Concept of Interlocking in PLC

Design a ladder logic for Go-Down wiring for 3-rooms using plc programming and explain the Concept of Interlocking in PLC programs.

I0.0 , I0.1 , I0.2 : Toggle switches of corresponding rooms 1, 2, 3.
Q0.0 , Q0.1 , Q0.2 : Output coils (Bulbs) in room 1, 2 ,3.

Interlocking in PLC

Seeing the ladder diagram we can infer that as soon as the person enters into room 1 and switches ON the toggle switch 1 (I0.0), the bulb of that room glows i.e, coil Q0.0 in rung 1 gets energized.

Now, the person moves into room 2 without switching OFF the toggle switch of room 1. As soon as he/she presses the toggle switch 2 (I0.1), the bulb of that room (Q0.1) starts glowing as its coil gets energized, while the bulb of room 1 itself goes OFF as its circuit is break by the interlock contact Q0.1 in rung 1 which is (N-C) type.

Similarly, when the goes into room 3 and presses switch 3 (I0.2) , the bulb of room 3 starts glowing and that of room 2 switches OFF keeping the bulb of room 1 already OFF. This is what we cover the going down from room 1 to room 2 to room 3.

Now as soon as the person wants to come back he/she again presses the toggle switch 3 (I0.2) which in turn breaks the circuit in rung 3 and the bulb (Q0.2) goes OFF. As soon as the bulb of room 3 goes OFF , the bulb of room 2 starts glowing again because here no longer the interlock contact Q0.0 remains open.

Similarly, pressing the toggle switch of room 2 (I0.1) switches OFF the bulb Q0.1 and in turn makes interlock contact Q0.l (N-C) from (N-O) and bulb of room 1 again start glowing by making the rung 1 logic TRUE. After pressing the toggle switch 1 (I0.0) again the bulb of room 1 i.e, Q0.0 goes OFF.

-END-

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