It is common practice to use IO modules and interface relays to isolate built-in PLC input and output circuits. The reason for this is PLC IO modules are relatively expensive and their replacement is not as easy as replacing a single external interface relay. A good example of replaceable relays are solid state interface relays that are designed for din rail mounting with a range of input and output voltages and currents. An alternative solution would be a miniature switching relay that provides full isolation between the input and output and uses a relay or optical coupling to achieve isolation. A particular problem arises when the switched current is high or inductive, and in these instances, a relay coupling with a high current rating is required. Another method is to use a PLC interface module, which is a terminal designed to accept small plug-in relays or optocouplers. The advantage of this solution is that replacement relays are easy to source. Suppliers of IO modules and interface relays include Finder, Omron, Phoenix Contact, Wago and Lumberg Automation.
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