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Lightning arresters, and why they sometimes fail
June, 1895.]
Lig/ztning Arresters.
439
L I G H T N I N G A R R E S T E R S , AND W H Y THEY SOMETIMES FAIL.* BY ALEXANDER JAY WURTS.
T h e lecturer was introduced by Prof. E. J. Houston, of the Institute, and spoke as follows: MEMBERS OF THE INSTITUTE, LADIES AND GENTLEMEN:
W h e n the first telegraph lines were installed it was noticed t h a t the instruments frequently became damaged during thunder-storms. The wires were charged with static electricity, which, instead of following m a n y turns of wire through the i n s t r u m e n t to earth, would puncture the insulation between consecutive convolutions and thus shortcircuit the coils. Besides this damage to the instruments, it was noticed that the static charge would sometimes spring from the line, or other metallic parts of the circuit, to neighboring conducting objects, such as nails, iron pipes and the like. It was also noticed that, when discharge occurred in this manner, the instruments were not damaged. In other words, these neighboring objects formed by-paths for the discharge and, in a measure, offered protection to the instruments. As a consequence of such observation, artificial by-paths were constructed, which consisted of two metal electrodes, separated by a small air space. One of these electrodes was connected to the line, the other to earth; and thus the first " l i g h t n i n g arrester" was constructed. It was then supposed that, when a line was provided with a " l i g h t n i n g arrester," the instruments would be "protected against lightning." T h e result, however, proved this conclusion to be erroneous. The discharges would sometimes pass to earth over the spark-gap of the lightning arrester, but not infrequently would quite ignore the arrester and, as before, puncture the insulating material of the * A lecture delivered before the Franklin Institute, March i5, I895.
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[J. F. I.,
instruments. In other words, the discharge was selective. The failure of the lightning arrester was a vexation; b u t at that time, owing to the small amount of apparatus in use, the damage reckoned in dollars and cents did not call for any special study of the reasons for the selective character of disruptive discharges. In these days, however, the vast amount of capital invested in electrical apparatus of various kinds, and the consequent increased annual loss directly dl~e to these disrnptive discharges, have called for a thorough investigation of the subject and tile designing of more efficient means of protection. It is a very significant fact that overhead wires, after being subjected to the influence of thunder-storms, do not show the damage to insulation which one would reasonably expect to find had the wires been actually struck by the lightning discharge. It is not uncommon, however, to h e a r of wires being struck by lightnm o ; in fact, Iinemen will frequently volunteer to point out the exact spot where the lightning entered the wire. But, so far as I have been able to learn, overhead wires are not struck by lightning. T h e points usually selected by lightning discharges are trees, lightning-rods, church steeples, tall chimneys, and the like. There seems to be no reason why lightning should strike a horizontal wire, especially when it is insulated from t h e earth. Overhead wires may become charged in three ways a n d combinations of these, viz.: by static induction from the clouds; by dynamic induction from a lightning discharge, and by conduction from the s u r r o u n d i n g charged atmosphere. T h e writer inclines to the theory of conduction. During thunder-storms, and in m a n y instances during fair weather, the atmosphere becomes charged with electricity at a constantly increasing potential as we recede from the earth. At the top of W a s h i n g t o n Monument, Washington, D. C., a potential of 3,ooo volts has been measured during thunder-storms, and at the top of the Eiffel Tower a potential of I0,ooo volts has been measured. Now, it is well known that lightning is oscillatory. T h e first oscillation, or discharge, makes a crack or hole t h r o u g h
June, I895.]
LightniJ~g Arresters.
44I
the atmosphere, and through this the succeeding oscillations take place. About ten to twelve oscillations can be observed, and the time interval is reckoned at about "ooooi of a second. The lightning, therefore, being oseillatory and the atmosphere charged, if an overhead wire be charged by conduction from the atmosphere--that is, become electrically a portion of it--then, with every lightning discharge it would seem as though the potential of the atmosphere would sympathize with the oscillatory character of the lightning, and that the charge in the atmosphere would also oscillate and produce oscillations in overhead wires; and, in fact, in all metallic conductors, such as wire fences, rails, etc. T h e fact that wires become heavily charged during fair weather offers further evidence that they are charged by conduction from the surrounding charged atmosphere. In some instances, lightning arresters have been .known, du.ring perfectly fair weather, to discharge overhead wires at the rate of 14o times a minute. In any case, whatever the method of charging m a y be, the discharges from overhead wires seem to be oscillatory, and during thunder-storms they are, in most eases, simultaneous with l i g h t n i n g flashes. If w e raise one end of a trough of water and then quickly lower it, the water will~quietly surge back and forth. If, w h e n the water is returning, we again raise the same end of the trough, a wave will be started forward, which, meeting the returning wave, will combine or collide with it, causing a piling up of the water at that point. Further eomplieations m a y be introduced by repeatedly, and at proper intervals, raising and lowering one end of the trough. If the crests of these waves be carefully examined, it will be noticed that they differ in height and are con. stantly shifting their positions. During thunder-storms the static eleetrieity in overhead conductors surges back and forth very m u c h as the water in a trough, and the indieations are that electric waves are set up which combine and :nterfere with each other in such a manner as to produce frequent and unequally distributed points of high and low pressure. T h e ends of wires are
I8/zlrt~ :
442
[J. F. I.,
points of reflection, a n d at these points the pressure is always great. W h a t e v e r m a y be t h e t r u e explanation of the a p p a r e n t idiosyncrasies of d i s r u p t i v e discharges, it is, after all, with facts t h a t we h a v e to deal. R e f e r r i n g to Fe~. z, let d A represent the terminals of a static i n d u c t i o n m a c h i n e L, a b a t t e r y of six one-half-gallon L e y d e n jars, I3, a spark gap of 12- and CI C2, etc., variable spark gaps, and let the c o n n e c t i o n s be as indicated. W i t h discharges at B, d i s c h a r g e s will also occur at one or more of the gaps CI, C2, etc. W h e n CI, C2, C3 are equal, t h e discharges occur across t h e s e gaps i n d i s c r i m i n a t e l y . W h e n discharge occurs over small gaps in t h e neighborh o o d of CI or C2, thread-like d i s c h a r g e s are always noticed a t Cn. _And even w h e n (2~ is increased to g~77, the thread-like discharge is still noticeable with every d i s c h a r g e at Ct or
~B~,
~
L
.
.
.
.
.
.
.
.
.
.
FIG. I.--t~xperiment with line arresters, illustrating selective character of discharge.
C2. If a large n u m b e r of Spark-gaps, v a r y i n g a m o n g e a c h other as m u c h as IOO per cent., be d i s t r i b u t e d as CI, C2, C3, etc., discharges will still occur i n d i s c r i m i n a t e l y , s o m e t i m e s across a large gap, s o m e t i m e s a small one, s o m e t i m e s across three or four c o n s e c u t i v e or w i d e l y d i s t r i b u t e d gaps. W h e n CI is equal to ~.7", ~ equal to ~7- and placed two, t h r e e or more feet a w a y from CI, and if C3, C4, etc., be omitted, t h e n a discharge will occur at C2 once in a b o u t 50o times w i t h o u t any discharge at Ct. T h e s e e x p e r i m e n t s d e m o n s t r a t e t h e selective c h a r a c t e r of t h e discharge. If, however, (?2 be increased to ~18-, t h e d i s c h a r g e s wilt cease a l t o g e t h e r a t t h a t point, and this last e x p e r i m e n t i n d i c a t e s the l i m i t of r a n g e over w h i c h selection may t a k e place u n d e r the conditions given. B u t it is perfectly e v i d e n t t h a t w i t h a l a r g e n u m b e r of gaps, the p r o b a b i l i t y of d i s c h a r g e across a n y
June, 1895.]
ZigAtning ~lrresters.
