the American Institute of Electrical Engineers, Schenectady, N. Y., May 17, 1912. Copyright 1912. By A.I.E.E. STUDIES OF PROTECTION AND PROTECTIVE BY E. E. F. CREIGHTON, F. R. SHAVOR AND R. P. CLARK The object of this paper is several-fold, first, to describe the effects of high frequencies on car wiring; second, to describe experiences with the application of the aluminum arresters to an electrical railway where lightning is especially severe; third, to describe additional devices applied to the d-c. aluminum arrester which give it greatly diminished deterioration with only a slight decrease in protective value; and, fourth, to describe experiments on the extinguishment of d-c. arcs and the changes that have resulted in the redesign of the old magnetic blow-out arrester. General. Some of the main points of past standard practise in the protection of railway apparatus and lines might to advantage be briefly summarized as a beginning. Cars using direct current are protected by one or two arresters of the series gap type, and a choke coil. To diminish the strains on the car arresters and apparatus, arresters are placed along the line in greater or less number, according to the type of arresters and the severity of lightning storms in the locality. These arresters are all of the series gap type. In fact, with the exception of the d-c. aluminum arrester, there is none that could be dignified by the name of " arrester," that has not a gap in series. All of these standard arresters contain more or less internal series resistance. Since, in a d-c. circuit, dynamic current must always follow the lightning spark across the gap of the arrester, another distinguishing feature is the method employed in extinguishing the arc in the arrester. In the older type the dynamic current energizes an electromag net which produces a magnetic field perpendicular to the arc and thus drives it up a porcelain arc-chute. The elongation of the arc increases its resistance, which reduces the current gradually until the arc is extinguished. This takes place when the energy of the arc is reduced to a value such that the rate of cooling makes it unstable. All this takes a measurable amount of time. In order that no serious damage shall be done to the electrodes of the gap during this time, some series resistance is necessary to limit the value of current. The series resistance is required also to furnish potential for the electromagnet. Since this resistance is in the path of the lightning, the best design of arrester is the one which reduces the resistance to the minimum value without jeopardizing the life of the arrester by the destructive action of the dynamic current. A second type of gap arrester prevents the continuance of the dynamic arc by an automatically variable series resistance which has a very high value to direct currents at 600 volts. It has a lesser resistance and impedance at higher potentials and high frequencies. As protectors, the efficiencies of both of these arresters are limited not only by the active values of resistance but also by the presence of the series gap. The detrimental effect of the gap is reduced to a minimum by the well known application of the lightning choke coil. The efficiency of the "gap-resistance" type varies according to the design, and they have usually proven sufficiently satisfactory, in all but extremely severe lightning districts, to warrant their standardization. A third type of arrester gap uses an electromagnet in parallel with a resistance, but the arc is interrupted by the mechanical movement of a plunger. This type has its application confined mostly to overhead trolley lines. It seems unnecessary to review the varied conditions of demand for protection according to the geographical location, also the variations in demand along any particular line which is more or less overshadowed by trees and houses, as these matters are common knowledge. THE RELATION OF CAR WIRING TO PROTECTION As far as the writers know, very little attention has been paid to the matter of car wiring from the standpoint of protection. In many cases the wiring is such as to jeopardize the insulation of the motors and other electrical car apparatus in spite of the application of the best lightning arresters. This objectionable condition consists in placing the trolley bus wire and the ground wire in the same cable with the controller wires of the motor. When a lightning charge comes down the trolley, a portion of it passes by induction from the trolley cable directly into the car controller cables of the motor and thence into the motor. An instant later the charge passes around the wiring to the motor, but the first stroke on the insulation of the motor comes from the bus wire in the cable. Where the charge is forced to follow the wiring it is possible to retard it for an instant by means of a choke coil, and thereby give the lightning arrester an opportunity to carry it off directly to ground. But when the charge is permitted to pass directly into the motor winding by induction the protective value of the arrester is not brought into play. Even the aluminum arrester with unusually short connections is placed at a great disadvantage, and the gap type arresters with their spark potentials of 2000 volts to 3000 volts and their dielectric spark-lags are utilized at a minimum of their effectiveness. Before going into this subject further, some isolated experiments illustrating the effect of induction will be given. EXPERIMENTS IN HIGH-FREQUENCY INDUCTION BETWEEN PARALLEL WIRES TO INDICATE THE RELATION OF CAR WIRING TO PROTECTION The possibility of a lightning stroke being induced between wires on a car and thus by-passing the protection given by a lightning arrester, prompted an experimental investigation of this subject. The available space on a car is so limited that wires are generally grouped in a cable, and this condition is the most favorable to the inducing of potentials at frequencies approximating lightning frequencies. The first tests were made to determine the values of induced potentials. Referring to Fig. 1, a large static machine with condensers consisting of four ordinary one-gallon leyden jars attached to each side of the machine was used as a source of potential for a number of these tests. The frequency of discharge was approximately 1,000,000 to 2,000,000 cycles per second, depending upon the size of the circuit and conductors used. For the first test, two rubber-covered stranded No. 6 B. & S. cables, each 6 ft. long, were laid parallel on a table. The cable OP was connected across the leyden jar terminals as shown in Fig. 1, and cable x y had a needle gap connected across its terminals. The area x y Q was maintained fairly constant for all positions of test. The length of leads to needle gap was 8 ft. (2.4 m.). The distance D between conductors was varied and the values of Q at which a discharge just fails to pass at the gap Q were observed and recorded. The machine terminal gap spacing (G gap) was maintained constant at 5 in. (12.7 cm.), as this setting gave the largest value of Q gap for any constant value of distance D. The values of induced potentials were first measured at Q for the wires unprotected by iron pipes. Subsequently the tests were repeated with either wire enclosed in a 1 in. (2.5 cm.) iron conduit pipe and finally with both wires in separate iron conduit pipes. The values obtained as noted in the following tables prove that an iron pipe around a conductor does not appreciably shield it from the effects of induction under certain conditions. In fact the induced potentials are so nearly the same for the four different conditions tested that it seemed reasonable to suspect that the area enclosed by the secondary circuit is an important factor. To check this point the following test was made. Two secondary circuits were made up, each of the No. 12 B. & S. solid cotton-covered wire. The first one was made to include the smallest possible area. This enclosed area was less than sq. in. (6.2 sq. cm.), not including the area near the needle gap, which was about 8 in. (20.3 cm.) from the primary circuit. The potential induced in this secondary circuit was not measurable on a fine needle gap. The other secondary circuit was made to include about 56 sq. |