Fig. 2,349.—Non-arcing multi-gap arrester. Based on the principle of employing for the terminals across which the arc is formed, such metals as are least capable of maintaining an alternating arc between them. This so called non-arcing property of certain metals was discovered by Alexander Wurtz. The action is such that the "line current" which follows the lightning discharge follows as an arc, but is stopped at the end of one alternation because of the property of the non-arcing metals to carry an arc in one direction, but requiring an extremely high voltage to start a reverse arc. The non-arcing metals ordinarily employed are alloys of zinc and copper. Plain multi-gap arresters as here shown operate satisfactorily with the smaller machines and on circuits of limited power, but for large machines of close regulation, and therefore of very large momentary overload capacity, especially when a number of such are operated in parallel, such arresters were found insufficient, the line current following the lightning discharge frequently was so enormous that the circuit did not open at the end of the half wave, that is the arrester held the arc and was destroyed. The introduction of synchronous motors made it necessary that the arc should be extinguished immediately, otherwise the synchronous motors and converters would drop out of step, and the system would in this way be shut down. To insure the breaking of the arc, resistance was introduced in the arrester, the modified device being known as the low equivalent arrester as shown in fig. 2,350.

FIG. 2,350.—Low equivalent arrester. This is a modification of the multi-gap arrester shown in fig. 2,349. About half of the total number of gaps are shunted by a resistance, and another resistance inserted between the cylinders and the earth. With this arrangement the middle point is at ground pressure, and there are between line and ground only one half of the total number of gaps. This is sufficient to prevent a bridging of the gaps under normal conditions.

Fig. 2,351.—General Electric 2,200 volt multi-gap arrester for station installation. It consists of fourteen ?" knurled cylinders and two shunt resistance rods mounted on a porcelain base. One of these rods has a low resistance, and shunts nine gaps; the other rod has a high resistance, and shunts eleven gaps. The effect of the shunt resistance in extinguishing the line current arc is the same, therefore, as that of an equal series resistance but is without the objectionable features of the latter. Series resistance limits the discharge current to such an extent that an arrester with series resistance fails to protect against destructive rises of voltage when the conditions are severest. Graded shunt resistance responds to all frequencies and opens a discharge path for excessive voltage when the frequencies are high as well as when they are low. Its further effect in withholding the line current from the gaps after the relieving discharge has occurred, is to aid the non-arcing quality of the metal cylinders in quickly suppressing the arc that follows a discharge. The arc is extinguished at the end of the half cycle of line current in which the discharge takes place.

Fig. 2,352.—General Electric 2,200 volt arrester in the act of discharging, and shunting the line current. The figure shows an actual discharge taking place. It will be seen that the heavy line current passes across only four of the gaps, and then goes through the resistance rods; while the static discharge passes straight across the entire series of thirteen gaps. When the gaps of an arrester are shunted by even a low resistance, discharges of very high frequency find it relatively difficult to pass through the resistance rods, owing to the impedance of the rods, but comparatively easy to pass across all the gaps, owing to the capacity effect in breaking down the gaps. The higher the frequency, the more pronounced is this effect, hence the discharges select different paths through gaps and resistances depending upon the frequency. By frequency is meant, not the frequency of the line current but the lightning frequency, which may run into hundreds of thousands, or into millions of cycles. The equivalent needle gap for this arrester is shown by tests to be nearly the same for all frequencies and quantities of discharge; that is, the arrester is equally responsive to all frequencies.

Figs. 2,353 to 2,355.—Oscillograph record of the phenomena that take place in the different circuits or selective paths of a multi-gap arrester during a discharge such as shown in fig. 2,352.

Figs. 2,356 and 2,357.—Diagram showing condenser action of cylinders and pressure gradient for static stress.

Fig. 2,358.—Arrangement of graded resistances on multi-gap arrester.