443
one gap is v e r y m u c h less t h a n if only two gaps were present. In o t h e r words, if C: represents a piece of apparatus w h i c h we desire to protect, the protection will be greater as we increase the n u m b e r of spark-gaps, C2, C3, etc., b u t in no case can there be certainty of protection unless t h e gap C~ be relatively so large--or, more broadly speaking, unless t h e path C~ offer such a h i g h resistance to the passage of disruptive d i s c h a r g e s - - a s to place it b e y o n d the limit of selection. T h e apparatus which we have used in the above e x p e r i m e n t s is different from t h a t employed by n a t u r e ; t h e results, however, so far as m y observations go, are identical, a n d the lesson to be learned is simple; it is this: the p r o b a b i l i t y of d a m a g e to electrical apparatus connected to o v e r h e a d wires is lessened as we increase the n u m b e r of o p p o r t u n i t i e s for discharge from the line. In s u m m i n g up thus far, we notice briefly that t h e line becomes charged, s u r g i n g s are set up, there are points of reflection w h i c h are always points of g r e a t electrical strain, there are points along t h e line of v a r y i n g intensity or tendency to discharge, these points are constantly shifting their positions, and spark gap l i g h t n i n g arresters are used to conduct disruptive discharges to earth. T h e l i g h t n i n g a r r e s t e r of t o day, as a protective device, differs from the l i g h t n i n g a r r e s t e r of t h e early telegraph in detail only. A simple spark-gap, one t e r m i n a l of which is connected to the line and the o t h e r to earth, is essentially the l i g h t n i n g a r r e s t e r n o w in use. But, w i t h the m o d e r n high-potential circuits, it is f o u n d that the d y n a m o c u r r e n t follows the static discharge across t h e spark-gap of the attester, c a u s i n g t h e r e b y a short-circuit or d a n g e r o u s g r o u n d on the line. In o r d e r to avoid this difficulty, a r c - r u p t u r i n g devices are a t t a c h e d to t h e l i g h t n i n g arrester, which have for t h e i r function t h e i m m e d i a t e i n t e r r u p t i o n of the d y n a m o current, w i t h o u t i n t e r f e r i n g w i t h the f u r t h e r operation of the l i g h t n i n g arrester as a d i s c h a r g i n g device. " A u t o m a t i c L i g h t n i n g Arresters," as t h e y are called, differ a m o n g one another in t h e various m e a n s adopted for r u p t u r i n g t h e d y n a m o arc. But t h e a r e - r u p t u r i n g a t t a c h m e n t has n o t h i n g w h a t e v e r to do with the a p p a r a t u s as a l i g h t n i n g arrester,
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so that although automatic lightning arresters vary in outward appearance, and are in general more cltmbersome and expensive than the original simple spark-gap arrester, yet, as lightning arresters, they are nothing more than sparkgaps. Various lightning-arrester attachments have beer~ designed for the automatic rupture of dynamo arcs, b u t owing to the very high potentials which are so often tlsed, t h e y are found to be not only undesirable, b u t inefficient, the lightning arresters being frequently destroyed b y the vicious dynamo arc. In practice, however, complaints are not so much t h a t lightning arresters are destroyed as that t h e y "fail to protect." It is not unusual to see several different kinds of lightning attesters in a single station, which have evidently b e e n installed with the idea of determining which one of t h e m would prove the most efficient. But such experiments have met with disappointment, for the reason that sometimes one lightning attester, and sometimes another, w o u l d receive the discharge, and the selective character of disruptive discharges was not understood. T h e failure of lightning arresters is not due to the particular k i n d of l i g h t n i n g arrester, or to the patent under which it is manufactured ; it is due largely to the peculiar conditions with which i t has to contend, namely, that the discharge is selective, and that, in order to protect apparatus, means m u s t be taken to control these selections, or, at least, to provide so m a n y paths to earth that t h e probable selection will be one of the many paths provided. In other words, lightning arresters do not " protect," they simply offer opportunities for discharge. These opportunities m a y or m a y not b e embraced, according to circumstances. But the failure of lightning arresters is not altogether due to the peculiar conditions already referred to. Poor ground connection, inductive resistance in the ground circuit, defective insulation in the apparatus to be protected, and a general misunderstanding of the subject, are not infrequently the cause of serious losses which might otherwise have been avoided. W e have noticed that lightning arresters do not " p r o t e c t , " that they simply offer opportunities for discharge. W e
June, I895.]
£i~r]zhzin~ Arresters.
445
have also noticed that discharges do not pass readily through coils of wire--coils therefore protect. Properly constructed choke coils, connected in the circuit, offer a high resistance to the passage of disruptive discharges, and when used in connection with lightning arresters the combination offers a very reliable means of protecting wellinsulated apparatus against lightning. Laboratory experiments, together with tests made under actual working conditions, indicate the advisability of using four choke coils in series, in each wire, with four lightning attesters intervening, as shown in Fig. 2. This arrangement is more particularly suitable for the protection of station apparatus. Coils can, however, be used to advantage on the line for the protection of the more expensive translating devices. CHOKECOILS.
,~ GENEBATOR
GROUND.
LIGHTNINGARRESTERS.
GROUNO.
FrG. 2.--Four choke coils with lightning arresters intervening.
The general construction of a choke eoilis a matter of
considerable importance. A flat spiral possesses some advantages over the helix, but for practical purposes these hardly compensate for the lower cost of the latter, which, for best results, should be wound over a wooden or other non-conducting core. Metal cores and eases should be avoided, because the induced currents in these parts would lower the choking effect of the coils. The n u m b e r of turns which can be used to advantage is limited. It is found by experiment that, with given conditions, the choking effect increases with the n u m b e r of turns up to a well defined critical point, after which additional turns fail to have any appreciable effect. It is probable that these critical points vary with the amount of electricity discharged ; that is, for h e a v y
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V/urts :
[J. F. I.,
discharges the critical point for m a x i m u m choking effect would embrace a larger n u m b e r of turns than would smalter discharges, and the choking effect for a given number of turns within the critical point would also be greater for heavy than for small discharges. For practical purposes the writer recommends from forty to fifty feet of wire wound either into a flat spiral having an internal diameter of three inches, or into a three-inch helix with a single layer. Summing up once more, we note that wires become charged, that lightning arresters offer opportunities for discharge, and t h a t coils protect. Bearing in mind these three points, let us note that, with the extensive systems of electric light and power distribution now in use, it would b e quite impractical to install the above.described system of choke coils and lightning arresters for the protection of each separate piece of apparatus. T h e first cost of such an installation would be prohibitory. It is evident, therefore, that the protection of line apparatus with choke coils being excluded there remains but one alternative, namely, that of providing such a large n u m b e r of opportunities for discharge, distributed over the entire system, that the selection of the discharge will be one or more of the paths provided for it. In other words, lithe a~'resters connected at frequent intervals offer the only practical method of protecting widely distributed apparatus. But, in order that such an installation m a y prove satisfactory from a commercial as well as an operative standpoint, the arresters m u s t be cheap, simple, free from moving parts and perfectly reliable without the necessity of regular inspection. Realizing that these points were not covered by any o n e of the m a n y automatic lightning arresters which had a l r e a d y been devised, I undertook to simplify the problem, and in m y early experiments proposed to equip the lines of electric systems with a multiplicity of spark-gaps, and, at the station, to provide an automatic circuit opening-and-closing device, which, upon the formation of a short circuit across one o r more of the spark-gaps on the line, would instantly o p e n and then immediately dose. While experimenting w i t h
June, 1895.]
Lig/ttninff Attesters.
447
this system, I d i s c o v e r e d that if the electrodes of a lightning arrester w e r e m a d e of zinc, the short-circuiting arc, which had h e r e t o f o r e t h r e a t e n e d the life of the l i g h t n i n g arrester, a b s o l u t e l y failed to be maintained. In other words, zinc p r o v e d to be w h a t is n o w c o m m o n l y known as one of the " n o n - a r c i n g " metals. S u b s e q u e n t i n v e s t i g a t i o n disclosed the fact that there were four other metals which exhibited similar characteristies, namely, bismuth, antimony, c a d m i u m and mercury.* The discovery of the non-areing metals at once solved
FIG. 3"--D°uble-p°le n°narcing metal line attester.
the p r o b l e m of a lightning" arrester a d a p t e d to the requirem e n t s of a l t e r n a t i n g current circuits, and w h i c h w o u l d m e e t the conditions already e n u m e r a t e d . T h e non-arcing m e t a l l i g h t n i n g arrester, w h i c h is illustrated in Figs. 3, 4 and y, consists, as will be seen, of seven non-arcing m e t a l cylinders, insulated from each other, each one and one.half inehes long and three-fourths of an inch in diameter. W h e n installed, the t w o o u t e r cylinders are A full description of the researches and experiments Which led to the discovery of the non-arcing metals is given inthe Transaclions oflhe Ameri. can Institulg of Electrical Engineers, for March I5, 1892.
