Fig. 2,358.—Arrangement of graded resistances on multi-gap arrester.
The Cumulative or "Breaking Back" Effect.—The graded shunt resistance gives a valuable effect, where the arrester is considered as four separate arresters. This is the "cumulative" or "breaking back" action.
When a lightning strain between line and ground takes place, the pressure is carried down the high resistance H (figs. 2,365 and 2,366), to the series gaps GS, and the series gaps spark over.
Although it may require several thousand volts to spark across an air gap, it requires relatively only a few volts to maintain the arc which follows the spark. In consequence, when the gaps GS spark over, the lower end of the high resistance is reduced practically to ground pressure.
If the high resistance can carry the discharge current without giving an ohmic drop sufficient to break down the shunted gaps GH, nothing further occurs—the arc goes out.
If, on the contrary, the lightning stroke be too heavy for this, the pressure strain is thrown across the shunted gaps, GH, equal in number to the previous set. In other words, the same voltage breaks down both of the groups of gaps, GS and GH, in succession. The lightning discharge current is now limited only by the medium resistance M, and the pressure is concentrated across the gaps, GM.
If the medium resistance cannot discharge the lightning, the gap GM spark, and the discharge is limited only by the low resistance.
The low resistance should take care of most cases but with extraordinarily heavy strokes and high frequencies, the discharge can break back far enough to cut out all resistance.
In the last steps, the resistance is relatively low in proportion to the number of shunt gaps, GL, and is designed to cut out the line current immediately from the gap, GL. This "breaking back" effect is valuable in discharging lightning of low frequency.