drycontact

A dry contact, also known as a potential-free contact or dry-switch contact, is a type of electrical contact whose terminals do not carry a supply voltage or signal within the device that contains the contact. The device only provides a switch closure or opening; the external circuitry supplies the voltage and current to be controlled. The term contrasts with wet contacts, where the device provides power or a signal to the circuit contacted.

Dry contacts are typically realized through mechanical switches or relays with isolated contacts, and sometimes through reed switches. They can be normally open (NO) or normally closed (NC), and can be used in various configurations to form signaling logic. They are often implemented as part of relays or switches and do not themselves source or sink power.

Applications include alarm and security systems, building automation, PLC inputs, process control, and safety interlocks. Dry contacts enable galvanic isolation between control equipment and the monitored circuit, reducing the risk of interference or unwanted voltage transfer between systems and allowing safe interconnection of disparate electrical domains.

Key considerations include the external circuit’s voltage and current limits, contact ratings, and the effects of contact wear and arcing. Dry contacts should be chosen with attention to contact life, bounce, and environmental conditions. They are commonly specified by their NO/NC ratings and may be used in coil-free or actuator-free configurations in relays or switches, emphasizing their role as signal closures rather than power paths.