Ques. Describe the construction of an end cell switch.
Ans. This is shown in [fig. 1,187]. The switch contact arm is made in two parts, A and B, which are insulated from each other as shown, and connected with each other through the protective resistance R. The end cell contacts are so spaced that when the main current carrying part A of the switch arm is squarely on one end cell contact such as X, the part B, does not touch any other contact such as Y, but when the switch arm is advanced for cutting into circuit another end cell, the part B, reaches the contact Y before the part A, leaves the contact X, thus keeping the battery circuit closed, while the resistance R, limits the current in the short circuited cell at the instant the switch arm passes from one end cell contact to the next.
Fig. 1,187.—Diagram of end cell switch. This form of switch controls several cells at one end of a storage battery and is used for regulating the voltage. The requirement of an end cell switch is that in switching from one end cell contact to another, the discharging circuit must not be opened, neither must the moving arm touch one contact before leaving the one adjacent, since the joining of two contacts will short circuit the cells connected thereto. To accomplish this, the spacings of the two arms and contacts are such that when the main arm A is squarely on an end cell contact, the advance or auxiliary arm B touches no other contact, but in passing from one point to the next, the advance arm reaches the contact toward which it is moving before the main arm leaves its contact. The resistance X, between the two points prevents short circuiting, and the current to the main circuit is never broken.
Ques. How should the conductors joining the end cells to the end cell switch contacts be proportioned?
Ans. They must have the same sectional area as the conductors of the main circuit.
The reason for this is that when any end cell is in use, the conductor connecting it to the switch becomes a part of the main circuit. An allowance of 1,000 amperes per sq. in., when the battery is discharging at the two-hour rate, is considered good practice.
Ques. Describe some of the features of end cell switch construction.
Ans. Those of small capacity are made circular; the larger sizes are made horizontal in form, and both types may be either operated by hand or motor driven.