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[ J. F. I.,
connected to e i t h e r ]e~ of an alternatin K c u r r e n t circuit and the central c y l i n d e r to earth, t h u s f o r m i n ~ a double-
FIG. 4.--Non-arcing metal line arrester.
I~'IG. 5.--Non-arcing metal line arrester.
pole l i g h t n i n g arrester. In practice, these are m a n u f a c t u r e d in units for i,ooo volts, so t h a t for 2,ooo volts two are con-
L@htnzng Attesters.
June, ~895.]
449
nected in series, for 3,ooo volts three are c o n n e c t e d in series. The d i a g r a m s in F~g-. 6 indicate t h e connections for each. potential. T h e r e are m a n y t h o u s a n d s of these l i g h t n i n g arresters in actual service, a n d the experience of three years~ has fully d e m o n s t r a t e d that t h e y are perfectly reliable w i t h out the n e c e s s i t y of r e g u l a r inspection. T h e simplicity of the device is at once apparent, and the cost to the user is a. mere fraction of w h a t was f o r m e r l y d e m a n d e d for the best. types of a u t o m a t i c l i g h t n i n g arresters. It will t h u s be seen that, at least for a l t e r n a t i n g current circuits, t h e require-. ments of a c o m m e r c i a l l y serviceable line l i g h t n i n g a r r e s t e r have been fully met. T h e success of the non-arcing metal l i g h t n i n g a r r e s t e r was such that I was i n d u c e d to m a k e f u r t h e r investigations. I0OO VOLTS
Station Arcestea
2OOO VOLTS
3000 VOLTY
-3N~..,,,
s
L;ne A rr,~ste.~.
E
"
Lt~£
LINE
MNI
FIG, 6.--Diagrams for lightning arrester connections on circuits of differ rot: potentials.
as to the p o s s i b i l i t y of c o n s t r u c t i n g an equally simple a n d efficient device for use on direct c u r r e n t circuits. In v i e w of the m a n y e x p e r i m e n t s w h i c h I had m a d e w i t h almost, every kind and c o m b i n a t i o n of the ordinary metals, I had b e c o m e c o n v i n c e d t h a t I should n o t be able to c o n s t r u c t a non-arcing l i g h t n i n g arrester for direct current circ~it~ which w o u l d derive its non.arcing p r o p e r t y from thenature of the m a t e r i a l of the electrodes, as is the case with the non-arcing m e t a l l i g h t n i n g arrester. Accordingly I directed m y a t t e n t i o n to the possible s u p p r e s s i o n of t h e d y n a m o arc b y v i r t u e of t h e construction or relation of the p a r t s . I had a l r e a d y o b s e r v e d the r e m a r k a b l e ease w i t h w h i c h VOL. C X X X I X .
No. 834.
29
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[J. F. I.,
disruptive discharges strike over i n s u l a t i n g surfaces, as compared to the difficulty wi'th which t h e y o r d i n a r i l y pierce the air. I~or example, disruptive discharges will strike over a glass surface, b e t w e e n two electrodes placed e i g h t inches apart, r a t h e r than pierce the air b e t w e e n electrodes placed one inch a p a r t and s h u n t i n g t h e electrodes in contact w i t h the glass. If g r o u n d glass be used and a pencil-mark be drawn between the widely s e p a r a t e d electrodes, the r a t i o between the distances over the glass and t h r o u g h the air is increased. W h i l e e n d e a v o r i n g to avail m y s e l f of this principle of " s u r f a c e discharge," as I h a v e called it, it occurred to me t h a t a d y n a m o arc, in order to exist, m u s t be fed b y the wtpors of its electrodes, a n d t h a t if such vapors c o u l d be suppressed, or if t h e i r f o r m a t i o n could be avoided, a n arc could not possibly exist. P l a c i n g these two i d e a s together, namely, " s u r f a c e discharge " and " suppression of the vapors," I at once h a d the f u n d a m e n t a l principles necessary for the construction of w h a t h a s since proved to be a most simple and efficient non-arcing l i g h t n i n g arrester f o r direct current circuits. t n the first e x p e r i m e n t s which led to a practical adaptation of these ideas to the c o n s t r u c t i o n of a lig'htning atrester, I placed two a l u m i n u m - f o i l t e r m i n a l s b e t w e e n t h e surfaces of two small blocks of marble, the surfaces of w h i c h had been carefully ground, and on one of w h i c h I h a d d r a w n a pencil-mark which s h o u l d bridge t h e d i s t a n c e between t h e foil electrodes. A t first these electrodes were placed a b o u t two inches apart, the two blocks of m a r b l e were f i r m l y bound t o g e t h e r w i t h twine, t h e t e r m i n a l s were c o n n e c t e d to the terminals of a 5oo-volt direct c u r r e n t generator, a n d disruptive discharges were t h e n caused to pass from o n e foil t e r m i n a l to the other, b e t w e e n the m a r b l e blocks a n d over the pencil-mark. It will at once be seen t h a t the function of one of these m a r b l e blocks was to f u r n i s h a s u r f a c e discharge plane, t h a t t h e pencil-mark was to f u r t h e r facilitate the passage of the disruptive discharge, and t h a t t h e function of the second block, which was firmly b o u n d to t h e first, was to suppress the f o r m a t i o n of vapors, which w o u l d otherwise serve to establish a short-circuit u n d e r the condi-
June, I895.]
Lig/ltnbzg~ ~trresters.
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tions of the test. N o arc being formed, the terminal foils: were b r o u g h t s u c c e s s i v e l y nearer to each other, until finally, when t h e d i s c h a r g e plane had been shortened to oneq u a r t e r of an inch, a short circuit was established. W i t h o u t e n t e r i n g into the details of s u b s e q u e n t experi-
FIG. 7.--Non-arcing railway lightning arrester (for car or line use).
m e n t s l e a d i n g to t h e p r a c t i c a l construction of a c o m m e r c i a l l i g h t n i n g arrester, it m a y briefly be stated t h a t t h e final form given to the arrester was, t h a t s h o w n in Fig. 7; In this form wood, h a v i n g b e e n f o u n d m o r e durable, h a s b e e ~ s u b s t i t u t e d for marble, and, in place of the pencil-marks,
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tVz~rts ."
[J. F. I.,
shallow g r o o v e s are b u r n e d longitudina]ly b e t w e e n the terminal electrodes, w h i c h are placed three-eighths of an inch apart. W h i l e s u b m i t t i n g this form of l i g h t n i n g a t t e s t e r t o long-continued tests with d i s r u p t i v e discharges, it was fotmd that there w a s a gradual w e a r i n g a w a y of the wood fibre, which eventually ended in the formation of a cavity in which v a p o r s formed in sufficient q u a n t i t y to establish a d y n a m o shortcircuit. T h e w e a r i n g a w a y of the fibre s e e m e d to be d u e to the fact t h a t no special vent, or o p p o r t u n i t y for displacement, was p r o v i d e d for t h e d i s r u p t i v e discharge. This evil was quickly cured b y slotting the u p p e r block at right angles to the charred grooves in such a m a n n e r as to open up the grooves or discharge plane to the air, without, however, uncovering the electrodes. It will n o w be proper to notice the d i s t i n c t i v e features of the t w o non-arcing l i g h t n i n g arresters w h i c h h a v e b e e n described. T h e non-arcing m e t a l l i g h t n i n g arrester is nonarcing b y v i r t u e of the material of its electrodes, and t h o u g h "non-arcing," nevertheless allows of t h e d y n a m o short-circuit. This is, however, so quickly i n t e r r u p t e d , p r o b a b l y b y virtue of non-conducting vapors w h i c h form b e t w e e n t h e electrodes d u r i n g t h e first rush of current, t h a t no appreciable b u r n i n g or d a m a g e to t h e electrodes can be noticed, and the m e l t i n g of fuses and c o n s e q u e n t i n t e r r u p t i o n to t h e service are entirely avoided. On the o t h e r hand, the nonarcing lightning arrester for direct c u r r e n t circuits is " n o n a r c i n g " b y v i r t u e of its construction ; and, m o r e t h a n this, is, strictly speaking, a discriminating l i g h t n i n g arrester, i n that it allows disruptive discharges to pass freely w i t h o u t allowing a n y d y n a m o c u r r e n t at all to follow. This l a s t feature is one which has e n a b l e d me to m a k e some interesting i n v e s t i g a t i o n s r e g a r d i n g the relative v o l u m e of discharges from overhead wires d u r i n g thunder-storms. W h i l e carrying on a series of e x p e r i m e n t s in Colorado, during the s u m m e r of I893, I c o n n e c t e d in series with a n u m b e r of these d i s c r i m i n a t i n g l i g h t n i n g arresters small spark-gaps, into w h i c h I inserted pieces of t i s s u e paper, which were i n t e n d e d to act as telltales, and which, b y t h e holes p u n c t u r e d b y the discharges, g a v e m e some record o f
June, i895.]
