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.
Horn Gap Arresters.—A horn gap arrester consists essentially of two horn shaped terminals forming an air gap of variable length, one horn being connected to the line to be protected and the other to the ground usually through series resistance as shown in fig. 2,378.
Ques. How does the horn gap arrester operate?
Ans. The arc due to the line current which follows a discharge, rises between the diverging horn and becoming more and more attenuated is finally extinguished.
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.
Ques. What is the objection to the horn gap on alternating current circuits?