If you want to play with line voltage you need to be a lot more careful compared to working with 5 or 12 volts. Viktor has built a nice compact system for easily interfacing line voltage powered items to his DIY projects. Instead of trying to breadboard the line voltages or make a temporary rig to switch line power he gutted an old power brick and mounted a large Solid State Relay inside. Now the AC power enters the housing using the power supplies original AC plug. The color codes in North America are different. Here the neutral line (white) and the ground (green) would be carried directly from input to output, the hot (black) would be interrupted using the SSR. While using this type of system there is no danger of having the line voltage feed back into your low voltage circuit since the control side SSR is optically isolated from the high voltage side.