Lig/lfning Attesters.
45 3
what was going on. Some of these tell-tale papers are shown in F@. & It will thus be seen that, although
Fro. 8.--Tell-tale papers.
the discharges which caused these holes were larger than is ordinarily obtained with a battery of Leyden jars, they
454
V/~lrts :
[J. F. I.,
are not to be compared with the holes w h i c h one w o u l d expect from the l i g h t n i n g flashes w h i c h are so often said to " s t r i k e t h e lines." T h e fact t h a t these telltale papers were not b u r n e d is sufficient evidence that t h e d y n a m o current did not follow the discharge. T h e r e is, however, a n o t h e r side to this m a t t e r of protection against l i g h t n i n g - - o n e w h i c h has been little discussed, and which is almost invariably overlooked b y those to w h o m interruptions d u e to l i g h t n i n g are a m a t t e r of vital importance. I refer to the insulation of t h e a p p a r a t u s to be protected. W h e n a l i g h t n i n g arrester "fails to p r o t e c t " it is condemned, the general opinion b e i n g t h a t the failure is due to some inherent fault in the l i g h t n i n g attester. B u t we have already learned that a l i g h t n i n g arrester is n o t h i n g more than a spark-gap. It w o u l d b e difficult, then, to conceive of a n y t h i n g f u n d a m e n t a l l y w r o n g with a l i g h t n i n g arrester, so far as offering an o p p o r t u n i t y for d i s c h a r g e is concerned. W e have also lear.ned t h a t d i s r u p t i v e d i s c h a r g e s do not always e m b r a c e the o p p o r t u n i t y for discharge w h i c h is offered b y a spark-gap l i g h t n i n g arrester. T h i s circumstance, a very f r e q u e n t l y occurring one, explains w h y lightning arresters s o m e t i m e s fail. A n o t h e r and all too f r e q u e n t cause is defective or i m p r o p e r l y applied i n s u l a t i n g m a t e rial. In a certain sense a l i g h t n i n g arrester is a s a f e t y valve. One w o u l d not expect to p r o t e c t a d e f e c t i v e or w e a k boiler with a safety valve set to blow at or near the b u r s t i n g strain of the boiler; no more s h o u l d we e x p e c t a s p a r k - g a p lightning arrester to protect w e a k or defective insulation. Defective insulation results either from w e a k i n s u l a t i n g material, or a f a u l t y a p p l i c a t i o ~ z of the i n s n ! a t i n g m a t e r i a l . Generally good insulating m a t e r i a l i s used. F a u l t y application m a y result in two w a y s : ( A ) T h r o u g h i m p r o p e r design ; (B) T h r o u g h carelessness or ignorance. E x a m p l e s under (A) are: (I) exposed surfaces offering opportunities for " surface discharge," and this is not an i n f r e q u e n t occurrence in connection w i t h i n s u l a t i n g materials w h i e h w o u l d o t h e r w i s e stand very high v o l t a g e s ; and, (;) insufficient allowance for a proper m a r g i n of safety. T h e effects of rough handling, h e a t and cold, d a m p and dry a t m o s -
June, i 895.]
L@lm)tg Arresters.
45 5
pheres, dirt and g r i t (this l a t t e r h a v i n g a particular attraction for electrical apparatus, etc.), d e m a n d a margin o f safety w h i c h is n o t always a p p r e c i a t e d even b y the designer. U n d e r (B) m i g h t b e m e n t i o n e d a long list of details, such a s bruises, cracks, pin holes, cuts, open joints, bits of metal i m b e d d e d in the insulation, s h a r p corners, etc., which will tend to l o w e r the insulation s t r e n g t h fifty, seventy-five or even IOO per cent. A n d at this point it should be observed that the w e a k e s t p o i n t in the insulation of a given piece of a p p a r a t u s (it m a y be a pin hole or m i n u t e crack invisible to the n a k e d eye) is a m e a s u r e of its insulation strength. R e p a i r w o r k in shops of local electric light and p o w e r companies is liable to b e m o r e or less defective, and b u t few such c o m p a n i e s are p r o v i d e d with testing sets. R e paired a r m a t u r e s and c o n v e r t e r s are placed in service and m i g h t s t a n d indefinitely the normal E.M.~'. of the circuit to which t h e y are connected, b u t field discharges, lightning, rise of potential and p r o x i m i t y to other circuits carrying high potentials , d e m a n d a m a r g i n of safety which cannot b e assured unless t h e insulation be actually t e s t e d with an E.M.F. from four to six times the normal. But, even t h o u g h the insulation of a p p a r a t u s be perfect and have its p r o p e r m a r g i n of safety when installed, deterioration may, nevertheless, occur from various causes, principal a m o n g w h i c h w o u l d be m o i s t u r e and overheating. In general, it m a y b e stated that if we undertake, b y means of spark-gaps, to p r o v i d e a b s o l n t e protection a g a i n s t static disr u p t i v e discharges, the insulation strength m u s t b e a r such a relation to the spark-gaps as to place it b e y o n d t h e limit of selection. Ordinarily, however, a b s o l u t e conditions do n o t occur in practice, we can b u t a p p r o x i m a t e to t h e m and t h e n philosophicaiIy accept a r e a s o n a b l e p e r c e n t a g e of failures .as inevitable. T h e failure of l i g h t n i n g arresters is too often d u e to careless i n s t a l l a t i o n . It m a y be instructive to note several examples : (i) One p l a n t is r e p o r t e d as having, for b e t t e r protection, c o n n e c t e d t w o a r r e s t e r s in series. T h i s w a s p r o b a b l y done with t h e i d e a t h a t if a little w a s good more w o u l d be better.
456
Wurts.
[L ~. l.,
(2) A large b a n k of station arresters was g r o u n d e d to an iron bolt a b o u t two feet long, driven into d r y sand. (3) Line arresters were g r o u n d e d b y p u s h i n g the g r o u n d w i r e s into the earth. (4) Line a t t e s t e r s were g r o u n d e d on iron poles, w h i c h were themselves set in P o r t l a n d cement. (5) An annual inspection of a u t o m a t i c l i g h t n i n g a t t e s t e r s d e v e l o p e d the fact that the arresters were nearly all b u r n e d o u t - - i n other words, t h a t the line was left unprotected. (6) T h e ground plate of a b a n k of arresters was t h r o w n into a n e i g h b o r i n g stream, w h i c h s u b s e q u e n t l y c h a n g e d its course, leaving the g r o u n d plate h i g h and dry. (7) T h e g r o u n d plate of a b a n k of station arresters w a s laid on the rock b o t t o m of a n e i g h b o r i n g stream. (8) In a large n u m b e r of cases a portion of the g r o u n d wire is w o u n d into a fancy coil (choke coil). A n d the list m i g h t be indefinitely extended, each such case f o r m i n g a ~source of complaint t h a t the a t t e s t e r s " fail to protect." But, when these curious m i s t a k e s are located and properly r e m e d i e d the complaints cease. 5"ummary.--Overhead wires b e c o m e charged. T h e y are d i s c h a r g e d t h r o u g h l i g h t n i n g arresters, w h i c h are sparkgaps. Shifting points of h~gh and low p r e s s u r e are f o r m e d along the line, so that the d i s c h a r g e does not n e c e s s a r i l y ,occur over the s h o r t e s t or easiest p a t h ; that is, the discharge is selective. L i g h t n i n g arresters offer o p p o r t u n i t i e s for discharge. Coils protect. A liberal d i s t r i b u t i o n of line arresters offers the only practical m e a n s of p r o t e c t i n g w i d e l y d i s t r i b u t e d apparatus. L i g h t n i n g arresters fail to " p r o t e c t ;" first, because of tile shifting high- and low-pressure points, or in o t h e r words, for lack of a sufficient n u m b e r of fine a r r e s t e r s ; second, b e c a u s e insulation is d e f e c t i v e ; and third, b e c a u s e l i g h t n i n g arresters are not properly installed.
