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Below shows two simple ladder logic examples written for a PLC:

 switches

As far as I understand, the -| |- and -|/|- above are the symbols we write for the PLC not the actual input switches.

But we also need to wire actual switches to inputs as field inputs. Is there any convention or good practice to wire NO switch in case (a) and NC switch for case (b)?

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The normally-open and normally-closed contact instructions you see in the logic do not correspond in any way to what type of switch is wired to the input. When writing the logic, you choose the instruction based on what you need to make the logic work correctly. In fact, you often have the same input used multiple times in the logic, using a normally-open instruction in some places and normally-closed in others.

The choice of what type of physical hardware switch to use is based on what the fail condition would be if a wire breaks or is disconnected. For example, a machine start push button should be a normally-open switch. If the connecting wire is disconnected, you will be unable to start the machine, which is safe. If you had used a normally-closed switch there and the wire broke, the machine would start by itself (unsafe).

Likewise, a machine stop push button should be a normally-closed switch. A wire break in that case would stop the machine (safe). If you incorrectly use a normally-open switch here, then a wire break would leave you unable to stop the machine (unsafe).

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An input is to be wired with an NO contact, should it be required to turn on an output, and with an NC contact, should it be required to turn off an output.

Then, in the ladder diagram, the input with the NO contact is to be represented thus -||- and the input with the NC contact also thus -||-. This is a fail-safe method.

Should the wire to the NO contact be disconnected the output would not turn on. The fault would be recognised during trouble-shooting and corrected.

Should the wire to the NC contact be disconnected the output would again not turn on, the fault would be recognised during trouble-shooting and corrected. Had an NO contact been wired then the output would have turned on and not turned off when required to.

To sum up, inputs are to be wired and programmed such that a wire disconnection would not result in an output turning on or an output not turning off.

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