Dry vs. Wet Contacts: The Passive-Active Divide in Industrial Control

Dry contacts and wet contacts remain two of the most confused basics in industrial automation, PLC wiring, and building control. The whole difference sits in one question: does the contact carry its own power?

A dry contact is a passive switch. It supplies no voltage or current, only opens or closes like a mechanical button, limit switch, or relay auxiliary contact. An external source must complete the loop. Its strengths are electrical isolation, no polarity, cross-voltage flexibility, strong noise immunity, and good performance over long cable runs. The trade-off is clear: it needs external power and slightly more wiring.

A wet contact is active. It already carries a defined level—typically DC24V, or NPN/PNP transistor output—so closing it sends “powered” or “unpowered” straight to a PLC input or small load. Proximity switches, temperature transmitters, and PLC transistor outputs are typical sources. It saves external supply and simplifies cables, but demands correct polarity, matched voltage, weaker isolation, and care against drop-out on long lines.

Field rule of thumb: for state feedback, isolation, and distance, pick dry; for direct sensor hookup, less wiring, and short spans, pick wet. Wrong choice means either no signal or, worse, two supplies colliding and burning the module.