Lig/ztning Arresters.
439
L I G H T N I N G A R R E S T E R S , AND W H Y THEY SOMETIMES FAIL.* BY ALEXANDER JAY WURTS.
T h e lecturer was introduced by Prof. E. J. Houston, of the Institute, and spoke as follows: MEMBERS OF THE INSTITUTE, LADIES AND GENTLEMEN:
W h e n the first telegraph lines were installed it was noticed t h a t the instruments frequently became damaged during thunder-storms. The wires were charged with static electricity, which, instead of following m a n y turns of wire through the i n s t r u m e n t to earth, would puncture the insulation between consecutive convolutions and thus shortcircuit the coils. Besides this damage to the instruments, it was noticed that the static charge would sometimes spring from the line, or other metallic parts of the circuit, to neighboring conducting objects, such as nails, iron pipes and the like. It was also noticed that, when discharge occurred in this manner, the instruments were not damaged. In other words, these neighboring objects formed by-paths for the discharge and, in a measure, offered protection to the instruments. As a consequence of such observation, artificial by-paths were constructed, which consisted of two metal electrodes, separated by a small air space. One of these electrodes was connected to the line, the other to earth; and thus the first " l i g h t n i n g arrester" was constructed. It was then supposed that, when a line was provided with a " l i g h t n i n g arrester," the instruments would be "protected against lightning." T h e result, however, proved this conclusion to be erroneous. The discharges would sometimes pass to earth over the spark-gap of the lightning arrester, but not infrequently would quite ignore the arrester and, as before, puncture the insulating material of the * A lecture delivered before the Franklin Institute, March i5, I895.
440
l,'Vzfrts ."
[J. F. I.,
instruments. In other words, the discharge was selective. The failure of the lightning arrester was a vexation; b u t at that time, owing to the small amount of apparatus in use, the damage reckoned in dollars and cents did not call for any special study of the reasons for the selective character of disruptive discharges. In these days, however, the vast amount of capital invested in electrical apparatus of various kinds, and the consequent increased annual loss directly dl~e to these disrnptive discharges, have called for a thorough investigation of the subject and tile designing of more efficient means of protection. It is a very significant fact that overhead wires, after being subjected to the influence of thunder-storms, do not show the damage to insulation which one would reasonably expect to find had the wires been actually struck by the lightning discharge. It is not uncommon, however, to h e a r of wires being struck by lightnm o ; in fact, Iinemen will frequently volunteer to point out the exact spot where the lightning entered the wire. But, so far as I have been able to learn, overhead wires are not struck by lightning. T h e points usually selected by lightning discharges are trees, lightning-rods, church steeples, tall chimneys, and the like. There seems to be no reason why lightning should strike a horizontal wire, especially when it is insulated from t h e earth. Overhead wires may become charged in three ways a n d combinations of these, viz.: by static induction from the clouds; by dynamic induction from a lightning discharge, and by conduction from the s u r r o u n d i n g charged atmosphere. T h e writer inclines to the theory of conduction. During thunder-storms, and in m a n y instances during fair weather, the atmosphere becomes charged with electricity at a constantly increasing potential as we recede from the earth. At the top of W a s h i n g t o n Monument, Washington, D. C., a potential of 3,ooo volts has been measured during thunder-storms, and at the top of the Eiffel Tower a potential of I0,ooo volts has been measured. Now, it is well known that lightning is oscillatory. T h e first oscillation, or discharge, makes a crack or hole t h r o u g h
June, I895.]
LightniJ~g Arresters.
44I
the atmosphere, and through this the succeeding oscillations take place. About ten to twelve oscillations can be observed, and the time interval is reckoned at about "ooooi of a second. The lightning, therefore, being oseillatory and the atmosphere charged, if an overhead wire be charged by conduction from the atmosphere--that is, become electrically a portion of it--then, with every lightning discharge it would seem as though the potential of the atmosphere would sympathize with the oscillatory character of the lightning, and that the charge in the atmosphere would also oscillate and produce oscillations in overhead wires; and, in fact, in all metallic conductors, such as wire fences, rails, etc. T h e fact that wires become heavily charged during fair weather offers further evidence that they are charged by conduction from the surrounding charged atmosphere. In some instances, lightning arresters have been .known, du.ring perfectly fair weather, to discharge overhead wires at the rate of 14o times a minute. In any case, whatever the method of charging m a y be, the discharges from overhead wires seem to be oscillatory, and during thunder-storms they are, in most eases, simultaneous with l i g h t n i n g flashes. If w e raise one end of a trough of water and then quickly lower it, the water will~quietly surge back and forth. If, w h e n the water is returning, we again raise the same end of the trough, a wave will be started forward, which, meeting the returning wave, will combine or collide with it, causing a piling up of the water at that point. Further eomplieations m a y be introduced by repeatedly, and at proper intervals, raising and lowering one end of the trough. If the crests of these waves be carefully examined, it will be noticed that they differ in height and are con. stantly shifting their positions. During thunder-storms the static eleetrieity in overhead conductors surges back and forth very m u c h as the water in a trough, and the indieations are that electric waves are set up which combine and :nterfere with each other in such a manner as to produce frequent and unequally distributed points of high and low pressure. T h e ends of wires are
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442
[J. F. I.,
points of reflection, a n d at these points the pressure is always great. W h a t e v e r m a y be t h e t r u e explanation of the a p p a r e n t idiosyncrasies of d i s r u p t i v e discharges, it is, after all, with facts t h a t we h a v e to deal. R e f e r r i n g to Fe~. z, let d A represent the terminals of a static i n d u c t i o n m a c h i n e L, a b a t t e r y of six one-half-gallon L e y d e n jars, I3, a spark gap of 12- and CI C2, etc., variable spark gaps, and let the c o n n e c t i o n s be as indicated. W i t h discharges at B, d i s c h a r g e s will also occur at one or more of the gaps CI, C2, etc. W h e n CI, C2, C3 are equal, t h e discharges occur across t h e s e gaps i n d i s c r i m i n a t e l y . W h e n discharge occurs over small gaps in t h e neighborh o o d of CI or C2, thread-like d i s c h a r g e s are always noticed a t Cn. _And even w h e n (2~ is increased to g~77, the thread-like discharge is still noticeable with every d i s c h a r g e at Ct or
~B~,
~
L
.
.
.
.
.
.
.
.
.
.
FIG. I.--t~xperiment with line arresters, illustrating selective character of discharge.
C2. If a large n u m b e r of Spark-gaps, v a r y i n g a m o n g e a c h other as m u c h as IOO per cent., be d i s t r i b u t e d as CI, C2, C3, etc., discharges will still occur i n d i s c r i m i n a t e l y , s o m e t i m e s across a large gap, s o m e t i m e s a small one, s o m e t i m e s across three or four c o n s e c u t i v e or w i d e l y d i s t r i b u t e d gaps. W h e n CI is equal to ~.7", ~ equal to ~7- and placed two, t h r e e or more feet a w a y from CI, and if C3, C4, etc., be omitted, t h e n a discharge will occur at C2 once in a b o u t 50o times w i t h o u t any discharge at Ct. T h e s e e x p e r i m e n t s d e m o n s t r a t e t h e selective c h a r a c t e r of t h e discharge. If, however, (?2 be increased to ~18-, t h e d i s c h a r g e s wilt cease a l t o g e t h e r a t t h a t point, and this last e x p e r i m e n t i n d i c a t e s the l i m i t of r a n g e over w h i c h selection may t a k e place u n d e r the conditions given. B u t it is perfectly e v i d e n t t h a t w i t h a l a r g e n u m b e r of gaps, the p r o b a b i l i t y of d i s c h a r g e across a n y
June, 1895.]
