The load is now a boiler. (See Figure 4, page 20.) This wiring diagram shows 120 V coming from L1 of a circuit breaker, through a switch, powering a boiler control and returning through L2, back to ...

This online article, “How to read PLC wiring diagrams for industrial automation” by Fathima Sadina, covers hardware, such as breaker switches, busbars, circuit breakers, switched-mode power supplies ...