It will be noted that this curve rises rather rapidly for a short time due to absorption and then rises very slowly for five or six years operation when it jumps very rapidly due to cracking.
In order to see the effects of this depreciation, it is necessary to study the effect upon operation.
Curve B shows the operating hazard or probable interruptions for the depreciation shown in curve A. It is assumed that a four-part insulator will fail when three parts become bad. Curve B, rises very rapidly after six or seven years as the hazard increases in direct proportion as the cube of the depreciation. This curve shows why cracking is often very serious before its effect is noticeable on the operation.
Curve C shows the operating hazard where all defective material is removed every four years. While the removal of faulty material greatly reduces the hazard, it is seen that the cracking during the tenth year would cause 14 probable breakdowns in a lot of 10,000, although there might have been only one or two the year before.
The insulators which crack are apparently affected in no way up to the instant of cracking, hence, it is impossible to anticipate their failure by any practical electrical test. Porous material on the other hand, can be detected and removed before it becomes valueless.
Owing to the small margin of safety and the nature of dielectrics used for insulators, it is not reasonable to assume that depreciation can be entirely eliminated. A low depreciation is well worth while, however, as it permits the use of insulators of fewer parts with their-smaller maintenance. Examination of many insulators which have cracked on the line indicates the following:
1. That the largest and apparently the strongest insulators crack soonest.
2. That the size, shape and number of the cemented joints effect the cracking.
Old insulators when heated up slowly will often crack before they reach 150 deg. fahr. The parts which fail are the tops or shells outside of large cement spaces.
When old insulators are heated up and then have their tops sprayed with water similar to a thunder storm, a very much larger per cent of cracking takes place. This, however, is usually confined to the head of the insulator.
Insulators like that shown in A, Fig. 6 may have a very high loss on a single heating and cooling cycle. Designs similar to B, 'however, stand very much more severe treatment without loss.
There is a very noticeable cracking on the heads of insulators similar to A after five or six years, weathering. If insulators are subject to the elements, it appears to make no difference whether they are in service or not.
When old insulators which are heated and sprayed with water, crack, the failure is announced by a sharp report, indicating a force of considerable magnitude. Insulators which have stood a heating and cooling cycle of higher range are little effected by a number of cycles of lower range.
These together with the absence of any electrical indication of weakness go to show that the cracking is due to a high stress rather than fatigue. It has been pointed out that the stress may increase due to a slight expansion of the cement with time. This stress in itself may not be serious, but combined with others may cause failure. The cement adheres more firmly to the porcelain with time so that any adjustment by slipping may become less. It is also possible that the modulus of elasticity of the cement may increase with time. The accumulation of dirt may cause a greater leakage of current, heating up the inner parts, or the insulators may get hotter in the sun. The poorer heat conductivity of the -cement is still another factor.
While it is true that porcelain or glass will fail at a lower ultimate if the stress is applied for a long time the stress required to cause failure is very high.
Under operating conditions an insulator may reach 1.50 or 160 deg. fahr. in the sun. Under these conditions insulator A, Fig. 6 will have a temperature gradient along the line 0-t like that shown in A Fig. 7. If rain falls on the upper surface, the temperature may be represented by A' after a few minutes. Under similar conditions, the temperature gradient for insulator B is shown by B and B'.
It will be noted that the inner parts of the older type of insulator get very much hotter than those in B. It is further notice-able that the radiation of heat from the inner parts is less in A than in B, so that when the heads of the insulators are cooled to the same temperature, the temperature gradient will be much steeper in A than in B, so that the stress set up between the porcelain parts regardless of the cement will be very much higher in the older type insulator.
The cement has a higher linear coefficient of expansion than the porcelain and will produce a very high stress if of large cross section. The practise then of reducing the size and area of the cement joint may reduce the stress set up by the cement to easily less than half that for the older insulators. The advantage in fewer cement joints and small heads on the insulators has been carried out extensively in the modern two-piece insulators for voltages up to 50 kv.
Much effort has been expended to incorporate the following in the modern pin-type insulator:
1. A few strong parts.
2. Small heads with corresponding cement sections and areas.
3. Minimum amount of nesting permissible with mechanical reliability.
4. Elasticity in the joint.
A few strong parts keep down the renewals when the line is cleared up for the loss will increase nearly as the product of the per cent loss on a part and the number of parts. This is important and will show up sooner or later on the line.