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443
one gap is v e r y m u c h less t h a n if only two gaps were present. In o t h e r words, if C: represents a piece of apparatus w h i c h we desire to protect, the protection will be greater as we increase the n u m b e r of spark-gaps, C2, C3, etc., b u t in no case can there be certainty of protection unless t h e gap C~ be relatively so large--or, more broadly speaking, unless t h e path C~ offer such a h i g h resistance to the passage of disruptive d i s c h a r g e s - - a s to place it b e y o n d the limit of selection. T h e apparatus which we have used in the above e x p e r i m e n t s is different from t h a t employed by n a t u r e ; t h e results, however, so far as m y observations go, are identical, a n d the lesson to be learned is simple; it is this: the p r o b a b i l i t y of d a m a g e to electrical apparatus connected to o v e r h e a d wires is lessened as we increase the n u m b e r of o p p o r t u n i t i e s for discharge from the line. In s u m m i n g up thus far, we notice briefly that t h e line becomes charged, s u r g i n g s are set up, there are points of reflection w h i c h are always points of g r e a t electrical strain, there are points along t h e line of v a r y i n g intensity or tendency to discharge, these points are constantly shifting their positions, and spark gap l i g h t n i n g arresters are used to conduct disruptive discharges to earth. T h e l i g h t n i n g a r r e s t e r of t o day, as a protective device, differs from the l i g h t n i n g a r r e s t e r of t h e early telegraph in detail only. A simple spark-gap, one t e r m i n a l of which is connected to the line and the o t h e r to earth, is essentially the l i g h t n i n g a r r e s t e r n o w in use. But, w i t h the m o d e r n high-potential circuits, it is f o u n d that the d y n a m o c u r r e n t follows the static discharge across t h e spark-gap of the attester, c a u s i n g t h e r e b y a short-circuit or d a n g e r o u s g r o u n d on the line. In o r d e r to avoid this difficulty, a r c - r u p t u r i n g devices are a t t a c h e d to t h e l i g h t n i n g arrester, which have for t h e i r function t h e i m m e d i a t e i n t e r r u p t i o n of the d y n a m o current, w i t h o u t i n t e r f e r i n g w i t h the f u r t h e r operation of the l i g h t n i n g arrester as a d i s c h a r g i n g device. " A u t o m a t i c L i g h t n i n g Arresters," as t h e y are called, differ a m o n g one another in t h e various m e a n s adopted for r u p t u r i n g t h e d y n a m o arc. But t h e a r e - r u p t u r i n g a t t a c h m e n t has n o t h i n g w h a t e v e r to do with the a p p a r a t u s as a l i g h t n i n g arrester,
444
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so that although automatic lightning arresters vary in outward appearance, and are in general more cltmbersome and expensive than the original simple spark-gap arrester, yet, as lightning arresters, they are nothing more than sparkgaps. Various lightning-arrester attachments have beer~ designed for the automatic rupture of dynamo arcs, b u t owing to the very high potentials which are so often tlsed, t h e y are found to be not only undesirable, b u t inefficient, the lightning arresters being frequently destroyed b y the vicious dynamo arc. In practice, however, complaints are not so much t h a t lightning arresters are destroyed as that t h e y "fail to protect." It is not unusual to see several different kinds of lightning attesters in a single station, which have evidently b e e n installed with the idea of determining which one of t h e m would prove the most efficient. But such experiments have met with disappointment, for the reason that sometimes one lightning attester, and sometimes another, w o u l d receive the discharge, and the selective character of disruptive discharges was not understood. T h e failure of lightning arresters is not due to the particular k i n d of l i g h t n i n g arrester, or to the patent under which it is manufactured ; it is due largely to the peculiar conditions with which i t has to contend, namely, that the discharge is selective, and that, in order to protect apparatus, means m u s t be taken to control these selections, or, at least, to provide so m a n y paths to earth that t h e probable selection will be one of the many paths provided. In other words, lightning arresters do not " protect," they simply offer opportunities for discharge. These opportunities m a y or m a y not b e embraced, according to circumstances. But the failure of lightning arresters is not altogether due to the peculiar conditions already referred to. Poor ground connection, inductive resistance in the ground circuit, defective insulation in the apparatus to be protected, and a general misunderstanding of the subject, are not infrequently the cause of serious losses which might otherwise have been avoided. W e have noticed that lightning arresters do not " p r o t e c t , " that they simply offer opportunities for discharge. W e
June, I895.]
£i~r]zhzin~ Arresters.
445
have also noticed that discharges do not pass readily through coils of wire--coils therefore protect. Properly constructed choke coils, connected in the circuit, offer a high resistance to the passage of disruptive discharges, and when used in connection with lightning arresters the combination offers a very reliable means of protecting wellinsulated apparatus against lightning. Laboratory experiments, together with tests made under actual working conditions, indicate the advisability of using four choke coils in series, in each wire, with four lightning attesters intervening, as shown in Fig. 2. This arrangement is more particularly suitable for the protection of station apparatus. Coils can, however, be used to advantage on the line for the protection of the more expensive translating devices. CHOKECOILS.
,~ GENEBATOR
GROUND.
LIGHTNINGARRESTERS.
GROUNO.
FrG. 2.--Four choke coils with lightning arresters intervening.
The general construction of a choke eoilis a matter of
considerable importance. A flat spiral possesses some advantages over the helix, but for practical purposes these hardly compensate for the lower cost of the latter, which, for best results, should be wound over a wooden or other non-conducting core. Metal cores and eases should be avoided, because the induced currents in these parts would lower the choking effect of the coils. The n u m b e r of turns which can be used to advantage is limited. It is found by experiment that, with given conditions, the choking effect increases with the n u m b e r of turns up to a well defined critical point, after which additional turns fail to have any appreciable effect. It is probable that these critical points vary with the amount of electricity discharged ; that is, for h e a v y
446
V/urts :
[J. F. I.,
discharges the critical point for m a x i m u m choking effect would embrace a larger n u m b e r of turns than would smalter discharges, and the choking effect for a given number of turns within the critical point would also be greater for heavy than for small discharges. For practical purposes the writer recommends from forty to fifty feet of wire wound either into a flat spiral having an internal diameter of three inches, or into a three-inch helix with a single layer. Summing up once more, we note that wires become charged, that lightning arresters offer opportunities for discharge, and t h a t coils protect. Bearing in mind these three points, let us note that, with the extensive systems of electric light and power distribution now in use, it would b e quite impractical to install the above.described system of choke coils and lightning arresters for the protection of each separate piece of apparatus. T h e first cost of such an installation would be prohibitory. It is evident, therefore, that the protection of line apparatus with choke coils being excluded there remains but one alternative, namely, that of providing such a large n u m b e r of opportunities for discharge, distributed over the entire system, that the selection of the discharge will be one or more of the paths provided for it. In other words, lithe a~'resters connected at frequent intervals offer the only practical method of protecting widely distributed apparatus. But, in order that such an installation m a y prove satisfactory from a commercial as well as an operative standpoint, the arresters m u s t be cheap, simple, free from moving parts and perfectly reliable without the necessity of regular inspection. Realizing that these points were not covered by any o n e of the m a n y automatic lightning arresters which had a l r e a d y been devised, I undertook to simplify the problem, and in m y early experiments proposed to equip the lines of electric systems with a multiplicity of spark-gaps, and, at the station, to provide an automatic circuit opening-and-closing device, which, upon the formation of a short circuit across one o r more of the spark-gaps on the line, would instantly o p e n and then immediately dose. While experimenting w i t h
June, 1895.]
Lig/ttninff Attesters.
447
this system, I d i s c o v e r e d that if the electrodes of a lightning arrester w e r e m a d e of zinc, the short-circuiting arc, which had h e r e t o f o r e t h r e a t e n e d the life of the l i g h t n i n g arrester, a b s o l u t e l y failed to be maintained. In other words, zinc p r o v e d to be w h a t is n o w c o m m o n l y known as one of the " n o n - a r c i n g " metals. S u b s e q u e n t i n v e s t i g a t i o n disclosed the fact that there were four other metals which exhibited similar characteristies, namely, bismuth, antimony, c a d m i u m and mercury.* The discovery of the non-areing metals at once solved
FIG. 3"--D°uble-p°le n°narcing metal line attester.
the p r o b l e m of a lightning" arrester a d a p t e d to the requirem e n t s of a l t e r n a t i n g current circuits, and w h i c h w o u l d m e e t the conditions already e n u m e r a t e d . T h e non-arcing m e t a l l i g h t n i n g arrester, w h i c h is illustrated in Figs. 3, 4 and y, consists, as will be seen, of seven non-arcing m e t a l cylinders, insulated from each other, each one and one.half inehes long and three-fourths of an inch in diameter. W h e n installed, the t w o o u t e r cylinders are A full description of the researches and experiments Which led to the discovery of the non-arcing metals is given inthe Transaclions oflhe Ameri. can Institulg of Electrical Engineers, for March I5, 1892.
448
I/Uzlrts ."
