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.
The single gap while adequate for telegraph line protection, was found insufficient for electric light and power circuits, because since the current in such circuits is considerable and usually at high pressure it would follow the lightning discharge across the gap. Thus the problem arose to devise means for short circuiting the line current resulting in various modifications of gap arrester.
Multi-gap Arresters.—The essential elements of an arrester of this type are a number of cylinders spaced with a small air gap between them and placed between the line to be protected and the ground, or between line and line.
In operation, the multi-gap arrester discharges at a much lower voltage than would a single gap having a length equal to the sum of the small gaps. In explaining the action of multi-gaps, there are three things to consider:
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.
1. The transmission of the static stress along the line of the cylinders; 2. The sparking at the gaps; 3. The action and duration of the current which follows the spark, and the extinguishment of the arc.
Ques. What is a spark?
Ans. The conduction of electricity by air.