What you'll learn
Connect three common volt-free field devices to PLC inputs: a maintained selector switch, a mechanical limit switch and a tank float switch. The devices do not create voltage; each simply passes the panel’s 24 V control supply to an input when its selected contact closes.
Lab time: ~12 minutes.
Lesson briefing
Dry-Contact Field Inputs
Connect three common volt-free field devices to PLC inputs: a maintained selector switch, a mechanical limit switch and a tank float switch. The devices do not create voltage; each simply passes the panel’s 24 V control supply to an input when its selected contact closes.
Why it is wired this way
Use the contact state that fails safely for the duty. A normally closed level or permissive loop reveals a broken cable as a lost signal, while a normally open command is appropriate where a cable break must not create a start request.
Commissioning standard
Before energising, prove protective earth and continuity, verify the drawing terminal-by-terminal, then energise through the protective device and measure the expected supply, signal and return values. Never use this training panel as a substitute for the machine manufacturer’s instructions, risk assessment, isolation procedure or applicable electrical standard.
Hints
Power the PLC first: +24 V to +24V and 0 V to 0V.
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This lesson uses 8 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.
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