Explanation: This system is commonly used for stair case wiring. It consists of two way switches (the switch operates always in one of the two possible positions) the circuit diagram is as shown in figure above. Assume that the lamp is in between ground floor and first floor with switch S1 is on ground floor and S2 is on first floor. When the position of the switches S1& S2 is as shown in figure then the lamps is ‘ON’. When a person reaches on first floor the lamp is required to be switched ‘OFF’ so the person will change the position of switch S2 such that the lamp will be switched ‘OFF’.
Application: 1. It is used for Stair case wiring 2. It is also used for Bedroom wiring 3. It used for corridors. 4. It is used for Hospital lighting