A dry contact is an electrically isolated switch or relay contact that does not provide its own voltage to the connected circuit. The external circuit supplies the voltage and current, while the dry contact simply opens or closes the circuit. A dry contact is also called a potential-free contact or volt-free contact and is widely used for interfacing PLCs, control systems, alarms, HVAC equipment, and industrial automation devices.
What Are Potential-Free Contacts and Dry Contacts?
A potential free contact is another term for the same concept. In potential free contact or dry contact, the contact itself does not provide a voltage source. It means that it is electrically isolated from the control circuit. When the potential-free contact closes, it completes a circuit using the voltage supplied by an external source.
A dry contact, upon closing, allows electrical current to flow through it. In other words, a dry contact describes a pair of electrical terminals that do not provide their own electrical voltage source but are used to control the flow of current from an external source.
Before going into the details of dry contact, let us briefly understand wet contact.
Dry Contact – The contact itself does not provide a voltage source. Any voltage measured across terminals X and Y depends on the external circuit connected to the contact.
Wet Contact – The output itself provides a voltage to the connected circuit. Depending on the contact state, the output may switch between 0 V and the specified supply voltage.
Generally, the controller or PLC has a wet contact with the same voltage level as that PLC supply. The external loads that are controlled through PLC are not driven by the PLC wet contact, and dry contacts are generated with the use of the relays. The relay contacts act as dry contacts. The dry contacts supply voltage is not taken from PLC; it is taken from the other source, and thus, the PLC output gets electrically isolated from the actual load.
Characteristics of Dry Contacts
- No Voltage or Current. Dry contacts do not carry any voltage or current, unlike typical electrical connections in a power outlet or a battery. They are electrically isolated from the circuit.
- Dry contacts are helpful in control functions, and they are primarily used to control the operation of other devices, such as switches (limit switches, proximity switches), relays, or electronic circuits. When the dry contact is closed (connected), the current flows, activating or deactivating the controlled device.
- Dry contacts have many common uses. Dry contacts are commonly found in applications where low-voltage or signal-level control is needed, such as in home automation systems, industrial control systems, and alarm systems. They are often used for interfacing between different systems or devices.
- There are so many examples related to home automation. A classic example of a dry contact is a light switch in home automation systems. When we close a switch, it is nothing but closing a dry contact, allowing electricity to flow to the light bulb and turning it on. Similarly, the doorbell button is another example of a dry contact; pressing it closes the contact, sending a signal to ring the doorbell.
- Dry contacts are flexible because they interface between devices with different voltage levels or electrical characteristics. This makes them suitable for various applications where electrical isolation is crucial.
- Dry contacts are known for their reliability and durability. This is because there are no voltages or current fluctuations across the contacts when they open or close. This minimizes wear and tear, making them suitable for applications where durability is essential.
Dry Contact vs, Wet Contact: Comparision Table
The table below highlights difference between dry vs wet contact.
| Feature | Dry Contact | Wet Contact |
| Voltage source | External circuit | Contact/output provides voltage |
| Own voltage at contact | No | Yes |
| Other name | Potential-free contact | Powered contact |
| Electrical isolation | Usually provides isolation | May not provide isolation |
| Typical device | Relay contact | PLC transistor/relay output |
| Main function | Opens or closes an external circuit | Supplies a voltage signal |
| Example | Relay NO/NC contact | 24 V DC PLC output |
Examples of Dry Contact Applications
- Industrial automation: In industrial settings, dry contacts are commonly used in control panels to manage the operation of various machines and processes. For example, they can start or stop conveyor belts, pumps, or motors.
- HVAC Systems: In heating, ventilation, and air conditioning systems, a thermostat with dry contacts can signal the HVAC system to turn on or off.
- Security Systems: Used to trigger an alarm in the monitoring station when the door opens.
- Fire Alarm Systems
- Elevator controls
Advantage of Dry Contact
The dry contact isolates the two circuits, and the problem in one part of the circuit does not affect the other parts. If the problem is in the dry contact circuit, it does not affect the wet contact circuit and vice versa.
Where Are Dry Contacts Used?
Dry contacts are widely used for control, signaling, monitoring, and equipment interfacing. They are particularly useful when two circuits need to communicate while maintaining electrical isolation.
Common applications include:
- PLC and industrial automation systems
- Motor control panels
- Alarm and security systems
- Fire alarm systems
- HVAC controls
- Elevator control systems
- Generator and switchgear panels
- Building management systems
- Pump and compressor control
- Remote status indication
- Interlocking circuits
What Is a Dry Contact Relay?
A dry contact relay is a relay whose output contacts are electrically isolated from the relay coil and do not supply voltage to the connected circuit. The NO or NC contacts act as an electrically isolated switch, allowing an external voltage source to control another circuit.
What Does Dry Contact Switch Mean?
A dry contact switch is a switch that does not supply its own voltage to the connected circuit. It simply opens or closes an externally powered circuit. A relay contact, push button, limit switch, or similar isolated contact can function as a dry contact.
Conclusion
A dry contact is an electrically isolated contact that opens or closes an external circuit. It does not provide its own voltage source. Instead, the external circuit supplies the required voltage and current through the contact.
Therefore, dry contacts are widely used in relays, PLCs, industrial automation, HVAC, alarm, and fire alarm systems. In addition, they provide electrical isolation between the control circuit and the external circuit, making them useful for many control and monitoring applications.
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