Figs. 2,359 to 2,364.—Westinghouse safety spark gaps. Fig. 2,359, indoor type; figs. 2,360 to 2,364, outdoor type. It is well known that with transformers, operating on high voltage lines and having large ratios of transformation, there may occur, on the low tension side, momentary voltages to ground greatly in excess of the normal. These momentary increases in voltage between the low tension circuits and ground are commonly called "static disturbances." In general they are the result of a change in the static balance of the high tension side and its connecting circuits. Unless certain precautions are taken, such a static disturbance on the low tension side may cause serious stresses in the secondary insulation of a transformer with a high ratio of transformation. This induced static voltage is independent of the ratio of transformation. The static stresses are more serious in a high ratio transformer simply because the insulation of its secondary is less able to withstand them. A method of relieving this disturbance is to connect a discharge spark gap between some point of the low tension side of the transformer to be protected (a middle or neutral point, if one be available) and the ground. The spark gap opening is such that any voltage very much in excess of the maximum normal will cause a discharge to ground, and thus the low tension side is practically tied to ground during such disturbance, while at other times it is ungrounded. The Underwriters recommend the grounding of the neutral point of low tension circuits when the conditions are such that the maximum normal voltage between the point connected and ground will not exceed 250 volts. The rule allows one side of a 250 volt circuit or the middle point of a 550 volt circuit to be grounded. The spark gaps shown above are designed for use on transformer secondary circuits and for protecting individual series arc lamps. These spark gaps are single pole, and consist of two cylinders of non-arcing metal with an air gap between. One of the cylinders is connected to the ground, the other to the line.

Figs. 2,365 and 2,366.—Graded shunt resistance arrester connections. Fig. 2,365, connections for 33,000 volt Y system with grounded neutral; fig. 2,366, connections for 33,000 volt delta or ungrounded Y systems. The type of arrester shown above may be considered as four arresters in one. First, for small discharges there are a few gaps in series with a high shunt resistance. This part of the arrester will safely discharge accumulated static and also all disruptive discharges of small ampere capacity. This path is shown through H (resistance) and GS (gaps). Second, there are a number of gaps in series with a medium shunt resistance which will discharge disruptive strokes of medium ampere capacity. This path is shown through M (resistance) and GH plus GS (gaps). Third, there are a greater number of gaps in series with a low shunt resistance which will discharge heavy disruptive strokes. This path is shown through L (resistance) and GM plus GH plus GS (gaps). Fourth, the total number of gaps has no series resistance, thus enabling the arrester to freely discharge the heaviest induced strokes. This path is shown through zero resistance and GH plus GM plus GH plus GS (gaps). In each of the above circuits the number of gaps and the resistance are so proportioned as to extinguish the line arc at the end of the half cycle in which the lightning discharge takes place.

Fig. 2,367.—Installation of a General Electric 12,500 volt, three phase, multi-gap lightning arrester in the Garfield Park sub-station of the West Chicago park common. The "V" unit multi-gap arrester, which is plainly seen in the illustration, is made up of "V" units consisting of gaps between knurled cylinders and connected together at their ends by short metal strips. The base is of porcelain, which thoroughly insulates each cylinder, and insures the proper functioning of the multi-gaps. The cylinders are made of an alloy that contains metal of low boiling point which gives the rectifying effect, and metals of high boiling point which cannot vaporize in the presence of the one of low boiling point. The cylinders are heavily knurled. As the arc plays on the point of a knurl it gradually burns back and when the metal of low boiling temperature is used up, the gap is increased at that point. The knurling, thus, insures longer life to the cylinder by forcing successive arcs to shift to a new point. When worn along the entire face, the cylinder should be slightly turned. The low resistance section of the graded shunt is composed of rods of a metallic alloy. These rods have large current carrying capacity, and practically zero temperature coefficient up to red heat. The medium and high resistance rods are of the same standard composition previously used. The contacts are metal caps shrunk on the ends; the resistances are permanent in value and the inductance is reduced to a minimum. The rods are glazed to prevent absorption of moisture and surface arcing.