The axial pressure tending to force the top out of the head in A is entirely eliminated in B by coating the ends of the shells by a wax w, or the use of a cushion. The grip surfaces S are sanded and as the grip is of very high efficiency, a minimum surface may be used for mechanical reliability. Since this surface grips in all directions equally well, there is a tendency for the insulator to hold together even when badly damaged mechanically. This is particularly important for an insulator where the parts are not deeply nested.
ELASTICITY OF THE CEMENT JOINT
In Fig. 8 are shown several types of joints which are of particular interest in considering cracking. In A is shown the ordinary cement joint with scored surfaces held together by cement C.
In B and C is shown an elastic or yielding insert r placed in the joint. In B the possible movement would be greatest across the joint and in C lengthwise of the joint. This then is one method for regulating relative movement in the joint either axially or radially.
Although a joint as shown in B or C will relieve any internal stress, it is not good for handling heavy loads and the danger of the 'insulator falling apart and dropping the line is increased so it is not used except in special cases.
The insulator joint will be satisfactory in so far as it fulfills the following:
1. Minimum area for a given grip.
2. Absence of slipping.
3. Elastic yield or ability to distribute a heavy load.
The coated sanded surface fulfills all these requirements to a marked degree, as will be seen by a consideration of D, E and F.
By reference to D, it is seen that any force between H and H' must be exerted through the small struts V. If the force is one of compression, the stress in the main members H and H' will be very small compared to that in the struts V, being in inverse proportion to their cross sectional areas. If there were a number of these struts, their area could be so proportioned that they would compress or crush and limit the stress at any point in H or H'. This method of connecting the different members would prevent looseness of the parts and would give a good stress distribution in practically any direction for the strain would occur in the small struts V.
In E is shown a practical method of accomplishing the stress distributing feature of D. The main insulating members H and H' are provided with ridges or projections t. If the joint is entirely filled with cement, conditions will be no better than in A so the surfaces are coated with paraffin or other material W which accumulates in the bottom of the grooves leaving the points only very thinly covered. The bearing of the cement is then confined to the tops of the projecting ridges or points which act in the same way as the struts in D. Under these conditions, it is seen that the stress per unit area at the points t and the adjacent cement will be many times that in H or H' so that the joint will give and relieve the main parts from heavy strain, produced by unequal expansion between the parts or cement.
A low coefficient of elasticity can be obtained in E by controlling the size and depth of the ridges or points and the coating used to regulate the effective area of contact between them and the cement.
While E fulfills the general requirements, the grooves tend to concentrate the stress and, the failure of a point may start a crack in the body of the piece. It also has the further objection in having a small give in the direction of the joint.
The sanded surface F provides the projections similar to E, but the space filled by the wax is much greater. This joint not only limits the stress much better in all directions but is easily made and is free from the objectionable feature of scoring.
Ordinary surfaces cannot be covered with wax or paint to relieve the stress unless the load is very light, as the bearing may be concentrated in one or two points, causing a low ultimate or cracking.
By proper control of the sanding, it is possible to eliminate hard spots or undue concentration of stress in many insulators, giving a higher ultimate in the suspension insulators without the danger of cracking.
CRACKING OF SUSPENSION INSULATORS
The cracking of insulators was covered to a large extent in a paper on Insulator Depreciation, December 1914. This discussion outlined very briefly, the causes of cracking and methods of reducing same. In order to insure the mechanical reliability of the insulator, it is advisable that the cap bear on the flange to prevent the escape of cement while setting. This contact area is shown at K in Fig. 9. Cutting the cap with a slot just above this point or the putting in of new cement at K showed that the insulator could be heated much higher without starting a crack at m. This proved that the stress was due to the elongation of the cap and expansion of the cement.
A higher assembly temperature would tend to eliminate cracks o, but would do little good at m, if the cement at k expanded, so the first improvement was to reduce the bearing area at K using a cap like that shown in insulator B. Insulators cemented in this way stood many cycles between freezing and boiling without showing damage. In practise, however, it was found that cement which filled in below the cap would not loosen and drop out; furthermore, the cap came in contact not on a 45-degree line as intended, but out on the flange in some cases.
To insure positive relief at point K, rubber gaskets were first used at the lower edge of the caps, or they were dipped or painted with wax or asphaltum but various types of paper felt gaskets were finally used. Some of these gaskets were left in and some removed, the present practise being to remove the gasket.
