- Dry Contact Definition: A dry contact is defined as a switch that controls electrical circuits without supplying the power itself, relying on an external source.
- Functionality: Dry contacts operate by opening and closing circuits, providing essential isolation and safety in electrical systems.
- Dry vs. Wet Contacts: Dry contacts are used for isolation, while wet contacts use the same power source for control and power supply, lacking isolation features.
- Applications of Dry Contacts: Typically found in low-voltage and safety-critical systems like alarms and industrial controls, demonstrating their versatility.
- Real-World Examples: From solid-state relays to compressor contactors, dry contacts enable varied voltage control, proving crucial in diverse electrical applications.
What is a Dry Contact?
A dry contact (also known as a volt free contact or potential-free contact) is a switch whose power/voltage does not come from the switching device. The switched circuit supplies its own source. The contacts are called passive: they do not generate energy. When the contact is closed, current from that external source still flows through the metal.
Electrically it is an ordinary switch. current flows when the contact is closed and stops when it is open.
On a relay, a dry contact is often an isolated output. It may switch a small alarm circuit or a full load. Dry versus wet is about whether the contact is sourced by the same supply as the coil or control electronics, not about auxiliary versus main. Isolation is the usual reason to use a dry contact. The figure is below.
Most relay outputs are dry in that sense: the coil has its own supply, and the contacts switch a separate circuit.
They appear in extra-low-voltage AC distribution circuits and in alarm loops (fire, burglar and power-system alarms). They are not limited to below 50 V. Contactor and RIB outputs are often rated hundreds of volts. Always stay inside the contact rating.
Dry Contact Vs. Wet Contact
The stored table below mixes two meanings of dry and wet. In controls, dry means volt-free and wet means the contact is sourced by the same supply as the control circuit. A second, older meaning is a dry (unwetted) relay contact versus a mercury-wetted contact. The last table rows use that second meaning and also put a dry-contact sentence in the wet column. The table was not edited.
| Dry Contact | Wet Contact |
| Dry contact is one in which the power is always being supplied by another source. | Wet contact is one in which the power is being supplied by the same power source that the control circuit is used to switch the contact. |
| Dry contacts function as simple ON/OFF switches, similar to a typical light switch, providing straightforward control. | It operates like a controlled switch. |
| It can be referred to as a secondary set of contacts of the relay circuit. | It can be referred to as the primary set of contacts. |
| Dry Contacts is used to provide isolation between devices. | Wet contacts provide the same power for controlling the device. Hence it does not provide isolation between devices. |
| Dry contacts are also known as “Passive” contacts. | Wet contacts are known as “Active” or “Hot” contacts. |
| It is commonly found in the relay circuit because the relay does not supply any intrinsic power to the contact. | It is used in the control circuit where the power is intrinsic to the device to switch the contacts. Example: Control Panel, temperature sensors, air-flow sensor, etc.. |
| Dry contacts mean a relay that does not use mercury-wetted contacts. | Wet contacts mean a relay that uses mercury-wetted contacts. |
| The main advantage of the dry contacts is that it provides complete isolation between the devices. | Dry contacts provide complete isolation between devices, enhancing safety and compatibility. Conversely, wet contacts simplify troubleshooting and wiring by using consistent voltage levels. |
Summary: A dry contact only opens or closes. The load supply is a separate source, so the switched circuit is isolated from the control supply. A wet contact is sourced by the same supply as the control circuit, so that isolation is not there.
Dry Contact Relay
A dry-contact relay still needs coil voltage. What it does not do is put that coil voltage onto the contacts. The contacts can switch another circuit, but only within their voltage and current rating. They cannot switch “any” voltage.
Functional Devices RIB “dry contact input” models are closed by an external switch, thermostat, relay or solid-state contact. Closing that input lets the RIB’s own Class 2 circuit energise the coil.
Coil power can come on a separate pair, or from a local mains feed. The output contacts are isolated from both the coil supply and the dry input. They can switch another load only inside the published contact rating, not any load.
The RIB02BDC below is an enclosed 20 A SPDT with a Class 2 dry-contact input and 208–277 Vac coil power. The input is the dry contact. The output is an isolated load contact.
The blower figure is a 24 V coil driving isolated contacts that switch the motor supply. When 24 V is applied to the coil, those contacts close and the motor circuit can run. The motor voltage is not the coil voltage.
Dry and Wet Contact Examples
Worked examples:
Dry Contact Examples
Electromechanical and some solid-state relays offer isolated (dry) outputs. A 24 V coil can then switch a different load voltage, up to the contact rating. A wet output is tied to the same supply as the control side, so the load voltage is that supply.
A compressor contactor is the same idea: a 24 V coil, and power contacts that switch the compressor line. Those power contacts are dry relative to the 24 V control transformer.
A PLC relay-output card is another dry output: the field circuit has its own supply. The 24 V field input and the processor’s 5 V logic are separate rails, not the dry-contact pair. The dry contacts are the isolated output terminals.
Wet Contact Examples
A typical 24 V HVAC thermostat is wet in the controls sense: it switches the same transformer that powers the thermostat. Some thermostats also offer a dry pair. Read the terminal diagram.
Three-wire PNP/NPN proximity, temperature and airflow sensors often switch the same DC rail that powers the sensor. That sourced output is a wet contact in the controls sense. Two-wire loop sensors are a different arrangement.
A GFCI (GFI) interrupts the same line that feeds the load. That is a series trip of the sourced circuit, not a dry-contact output. Treat it as a wet/sourced path, not as a volt-free relay contact.