[ J. F. I.,
connected to e i t h e r ]e~ of an alternatin K c u r r e n t circuit and the central c y l i n d e r to earth, t h u s f o r m i n ~ a double-
FIG. 4.--Non-arcing metal line arrester.
I~'IG. 5.--Non-arcing metal line arrester.
pole l i g h t n i n g arrester. In practice, these are m a n u f a c t u r e d in units for i,ooo volts, so t h a t for 2,ooo volts two are con-
L@htnzng Attesters.
June, ~895.]
449
nected in series, for 3,ooo volts three are c o n n e c t e d in series. The d i a g r a m s in F~g-. 6 indicate t h e connections for each. potential. T h e r e are m a n y t h o u s a n d s of these l i g h t n i n g arresters in actual service, a n d the experience of three years~ has fully d e m o n s t r a t e d that t h e y are perfectly reliable w i t h out the n e c e s s i t y of r e g u l a r inspection. T h e simplicity of the device is at once apparent, and the cost to the user is a. mere fraction of w h a t was f o r m e r l y d e m a n d e d for the best. types of a u t o m a t i c l i g h t n i n g arresters. It will t h u s be seen that, at least for a l t e r n a t i n g current circuits, t h e require-. ments of a c o m m e r c i a l l y serviceable line l i g h t n i n g a r r e s t e r have been fully met. T h e success of the non-arcing metal l i g h t n i n g a r r e s t e r was such that I was i n d u c e d to m a k e f u r t h e r investigations. I0OO VOLTS
Station Arcestea
2OOO VOLTS
3000 VOLTY
-3N~..,,,
s
L;ne A rr,~ste.~.
E
"
Lt~£
LINE
MNI
FIG, 6.--Diagrams for lightning arrester connections on circuits of differ rot: potentials.
as to the p o s s i b i l i t y of c o n s t r u c t i n g an equally simple a n d efficient device for use on direct c u r r e n t circuits. In v i e w of the m a n y e x p e r i m e n t s w h i c h I had m a d e w i t h almost, every kind and c o m b i n a t i o n of the ordinary metals, I had b e c o m e c o n v i n c e d t h a t I should n o t be able to c o n s t r u c t a non-arcing l i g h t n i n g arrester for direct current circ~it~ which w o u l d derive its non.arcing p r o p e r t y from thenature of the m a t e r i a l of the electrodes, as is the case with the non-arcing m e t a l l i g h t n i n g arrester. Accordingly I directed m y a t t e n t i o n to the possible s u p p r e s s i o n of t h e d y n a m o arc b y v i r t u e of t h e construction or relation of the p a r t s . I had a l r e a d y o b s e r v e d the r e m a r k a b l e ease w i t h w h i c h VOL. C X X X I X .
No. 834.
29
450
PI~trts :
[J. F. I.,
disruptive discharges strike over i n s u l a t i n g surfaces, as compared to the difficulty wi'th which t h e y o r d i n a r i l y pierce the air. I~or example, disruptive discharges will strike over a glass surface, b e t w e e n two electrodes placed e i g h t inches apart, r a t h e r than pierce the air b e t w e e n electrodes placed one inch a p a r t and s h u n t i n g t h e electrodes in contact w i t h the glass. If g r o u n d glass be used and a pencil-mark be drawn between the widely s e p a r a t e d electrodes, the r a t i o between the distances over the glass and t h r o u g h the air is increased. W h i l e e n d e a v o r i n g to avail m y s e l f of this principle of " s u r f a c e discharge," as I h a v e called it, it occurred to me t h a t a d y n a m o arc, in order to exist, m u s t be fed b y the wtpors of its electrodes, a n d t h a t if such vapors c o u l d be suppressed, or if t h e i r f o r m a t i o n could be avoided, a n arc could not possibly exist. P l a c i n g these two i d e a s together, namely, " s u r f a c e discharge " and " suppression of the vapors," I at once h a d the f u n d a m e n t a l principles necessary for the construction of w h a t h a s since proved to be a most simple and efficient non-arcing l i g h t n i n g arrester f o r direct current circuits. t n the first e x p e r i m e n t s which led to a practical adaptation of these ideas to the c o n s t r u c t i o n of a lig'htning atrester, I placed two a l u m i n u m - f o i l t e r m i n a l s b e t w e e n t h e surfaces of two small blocks of marble, the surfaces of w h i c h had been carefully ground, and on one of w h i c h I h a d d r a w n a pencil-mark which s h o u l d bridge t h e d i s t a n c e between t h e foil electrodes. A t first these electrodes were placed a b o u t two inches apart, the two blocks of m a r b l e were f i r m l y bound t o g e t h e r w i t h twine, t h e t e r m i n a l s were c o n n e c t e d to the terminals of a 5oo-volt direct c u r r e n t generator, a n d disruptive discharges were t h e n caused to pass from o n e foil t e r m i n a l to the other, b e t w e e n the m a r b l e blocks a n d over the pencil-mark. It will at once be seen t h a t the function of one of these m a r b l e blocks was to f u r n i s h a s u r f a c e discharge plane, t h a t t h e pencil-mark was to f u r t h e r facilitate the passage of the disruptive discharge, and t h a t t h e function of the second block, which was firmly b o u n d to t h e first, was to suppress the f o r m a t i o n of vapors, which w o u l d otherwise serve to establish a short-circuit u n d e r the condi-
June, I895.]
Lig/ltnbzg~ ~trresters.
45 !
tions of the test. N o arc being formed, the terminal foils: were b r o u g h t s u c c e s s i v e l y nearer to each other, until finally, when t h e d i s c h a r g e plane had been shortened to oneq u a r t e r of an inch, a short circuit was established. W i t h o u t e n t e r i n g into the details of s u b s e q u e n t experi-
FIG. 7.--Non-arcing railway lightning arrester (for car or line use).
m e n t s l e a d i n g to t h e p r a c t i c a l construction of a c o m m e r c i a l l i g h t n i n g arrester, it m a y briefly be stated t h a t t h e final form given to the arrester was, t h a t s h o w n in Fig. 7; In this form wood, h a v i n g b e e n f o u n d m o r e durable, h a s b e e ~ s u b s t i t u t e d for marble, and, in place of the pencil-marks,
452
tVz~rts ."
[J. F. I.,
shallow g r o o v e s are b u r n e d longitudina]ly b e t w e e n the terminal electrodes, w h i c h are placed three-eighths of an inch apart. W h i l e s u b m i t t i n g this form of l i g h t n i n g a t t e s t e r t o long-continued tests with d i s r u p t i v e discharges, it was fotmd that there w a s a gradual w e a r i n g a w a y of the wood fibre, which eventually ended in the formation of a cavity in which v a p o r s formed in sufficient q u a n t i t y to establish a d y n a m o shortcircuit. T h e w e a r i n g a w a y of the fibre s e e m e d to be d u e to the fact t h a t no special vent, or o p p o r t u n i t y for displacement, was p r o v i d e d for t h e d i s r u p t i v e discharge. This evil was quickly cured b y slotting the u p p e r block at right angles to the charred grooves in such a m a n n e r as to open up the grooves or discharge plane to the air, without, however, uncovering the electrodes. It will n o w be proper to notice the d i s t i n c t i v e features of the t w o non-arcing l i g h t n i n g arresters w h i c h h a v e b e e n described. T h e non-arcing m e t a l l i g h t n i n g arrester is nonarcing b y v i r t u e of the material of its electrodes, and t h o u g h "non-arcing," nevertheless allows of t h e d y n a m o short-circuit. This is, however, so quickly i n t e r r u p t e d , p r o b a b l y b y virtue of non-conducting vapors w h i c h form b e t w e e n t h e electrodes d u r i n g t h e first rush of current, t h a t no appreciable b u r n i n g or d a m a g e to t h e electrodes can be noticed, and the m e l t i n g of fuses and c o n s e q u e n t i n t e r r u p t i o n to t h e service are entirely avoided. On the o t h e r hand, the nonarcing lightning arrester for direct c u r r e n t circuits is " n o n a r c i n g " b y v i r t u e of its construction ; and, m o r e t h a n this, is, strictly speaking, a discriminating l i g h t n i n g arrester, i n that it allows disruptive discharges to pass freely w i t h o u t allowing a n y d y n a m o c u r r e n t at all to follow. This l a s t feature is one which has e n a b l e d me to m a k e some interesting i n v e s t i g a t i o n s r e g a r d i n g the relative v o l u m e of discharges from overhead wires d u r i n g thunder-storms. W h i l e carrying on a series of e x p e r i m e n t s in Colorado, during the s u m m e r of I893, I c o n n e c t e d in series with a n u m b e r of these d i s c r i m i n a t i n g l i g h t n i n g arresters small spark-gaps, into w h i c h I inserted pieces of t i s s u e paper, which were i n t e n d e d to act as telltales, and which, b y t h e holes p u n c t u r e d b y the discharges, g a v e m e some record o f
June, i895.]
