As discussed in earlier posts, the two way switch has 3 terminals, there should be a minimum of three wires running to each of the switch.
Remember also that the terminals are indicated by the signs, L1, L2 and L3. It might differ among manufacturers. Some like to refer the terminals as L1, and C which stands for “common”. First we have to understand how the wires are installed at both locations.
Now we already know that a normal switch has a live wire (the one from the mains) and a switch wire (the one going to the lights) When the live wire is connected to the switch wire, the circuit is closed thereby lighting up the light. The two way switch also acts in the same way except it can be switched on at one end and switched off at the other. This is achieved by installing 2 extra wires between the switches. We shall call this 2 wires the common wire.
At the first switch, there will be a live wire, but instead of running a switch wire to the lights in the normal way, we run 2 common wires from the first switch to the second switch. It is only from the second switch that we run the switch wire up the light. It is just like splitting on switch into two. On one end we have the live wire and on the other end we have the switch wire. Both ends of the two way switch are then connected by 2 common wires.
To understand how it works, imagine the 3 terminals. L1 is the terminal where the Live wire is going in. That means electricity is always residing on L1. Now imagine L1 has a pendulum swinging below it. One end of the pendulum is locked to L2 in normal circumstances so electrical current passes through one of the common wire which is connected to L3 of the second switch at the other end, which is always at the “normally open” state. (remember the L2 is at the “normally closed” state with L1) What we have now is an “open” circuit.
When someone presses the switch to the ON position at the first switch, the pendulum will swing over to L3 which was “open” by default. This closes the circuit between L1 and L3. The current is then passed through the common wire which is correctly connected to L2 at the switch on the other end, (which is in a “normally closed” state) thereby allowing the current through and therefore lighting up the light. When he reaches the other end, he presses the switch again to switch off the lights. The terminals of that switch between L1 and L2 are then “opened” again thereby switching off the light.