Most of what we deal with here at BTS is 24vdc Controls. Operating inductive loads on our PLC Outputs such as relays can shorten the life of the PLC if you do not use a freewheel diode to mitigate the arc caused by the collapsing field of the magnet in the relay.
When you open the PLC “switch”, there is an ARC as induction creates a reverse voltage. The ARC does not want to go towards the 0vdc so it heads for the PLC Output. Often it can be larger than the supplied voltage.
Put in a Freewheeling Diode and rather than an arc at the switch, the excess voltage travels around the relay circuit.
If you are using a PLC to operate the Relay or other inductive load, the excess voltages can damage the Output Transistor or relay in the PLC. In a PLC, a spike in voltage from the Inductive Load such as a relay, transformer, indicator lamp, etc. can damage the PLC Output.