Lig/lfning Attesters.
45 3
what was going on. Some of these tell-tale papers are shown in F@. & It will thus be seen that, although
Fro. 8.--Tell-tale papers.
the discharges which caused these holes were larger than is ordinarily obtained with a battery of Leyden jars, they
454
V/~lrts :
[J. F. I.,
are not to be compared with the holes w h i c h one w o u l d expect from the l i g h t n i n g flashes w h i c h are so often said to " s t r i k e t h e lines." T h e fact t h a t these telltale papers were not b u r n e d is sufficient evidence that t h e d y n a m o current did not follow the discharge. T h e r e is, however, a n o t h e r side to this m a t t e r of protection against l i g h t n i n g - - o n e w h i c h has been little discussed, and which is almost invariably overlooked b y those to w h o m interruptions d u e to l i g h t n i n g are a m a t t e r of vital importance. I refer to the insulation of t h e a p p a r a t u s to be protected. W h e n a l i g h t n i n g arrester "fails to p r o t e c t " it is condemned, the general opinion b e i n g t h a t the failure is due to some inherent fault in the l i g h t n i n g attester. B u t we have already learned that a l i g h t n i n g arrester is n o t h i n g more than a spark-gap. It w o u l d b e difficult, then, to conceive of a n y t h i n g f u n d a m e n t a l l y w r o n g with a l i g h t n i n g arrester, so far as offering an o p p o r t u n i t y for d i s c h a r g e is concerned. W e have also lear.ned t h a t d i s r u p t i v e d i s c h a r g e s do not always e m b r a c e the o p p o r t u n i t y for discharge w h i c h is offered b y a spark-gap l i g h t n i n g arrester. T h i s circumstance, a very f r e q u e n t l y occurring one, explains w h y lightning arresters s o m e t i m e s fail. A n o t h e r and all too f r e q u e n t cause is defective or i m p r o p e r l y applied i n s u l a t i n g m a t e rial. In a certain sense a l i g h t n i n g arrester is a s a f e t y valve. One w o u l d not expect to p r o t e c t a d e f e c t i v e or w e a k boiler with a safety valve set to blow at or near the b u r s t i n g strain of the boiler; no more s h o u l d we e x p e c t a s p a r k - g a p lightning arrester to protect w e a k or defective insulation. Defective insulation results either from w e a k i n s u l a t i n g material, or a f a u l t y a p p l i c a t i o ~ z of the i n s n ! a t i n g m a t e r i a l . Generally good insulating m a t e r i a l i s used. F a u l t y application m a y result in two w a y s : ( A ) T h r o u g h i m p r o p e r design ; (B) T h r o u g h carelessness or ignorance. E x a m p l e s under (A) are: (I) exposed surfaces offering opportunities for " surface discharge," and this is not an i n f r e q u e n t occurrence in connection w i t h i n s u l a t i n g materials w h i e h w o u l d o t h e r w i s e stand very high v o l t a g e s ; and, (;) insufficient allowance for a proper m a r g i n of safety. T h e effects of rough handling, h e a t and cold, d a m p and dry a t m o s -
June, i 895.]
L@lm)tg Arresters.
45 5
pheres, dirt and g r i t (this l a t t e r h a v i n g a particular attraction for electrical apparatus, etc.), d e m a n d a margin o f safety w h i c h is n o t always a p p r e c i a t e d even b y the designer. U n d e r (B) m i g h t b e m e n t i o n e d a long list of details, such a s bruises, cracks, pin holes, cuts, open joints, bits of metal i m b e d d e d in the insulation, s h a r p corners, etc., which will tend to l o w e r the insulation s t r e n g t h fifty, seventy-five or even IOO per cent. A n d at this point it should be observed that the w e a k e s t p o i n t in the insulation of a given piece of a p p a r a t u s (it m a y be a pin hole or m i n u t e crack invisible to the n a k e d eye) is a m e a s u r e of its insulation strength. R e p a i r w o r k in shops of local electric light and p o w e r companies is liable to b e m o r e or less defective, and b u t few such c o m p a n i e s are p r o v i d e d with testing sets. R e paired a r m a t u r e s and c o n v e r t e r s are placed in service and m i g h t s t a n d indefinitely the normal E.M.~'. of the circuit to which t h e y are connected, b u t field discharges, lightning, rise of potential and p r o x i m i t y to other circuits carrying high potentials , d e m a n d a m a r g i n of safety which cannot b e assured unless t h e insulation be actually t e s t e d with an E.M.F. from four to six times the normal. But, even t h o u g h the insulation of a p p a r a t u s be perfect and have its p r o p e r m a r g i n of safety when installed, deterioration may, nevertheless, occur from various causes, principal a m o n g w h i c h w o u l d be m o i s t u r e and overheating. In general, it m a y b e stated that if we undertake, b y means of spark-gaps, to p r o v i d e a b s o l n t e protection a g a i n s t static disr u p t i v e discharges, the insulation strength m u s t b e a r such a relation to the spark-gaps as to place it b e y o n d t h e limit of selection. Ordinarily, however, a b s o l u t e conditions do n o t occur in practice, we can b u t a p p r o x i m a t e to t h e m and t h e n philosophicaiIy accept a r e a s o n a b l e p e r c e n t a g e of failures .as inevitable. T h e failure of l i g h t n i n g arresters is too often d u e to careless i n s t a l l a t i o n . It m a y be instructive to note several examples : (i) One p l a n t is r e p o r t e d as having, for b e t t e r protection, c o n n e c t e d t w o a r r e s t e r s in series. T h i s w a s p r o b a b l y done with t h e i d e a t h a t if a little w a s good more w o u l d be better.
456
Wurts.
[L ~. l.,
(2) A large b a n k of station arresters was g r o u n d e d to an iron bolt a b o u t two feet long, driven into d r y sand. (3) Line arresters were g r o u n d e d b y p u s h i n g the g r o u n d w i r e s into the earth. (4) Line a t t e s t e r s were g r o u n d e d on iron poles, w h i c h were themselves set in P o r t l a n d cement. (5) An annual inspection of a u t o m a t i c l i g h t n i n g a t t e s t e r s d e v e l o p e d the fact that the arresters were nearly all b u r n e d o u t - - i n other words, t h a t the line was left unprotected. (6) T h e ground plate of a b a n k of arresters was t h r o w n into a n e i g h b o r i n g stream, w h i c h s u b s e q u e n t l y c h a n g e d its course, leaving the g r o u n d plate h i g h and dry. (7) T h e g r o u n d plate of a b a n k of station arresters w a s laid on the rock b o t t o m of a n e i g h b o r i n g stream. (8) In a large n u m b e r of cases a portion of the g r o u n d wire is w o u n d into a fancy coil (choke coil). A n d the list m i g h t be indefinitely extended, each such case f o r m i n g a ~source of complaint t h a t the a t t e s t e r s " fail to protect." But, when these curious m i s t a k e s are located and properly r e m e d i e d the complaints cease. 5"ummary.--Overhead wires b e c o m e charged. T h e y are d i s c h a r g e d t h r o u g h l i g h t n i n g arresters, w h i c h are sparkgaps. Shifting points of h~gh and low p r e s s u r e are f o r m e d along the line, so that the d i s c h a r g e does not n e c e s s a r i l y ,occur over the s h o r t e s t or easiest p a t h ; that is, the discharge is selective. L i g h t n i n g arresters offer o p p o r t u n i t i e s for discharge. Coils protect. A liberal d i s t r i b u t i o n of line arresters offers the only practical m e a n s of p r o t e c t i n g w i d e l y d i s t r i b u t e d apparatus. L i g h t n i n g arresters fail to " p r o t e c t ;" first, because of tile shifting high- and low-pressure points, or in o t h e r words, for lack of a sufficient n u m b e r of fine a r r e s t e r s ; second, b e c a u s e insulation is d e f e c t i v e ; and third, b e c a u s e l i g h t n i n g arresters are not properly installed.