Ques. What is an arc.
Ans. The conduction of electricity by vapor of the electrode.
Distribution of Static Stress.—The cylinders of the multi-gap arrester act like plates of condensers in series. This condenser function is the essential feature of its operation.
When a static stress is applied to a series of cylinders between line and ground, the stress is immediately carried from end to end.
If the top cylinder be positive it will attract a negative charge on the face of the adjacent cylinder and repel an equal positive charge to the opposite face and so on down the entire row.
The second cylinder has a definite capacity relative to the third cylinder and also to the ground; consequently the charge induced on the third cylinder will be less than on the second cylinder, due to the fact that only part of the positive charge on the second cylinder induces negative electricity on the third, while the rest of the charge induces negative electricity to the ground. Each successive cylinder, counting from the top of the arrester, will have a slightly smaller charge of electricity than the preceding one.
Fig. 2,351.—General Electric 2,200 volt multi-gap arrester for station installation. It consists of fourteen ⅝" 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.
Sparking at the Gaps.—The quantity of electricity induced on the second cylinder is greater than on any lower cylinder and its gap has a greater pressure strain across it as shown in fig. 2,357. When the voltage across the first gap is sufficient to spark, the second cylinder is charged to line voltage and the second gap receives the static strain and breaks down. The successive action is similar to overturning a row of ten-pins by pushing the first pin against the second. This phenomenon explains why a given length of air gap concentrated in one gap requires more voltage to spark across it than the same total length made up of a row of multi-gaps.