The present practise is shown in B, Fig. 9. The head of the insulator is covered with a wax or paraffine w, a disk C is used at the end of the pin and a gasket at G, the latter being removed after the cement has hardened.
All grip surfaces are sanded and a moderate assembly temperature used so that dangerous shearing stresses will not be set up in cold weather. With machined or highly refined caps, it is possible to go to very high assembly temperatures. Without these refinements, however, a high assembly temperature is not advisable.
The use of the elastic sanded surface for limiting the stress together with the assembly shown in B, so materially improves conditions that the danger of cracking is certainly very much reduced or practically eliminated at least for a very much longer period.
The higher the mechanical ultimate in the insulator, the more difficult it becomes to design or manufacture so as to keep down the internal stress.
Although the sanded surface permits of the design of very much higher ultimates in the cemented type of strain insulators, it is probably best practise for very heavy loads to use insulators which place all the stress on a wood or fiber core similar to those shown in Fig. 10. Insulators of this type are in operation on the highest voltages and on loads up to 35,000 pounds and can be designed for practically any mechanical ultimate without making the conditions any more severe as to the dielectric.
While a close selection is made as to vitrification, it might be well to note that the heating of the insulator will melt the paraffine or wax on the head of the insulator which will impregnate the cement and porcelain, should it be porous.
Further refinements will be made from time to time, but it would seem that the present standard is such that they will have to be made without materially increasing the cost.
The coated sanded surface still offers refinements at minimum cost and it would seem that improvements in the near future will be along this line, particularly if it seems necessary to provide elasticity in the pin for better stress distribution.
ABSORPTION
The loss by absorption or porosity may vary widely on different systems and for insulators manufactured at different times. Since most of the porous material can be detected by the trained eye, the question naturally arises as to the conditions which permitted its installation. It is even more important to know how this can be guarded against in the future.
The routine tests made on insulators up until 1903 were made with dry electrodes and were often made on the nested parts at very low voltages as compared to present practise. Under these conditions very porous material would pass tests successfully and it is certain that some lots of insulators included fifty per cent of material which would be classed as porous with present standards.
Although tests were made from 1903 on, with water inside and outside the insulator parts, most of the insulator parts were tested inside of two minutes after applying the water and then packed and shipped to be assembled in the field later and installed without further test. Under these conditions some of the poorest material was eliminated but large percentages of insulator parts were frequently installed which furnished but little insulation by the time they were in place.
Much material was installed as late as 1905 which showed a fifty per cent loss on second test after soaking 24 hours. Most of this material was used on wood construction and on the Pacific coast where conditions were favorable; otherwise, very serious trouble would have resulted.
By the end of 1906 tests had not only been raised, but insulators were assembled in the factory and were beginning to be given a final test. This did much in some instances to eliminate porous material, but as assembled tests were seldom over 70 per cent of the flashover of the insulator, many pieces of porous material could pass the assembled test which would have been detected on a higher test voltage.
Many different designs were made in the next few years and in some instances desirable forming properties were developed in the clay to facilitate manufacture. These desirable properties were developed at the expense of vitrification with the result that some large lines were equipped with material which, although standing a high unassembled part test, was very weak dielectrically after installation on the line.
While there was a general improvement in the vitrification up to 1910, operating conditions were becoming more severe, which had the tendency to offset this improvement.
Thicker material was used on parts to give greater dielectric strength, and as it takes a considerably longer time for absorption to take place with increasing thickness, the greater weeding out of the higher test voltages was largely offset.
Much of the ware put out in 1909 and 1910 showed a loss of less than two per cent by absorption when examined several years later, although material was considered as vitrified over approximately a four-cone range, 80 deg. cent.
To obtain properly vitrified material which will not fail by absorption it is necessary,
1. That the body composition be such that it will vitrify to the extent that the pores will be sealed against absorption.
2. That all material be fired to the proper temperature and time for the body composition used.
To obtain the first condition many checks are necessary in order that uniformity be maintained. The general composition may be entirely satisfactory and spoiled in the manipulation due to a streak of off mixture from settling. This streak or a path left by the burning out of lint may absorb water and render the insulator worthless later. Material improvements have been made in the compounding and working of the clay in the last few years, so that some of the worst hazards cause but little concern at the present time. Better equipment had to be designed and placed in operation to eliminate the most serious cases of trouble, while additional checks were used to cut down others.
