When a controls cabinet is designed and constructed ladder diagrams are used to document the wiring. A basic PLC wiring diagram is shown in below Figure. In this example the system would be supplied with AC power (120Vac or 220Vac) on the left and right rails.
The lines of these diagrams are numbered, and these numbers are typically used to number wires when building the electrical system. The switch before line 010 is a master disconnect for the power to the entire system.
A fuse is used after the disconnect to limit the maximum current drawn by the system. Line 020 of the diagram is used to control power to the outputs of the system.
The stop button is normally closed, while the start button is normally open. The branch, and output of the rung are CR1, which is a master control relay. The PLC receives power on line 30 of the diagram.
The inputs to the PLC are all AC, and are shown on lines 040 to 070. Notice that Input I:0/0 is a set of contacts on the MCR CR1. The three other inputs are a normally open push button (line 050), a limit switch (060) and a normally closed push button (070).
After line 080 the MCR CR1 can apply power to the outputs. These power the relay outputs of the PLC to control a red indicator light (040), a green indicator light (050), a solenoid (060), and another relay (080).
The relay on line 080 switches a relay that turn on another device drill station.
In the wiring diagram the choice of a normally close stop button and a normally open start button are intentional.
Consider line 020 in the wiring diagram. If the stop button is pushed it will open the switch, and power will not be able to flow to the control relay and output power will shut off.
If the stop button is damaged, say by a wire falling off, the power will also be lost and the system will shut down - safely.
If the stop button used was normally open and this happened the system would continue to operate while the stop button was unable to shut down the power.
Now consider the start button. If the button was damaged, say a wire was disconnected, it would be unable to start the system, thus leaving the system unstarted and safe.
In summary, all buttons that stop a system should be normally closed, while all buttons that start a system should be normally open.