Comment by alright2565

For building mains devices, you should start by learning about creepage, clearance, double-insulation, the role of earth connections, the requirements for isolation between low voltage and high voltage, etc. Basically a good bit of electrical engineering.

This is different from what you need to know to do house wiring. That's done by following electrical code, which is basically a distillation of the engineering above into a set of relatively easy-to-follow rules.