Uniform viscosity in the slip and time of pumping the filter presses were prime factors in eliminating streaks of off mixture.
FIRING OR BURNING OF THE WARE
The important factors in obtaining satisfactorily vitrified ware are:
a. A means of determining the firing history of each individual piece.
Where a glaze is used for the insulator which has a wide range of color for a slight difference in temperature, the selection as to temperature or firing history is fairly easy for the trained eye. Glazes such as slate, white, yellow, caramel or Albany slip glazes where the calcium content is too high, change little over a wide firing range and make selection as to firing difficult or practically impossible.
b. A body composition that is satisfactorily vitrified over as wide a temperature range as possible.
In guarding against porous material, the use of a body composition of such range of vitrification that proper selection can be safely and economically made inside this range is an important factor.
c. Close control of the firing of the ware.
Some body compositions have such a very short range of vitrification that a selection extending not over a range of two cones may include both over and under fired material.
Only a few years ago, the use of a glaze which had too uniform color over a wide temperature range together with a body composition of short range of vitrification, while apparently very satisfactory at the time was largely responsible for a considerable number of insulators becoming bad through absorption.
It is always possible to obtain good ware by selection where factors a and b are favorable although factor c is anything but good. Unless the firing control is good, it is not possible to make a close selection of ware without a heavy rejection which would materially affect the cost.
Much work was required to improve firing conditions so that the selection could be reduced from a four-cone range, 80 deg. cent., to practically a two-cone range, 40 deg. cent. Kilns had to be reconstructed so as to have less difference between the hottest and coolest places, and a method of feeding the fuel used which was largely independent of the personal element.
Recording mercury thermometers were used to indicate temperatures up to 1000 deg. fahr. These thermometers do not control the firing, but serve as a check of the method and give data for correction of same.
Reference to the chart in Fig. 11 shows that the rise in temperature is under perfect control, which is very desirable so as to eliminate firing checks in heavy ware. In addition to the use of mercury recording thermometers up to 800 or 1000 deg. fahr., a recording electric pyrometer is used to indicate the rate of temperature change in firing and cooling.
For indicating the finishing temperature, pyrometric cones are used and also disks or rings whose shrinkage can be accurately measured.
Fig. 12 shows a set of records and trials now used for a single firing. Until two years ago, the Seger cones and the experience of the kiln burner were the only guides in firing. Highly skilled firemen, even where gas was used for fuel were unable to produce a satisfactory result for the narrower limits until a system was adopted that was largely independent of the personal factor. Consequently the control was not nearly as good as at present.
The fact that much ware put out some years ago selected over twice the firing range of that at present had an exceedingly small percentage of porous material, would indicate that there is little cause for concern with present material, if factors a and b are at all favorable and a careful selection made.
The present practise of using much thicker porcelain tends to minimize the danger due to absorption, for it is possible that a drying out action may start before moisture has extended, entirely through the part. The time of complete penetration is controlled by many factors, but may be regarded as increasing as some power of the relative thickness when comparing two pieces of the same vitrification.
Unless conditions are favorable, it is seen that resistance tests which require that moisture extend entirely through the piece to detect porosity may require entirely too much time unless made on thin pieces. Before tests could be made on thick pieces, they would be on the line.
It is the general factory practise to depend largely upon dielectric tests made before and after soaking or assembly. These tests indicate the point where even slight absorption takes place and the selection for vitrification is such as to keep away from the danger point.
While a thorough discussion of tests is beyond the scope of this paper, those in general use deserve some attention.
With the thickness of material used in the present insulator, tests just below flashover are hardly severe enough. If, however, the regulation of the circuit is poor and the voltage is raised so that a violent flashover is produced, a very effective weeding out of poor material is obtained.
The test in Fig. 3 is of this nature, and is undoubtedly the best all around test method in use. A few insulators or a great many can be tested with certainty, which is no small advantage where thousands of insulators have to be tested daily.
A brief analysis of conditions in Fig. 13 will show why this test is so efficient in detecting material which is porous. In A, the section of an insulator is shown which has absorbed moisture so that there are two wet zones 1 and 3 adjacent to the cement surfaces with a dry zone 2 between.
The porous zones may be regarded as condensers and resistances in multiple, which are in series with a condenser represented by the dry zone. The arrangement is shown diagrammatically in B.
