The C3 Inputs can be used to sense conditions where equipment should be on / off, or to generate alarms to be sent via email / SMS. Typically, these triggers are based on a stateful input.
Simple voltage free contacts from pressure or flow switches can be wired into a C3 input.
Pressure switch electrical connection:
Flow switch electrical connection:
High Voltage Isolation Relay
Relays can be used to isolate higher voltages from C3 inputs, or example when detecting the absence of mains power