Fig. 992.—Two way lighting circuit permitting control from two points. This is the usual arrangement for hall when it is desired that the lights may be turned on or off from either floor. The circuit contains two two way switches connected by "travellers." From the diagram it is seen that the light may be controlled from either switch. It is a bad arrangement to have travellers and return wire located near each other, as it is possible by this method to connect two individual circuits together and possibly overload one of the two feed wires of each circuit. However, should each feed wire run to a fuse direct, without any other lamps than those contained in the three way circuit being connected, it is not objectionable and becomes a convenient method in many cases.
Fig. 993.—Four way lighting circuit, permitting control from three points. This arrangement consists of a four way switch connected between two two way switches as shown. In making the connections it should be noted that the travellers connecting one side of the four way switch to the two way switches should be crossed. On the opposite side of the four way switch, the connections are direct. For the various positions of the switches, the corresponding circuits through them are as follows: ABDGHJ—ABDEIJ—ACFGHJ—ACFEIJ.
Ques. How may a group of lights be controlled from three points?
Ans. By the use of a 4 way switch and two 2 way switches connected as shown in [fig. 993].
Ques. Before laying out the wiring system for a building, what should be done?
Ans. It is necessary to ascertain whether power will be supplied from the central station, or whether a private plant is to be installed.