Figs. 2,368 to 2,370.—Multi-gap or low equivalent lightning arrester. It consists of: 1, a number of gap units in series; 2, a number of gap units in shunt with a resistance; and 3, a series resistance. All resistances are wire wound and the series resistance is non-inductive. The shunt resistance and gap units are mounted on marble. When a discharge occurs, the series gaps are broken down, and if the discharge be heavy enough, it will meet opposition in the shunt resistance and pass over the shunted gaps, through the series resistance to the ground. The arc which tends to follow the discharge is then withdrawn from the shunted gaps by the shunt resistance, and aided by both resistances is suppressed by the series gaps. The pressure of discharge is determined by the number of series gaps as sufficient number is used to withstand the normal voltage and yet give a proper factor of safety for the severest service.

Fig. 2,371.—Westinghouse three pole or four pole arrester in weather proof wooden case which protects the arrester units from rain and snow when they are installed in exposed locations, as on poles or buildings.

Figs. 2,372 and 2,373.—Westinghouse multi-gap lightning arrester and views showing parts. In construction, a series of gaps, between non-arcing metal cylinders arranged in a row, is connected between line and ground in series with a composition stick resistor having a resistance of something between 80 and 120 ohms. In operation, if an excessive pressure be developed on a line, electric discharge arcs form between the metal cylinders, and the charge of electricity flows to ground, relieving the excessive stress. The resistance of the stick resistor limits the flow so that an excessive power current cannot pass through the arrester. The tendency for a destructive power arc to follow the discharge arc is thus counteracted. The composition resistors and the gap cylinders are mounted in pairs on a porcelain base, and complete units are arranged within weather proof wooden boxes as indicated. For two pole arresters, one unit is mounted on the back of the box. For three pole and four pole arresters, two units are used; one is secured on each side of the box.

Figs. 2,374 to 2,376.—Connections for Westinghouse multi-gap (type G) arresters. These arresters may be installed outside on poles or buildings, or indoors on station walls. The weather proof wooden case (as shown in fig. 2,371) protects the arrester units from rain and snow when installed in exposed places. Fig. 2,374 shows single phase installation, fig. 2,375, two phase installation, and fig. 2,376, three phase installation. On a two pole circuit one line wire is connected to the top of each of the composition resistors of each arrester unit, as shown in fig. 2,374, and the ground wire is connected to the middle point of the gap series. On four pole circuits, fig. 2,375, the same scheme of connections is used, but two arrester units are necessary and the connections of both are the same. On three pole circuits, two arrester units are used, with the same connections as for four pole circuits, except that there are but three line connections instead of four as in fig. 2,376.

Fig. 2,377.—Horn gap arrester, diagram showing arrester and connections between line and ground. The horn type arrester was invented by Oelschlaeger for the Allgemeine Electricitaets Gesellschaft, and like the Thomson arc circuit arrester, its operation is based on the fact that a short circuit once started at the base, the heat generated by the arc will cause it to travel upward until it becomes so attenuated that it is ruptured. On circuits of high voltage this rupture sometimes takes a second or two, but seems to act with little disturbance of the line. Sometimes a water resistance is used, a choke coil being inserted in the circuit in series. In one installation for a 40,000 volt line, the horns were made of No. 0,000 copper wire with gap knees 2¼ to 3 or 3¼ inches. The capacity of the water resistance receptacle was 15 gallons. Users differ as to whether the water should contain salt. The choke coil can be made of about 18 turns of iron wire wound on a 6 inch cylinder.

