What is the second function of circuit breaker?

Answer 1

The two functions a circuit breaker can do is protect the circuit from a high current short circuit and through its thermal trip it can protect the circuit from overload conditions. A circuit breaker will trip if too large a draw or current flow occurs across a thermal shunt inside, it can also have a ground fault circuit interrupter integrated internally in case of a difference in potential between neutral and ground resulting from voltage leakage from appliances or to protect against potential electrocutions

A double circuit breaker provides access to the 2 legs of 120V in the back plane on the breaker panel. There are now typically 4 wires away from this breaker, a bare ground or earth ground, a white wire for neutral or bonded ground, a black wire for 1 leg of 120 and a red or blue wire for the 2nd leg of 120. You can use either leg and the white wire to access 120 or use the black and red/blue to access 240, white would then be used on the 3rd plug and ground goes to the ground lug or if missing it ties with the white and goes on the 3rd leg.