Dry Contact vs Wet Contact: What Is the Difference?

A dry contact is a pair of switch contacts with no voltage of its own. It might be the output relay of a fire alarm panel, a generator’s “running” contact, a float switch or a door reed switch. It simply opens or closes, and whatever is connected to it has to provide the voltage and current that flow when it closes. Building services engineers call it a volt-free or potential-free contact.

A wet contact carries a voltage supplied by the device it belongs to. When it closes, it delivers that voltage, often 24 V DC or 24 V AC, to whatever is wired to it. A thermostat output that switches 24 V AC to a valve, or a sensor whose output switches its own 24 V supply, is wet.

The terms describe the signal source, not the switch inside. The same relay can be a dry contact on one panel and a wet one on another, depending on whether the manufacturer wired its common terminal to an internal supply.

The words come from wetting current, the minimum current a contact needs to stay reliable. Metal contacts build up a thin film of oxide or contamination, and a small current at a reasonable voltage breaks through it each time they close. Too little current, and a closed contact can read as open.

So an input designed for dry contacts provides a wetting voltage and current from its own supply, typically a few milliamps at 12 V or 24 V, and switches made for low-level signals often use gold-plated contacts that resist the film. Utility substations, with long cable runs and harsh conditions, commonly wet status contacts from the station battery at 48 V or 125 V DC for the same reason. Confusingly, relay makers use “dry circuit” for very low-level switching with no arcing, which is a different idea.

Relay and switch contacts are described by a standard set of forms, and the form decides what happens when power is lost.

On a datasheet, the dry or wet question usually appears as the type of digital output. The table compares the three common kinds.

Transistor outputs are a form of wet signal, because the sensor switches its own supply. Whether they sink or source current is a separate question, covered below.

A digital input on a programmable logic controller or industrial computer is built for one or both. A dry contact input has an internal, usually isolated, wetting supply, so the field device only needs to close a contact. A wet contact input expects a voltage, often 12 V to 24 V DC, and reads it through an optocoupler. Our guide to isolated digital inputs sets out the input side in more detail.

Many isolated inputs take both. Several of the industrial IoT gateways we list have isolated digital inputs that accept dry contact, sinking or sourcing signals. On the output side, some of our PC/104 digital I/O modules carry banks of SPDT Form C relays for switching lamps, solenoids and alarms. Even network equipment uses dry contacts: several of our unmanaged industrial switches have an alarm relay contact, rated 1 A at 24 V DC, that tells a PLC or alarm panel when a power feed is lost.

When wiring either type, check the contact rating against the load, fit a diode or snubber across inductive loads such as relay coils and solenoids, and never connect a wet signal to an input that supplies its own wetting voltage, or one supply can feed back into the other. Writing “dry” or “wet” against every signal on the I/O list avoids most of these mistakes.

We wire and test the digital I/O on the industrial computers and gateways we supply as standard, matched to whether each field device provides a dry or a wet contact.