Figs. 2,368 to 2,370.—Multi-gap or low equivalent lightning arrester. It consists of: 1, a number of gap units in series; 2, a number of gap units in shunt with a resistance; and 3, a series resistance. All resistances are wire wound and the series resistance is non-inductive. The shunt resistance and gap units are mounted on marble. When a discharge occurs, the series gaps are broken down, and if the discharge be heavy enough, it will meet opposition in the shunt resistance and pass over the shunted gaps, through the series resistance to the ground. The arc which tends to follow the discharge is then withdrawn from the shunted gaps by the shunt resistance, and aided by both resistances is suppressed by the series gaps. The pressure of discharge is determined by the number of series gaps as sufficient number is used to withstand the normal voltage and yet give a proper factor of safety for the severest service.
At this instant the medium resistance, M, aids the rectifying quality of the gaps, GM, by shunting out the low frequency current of the alternator.
On account of this shunting effect the current dies out sooner in the gaps, GM, than it otherwise would.
In the same manner, but to a less degree, the high resistance, H, draws the line current from the gaps, GH.
This current now being limited by the high resistance, the arc is easily extinguished at the end of the first one-half cycle of the alternator wave.
Ques. What is the difference between arrester for grounded Y and non-grounded neutral systems?
Ans. The connections are shown in figs. 2,365 and 2,366. The difference in design lies in the use of a fourth arrester leg between the multiplex connection and ground or ungrounded system.
Ques. Why is the fourth leg introduced?