Ans. [Fig. 461] illustrates a knife switch of the double-pole, single-throw type. It consists of the following parts: base, hinges, blades, contact jaws, insulating cross bar, and handle, as shown.
Ques. How should knife switches be installed?
Ans. They should be placed so that gravity tends to open them.
Otherwise if the hinges become loose, the weight of the blades and handle would tend to close the switch, thus closing the circuit and possibly resulting in considerable damage.
Fig. 461.—A single-throw, two pole knife switch. As usually constructed it is made of hard-drawn copper with cast terminal lugs and fibre cross bar.
Ques. How should switches be proportioned?
Ans. The minimum area of the contact surfaces should not be less than .01 square inch per ampere, and in those used on arc lighting or other high voltage circuits where the current is usually small, the area of the contact surfaces are usually from .02 to .05 inch per ampere. Since dirt or oxidation would prevent good contact under a simple pressure between the contact surfaces, the mechanism of a switch provides a sliding contact.
In the general design of switches, all parts which carry current are given a cross sectional area of at least one square inch per 1,000 amperes if they be made of copper, and about three times as much if made of brass, as the conductivity of the latter is only one-third that of the former. Furthermore, the current should never be permitted to pass through springs, as the heat generated will destroy their elasticity.