The impressed voltage under different conditions is shown in C. t is the voltage on the series of zones at 60 cycles, the maximum voltage being just under flashover.
If the voltage is raised until the insulator flashes the effective voltage on the series will be represented by t2.
The resistances ri and r2 may be very high so that the porous zones will carry an appreciable portion of the total voltage on the piece. Owing to the phase displacement of the voltage over the different zones, the dry zone will be subjected to a large percentage of the total voltage.
It is evident that where 12 represents the impressed voltage, conditions will be much more severe on the dry zone for the leakage over the resistance will be greater. That this is necessarily true is seen, for if the frequency was zero, zones 1 and 3 would, of course, carry no voltage, the entire voltage being concentrated on zone 2.
This shows that the lower the frequency or the flatter the wave, the greater the stress on the dry zone. In t2 the rise in voltage is more rapid than in t1 which tends to place a higher stress on zones 1 and 3 than in t1, but owing to the greater length of time the maximum voltage is applied the dry zone will also receive more stress than in t1.
It is evident that increasing the frequency to 200,000 cycles will give little time for the leakage of charging current through r1 and r2, so that a porous piece will act as though it were a perfectly good piece and the chance of elimination on test by failure of the dry zone is very much less.
In t2 the insulator flashes and is subject to several surges during each alternation. This affects the concentration of stress on the dry zone C2, but little, while subjecting the wet zones C1 and C3 to their proportion of the voltage, which they would carry if they were dry.
From this, it follows that a test made at normal frequency so that the insulator flashes heavily without picking up a power arc tends to place maximum stress on all zones.
The fact that the conditions of normal frequency as well as high frequency are present at the same time undoubtedly accounts for the effectiveness of this method of test in eliminating material which tends to become faulty under line conditions.
CONCLUSIONS
It is hoped that the above brief outline of insulator practise will indicate the possible differences between an old and a modern insulator, for it would appear that much concern is felt over what are chiefly the defects of old insulators.
The insulator is subject to many hazards and as the design is a compromise at best, it is not surprising that certain properties should be developed at the expense of others. It is gratifying, however, to note that the chief troubles have been along lines not looked for.
The rapid development of the art has rendered much transmission apparatus obsolete, and the insulator is by no means an exception. When in addition to this, we take into consideration that little is generally known even at the present time as to the conditions under which the insulator has to operate and the necessary properties in same to withstand these conditions, it is surprising that the insulator has reached its present state of perfection.
It has been a number of years since the operating voltage has been limited by the insulator, and the performance of many lines put in during recent years shows that the modern insulator has more than justified expectations even in the face of higher standards of operation and increasing severity of line conditions due to the growth of the system or mechanics of the line.
The fact that the design is a compromise calls for the most careful thought in the design and manufacture, and the successful line will be constructed with insulators which have been improved along lines which are shown to be necessary by time and experience rather than along radical lines that may incorporate elements which will cause serious trouble in unlooked for quarters after a few years operation.
It is much easier to locate the cause of trouble than it is to effect a cure, for the insulator works on exceedingly narrow margins, and it is difficult to find a means of strengthening the insulator for one set of conditions without materially affecting another.
It is well to note that the above paper deals chiefly with the means which have been employed to eliminate some of the worst defects without sacrificing the properties in other respects proven to be good.
The abnormally low depreciation and successful performance of well insulated lines under the severest conditions show that the modern insulator is very successful compared to the earlier product, and if properly made, the standard is such that further improvements are not warranted 'if they greatly increase the cost.
For old lines, it is well to consider the increasing hazard with time due to an increasing rate of depreciation. To maintain a given standard of performance, it is then necessary to either split up the system into smaller sections as the line becomes older or to go over the line at decreasing intervals of time to remove the faulty material. It is possible that considerable benefit can be obtained on some systems by putting the old insulators through a temperature cycle, as well as the usual tests, to eliminate faulty material.
The fact that some of the weakest points in the insulators were greatly improved by design or manufacture without materially affecting the cost, did much to insure progress for a considerable period when troubles were only occasionally noticeable. The present situation shows that these improvements give every promise of being not only very valuable, but necessary for a long life and are worth many times the effort required to produce same.
Manuscript of this paper was received April 28, 1917. To be presented at the 34th Annual Convention of the American Institute of Electrical Engineers, Hot Springs, Va., June 27, 1917.
