Wiring 18 — Dry-Contact Field Inputs

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.

The device in the real world

A dry contact is a plain mechanical switch that supplies no voltage of its own. Selector switches on a control desk, limit switches on a gate or slide, and float switches in a tank are all dry contacts. The PLC input card does the sensing: it sees 24 V on the input when the contact is closed and nothing when it is open. Each device has a common terminal and one or two output contacts, marked NO for normally open and NC for normally closed, so you decide which one carries the signal.

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.

Worked example

Take a day tank with a float switch. If the tank is healthy the float sits high, and you want the transfer pump to be allowed to run. Use the NC contact: at healthy level it is closed, so the input X2 sees 24 V and the program lets the pump run. If someone cuts the float cable, the input drops to 0 V, the program sees a low level, and the pump stops. A normally open contact would give the same reading for a cut cable as for an empty tank only by accident, and would hide the break. For the selector switch you want the opposite habit: a start request should be NO so that a cut cable can never create a start.

Common mistakes

• Feeding the common terminal of each device from 0 V instead of +24 V, so the input never sees a voltage.
• Taking both contacts of one device to PLC inputs when the drawing needs only one.
• Choosing NO for a safety-relevant permissive, which hides a broken cable.
• Forgetting to power the PLC itself, then suspecting every field switch.

What you can now do

You can connect volt-free switches to PLC inputs from a 24 V distribution block, and choose the NO or NC contact according to what a broken cable should do.

Commissioning standard

Operate each switch by hand in turn and watch only its own input LED change, then break the float cable on purpose in a safe test and confirm the permissive drops out.