Fig. 2,378.—General Electric horn gap with charging resistance for cable system. Arresters for cable systems differ from arresters for overhead circuits only in the construction of the horn gaps. The necessity for this difference is due to the fact that a cable system has a very much higher electrostatic capacity and much less inductance than an overhead system. In consequence, the currents which flow into the arrester during charging are somewhat higher. It is desirable to avoid these heavier currents, especially during the time of breaking the arc at the horn gap. This is accomplished by using a special horn gap and resistance. This consists of an auxiliary horn mounted above and insulated from the regular horn in such a manner as to intercept the arc if it rise on the regular horns. Enough resistance is connected in series with this auxiliary horn so that the current flow and arc across this gap are always limited to a moderate value. Such a device has several advantages. Since the mechanism is so arranged that the charging is always done through the auxiliary horn the current rush is limited during the charging and thus troubles from carelessness or ignorance are avoided. It also gives a nearer uniform charging current. In the use of this auxiliary horn gap and resistance there are three successive stages, as follows: 1, light discharges will pass across the smaller gaps to the auxiliary horn and through the series resistance to the cells; 2, if the discharge be heavy, the resistance offers sufficient impedance to cause the spark to pass to the main horn. This is accomplished with only a slight increase in pressure because the gap is already ionized. If the cells be in normal condition, the spark at the gap is immediately extinguished, without any flow of line current; 3, if the cells be in poor form, the line current may follow the discharge across the main gap and the arc will rise to the safety horn and be extinguished through a resistance. For mixed overhead and cable systems the choice of arrester will be a matter of judgment. If there be a comparatively short length of cable, the usual practice for overhead systems may be adopted. For direct connection to busbars, arresters with charging resistance should be used.

Figs. 2,379 and 2,380.—Diagram showing connections of horn type lightning arresters on series circuits.

Fig. 2,381.—General Electric horn type arrester, mounted for 15 light series arc circuit. The horn type arrester consists of a horn gap with series resistance between each line and ground. The resistances and horn gaps are mounted on porcelain bases and the latter on insulating wooden supports. The supports have asbestos barriers (except for lowest voltages), and backs to eliminate liability of damage from the arc which forms in the horn gap at the time of the discharge. The spark gaps are adjusted to give a low spark pressure relative to the voltage of the circuit. The number and ohmic value of the resistance rods used in the various arresters depend upon the voltage and current of the circuit.

Fig. 2,382.—General Electric horn arrester for pole installation. Quite frequently series circuits are run underground in cables for some distance from the generating station. In order to protect the cables it is advisable to place horn arresters at the points where the cable joins the overhead wires. The resistance units are mounted in the wooden box. This design is used to economize space, since if the horn gaps be placed in the box the latter would have to be made very large to accommodate the asbestos barriers and backs. In installing this type of arrester it is advisable to place it as near as possible to the top of the pole so that the arc may rise unobstructed and thus avoid the likelihood of live wires coming in contact with the horns which, during the operation of the series current, are alive.

Figs. 2,383 and 2,384.—Elevation and plan of General Electric horn gaps and operating stand for high voltage arresters.

Fig. 2,385.—Cross section of General Electric aluminum (electrolytic) lightning arrester.

FIG. 2,386 to 2,390.—Parts of General Electric 15,000 volt aluminum lightning arrester, not including horn gaps, etc.

Fig. 2,391.—Volt ampere characteristic curve of a General Electric aluminum (electrolytic) cell on alternating current. The permanent critical voltage is between 335 and 360 volts. With alternating current, the cell acts as a fairly good condenser, and there is not only the leakage through the film, but also a capacity current flowing into the cell. The phase of this current, then, is nearly 90 degrees ahead of the pressure and represents a very low energy factor.

Fig. 2,392.—Westinghouse electrolytic lightning arrester, for three phase ungrounded neutral service, 25,000 maximum voltage. These arresters are designed for the protection of alternating current circuits from all kinds of static disturbances. They have been standardized for installation on three phase circuits of voltages of 2,200 to 110,000. They cannot be used for voltages of less than 13,500. For voltages below this the horn gaps cannot, with safety, be set close enough together, out of doors, to take advantage of the freedom of discharge of the electrolytic element. If the horn gaps be set too close together they may be short circuited by rain. A shelter should be built for arresters of 13,500 volts and below for their protection when installed outside.

Figs. 2,393 and 2,394.—Aluminum trays for Westinghouse electrolytic lightning arresters.