 If there is another doorbell button (typically near a back door), it is connected between the transformer and the third terminal.The transformer primary winding, being energized continuously, does consume a small amount (about 1 to 2 W) of standby power constantly; systems with lighted pushbutton switches may consume a similar amount of power per switch.The tradeoff is that the wiring to the button carries only safe, low voltage isolated from earth ground.