What is a Dry Contact?
A dry contact (also known as a volt free contact or potential-free contact) is defined as a contact in which power/voltage is not directly provided from the switch but is instead always being supplied by another source. Dry contacts are known as passive contacts, as no energy is applied to the contacts.
Dry contact simply operates like an ordinary switch that opens or closes the circuit. When the contacts are closed the current flows through the contacts and when the contacts are opened no current flows through the contacts.
It can be referred to as the secondary sets of contacts of a relay circuit which does not make or break the primary current being controlled by the relay. Hence dry contacts are used to provide complete isolation. The dry contact is shown in the below figure.
Dry contacts are commonly found in the relay circuit. As in a relay circuit, there is no external power directly applied to the contacts of the relay, the power is always being supplied by another circuit.
Dry contacts are primarily used in low-voltage (less than 50 V) AC distribution circuits. It can also be used to monitor alarms such as fire alarms, burglar alarms and alarms used power systems.
Dry Contact Vs. Wet Contact
The differences between dry contact and wet contact are discussed in the table below.
|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.
|It can operate as an ordinary single-pole ON/OFF switch.
|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.
|The main advantage of the wet contact is that it makes troubleshooting much easier because of the simplicity of wiring and the same voltage level.
Summary: The dry contacts open or close the circuit and provides complete isolation between the devices hence, the output power is completely isolated from the input power. Whereas, the wet contacts do not provide complete isolation hence output power is immediately supplied along with the input power whenever the switch is energized.
Dry Contact Relay
In a dry contact relay, the contacts are opened or closed without using any voltage. Hence, we can control the dry contact relay at any voltage level.
The RIB series dry contact input relay uses different dry contacts such as switches, thermostats, relays, and solid-state switches, etc. The dry contact input RIB provides the low-voltage signal to operates the relay by closing the dry contact.
The power to energize the relay can be provided by using a separate wire. The relay contacts and the dry contacts are isolated from the input power hence they can be wired to switch any load.
The RIB02BDC dry contact relay is shown in the figure below. This relay has dry contacts and it can be used in different types of power applications.
Another example of a dry contact relay that is used to control the blower motor is shown in the figure below. When 24 V applied to the relay coil the dry contact is close and it operates the blower motor.
Dry and Wet Contact Examples
Some of the examples of dry contacts and wet contacts are discussed below.
Dry Contact Examples
Dry contacts are used in all types of relay, including a solid-state relay. One of the advantages of using dry contact in a relay that the relay provides a wide variety of output voltage levels. For example, a relay with a 24 V coil, a dry contact will allow controlling the load at any voltage level. This cannot be achieved by the wet contacts because the wet contacts use the same voltage level to control the load.
Another example is the dry contact in a compressor contactor. The compressor contactor has a separate 24 V coil and the power supply does not provide power directly to the compressor contactor. Hence, dry contacts are mostly used because it provides complete isolation between devices.
Dry contacts are used in PLC modules in which input voltage 24 V is provided to the input of PLC modules and output is provided with separate control voltage 5 V from the processor.
Wet Contact Examples
A thermostat is the most common example of a wet contact. A thermostat has the same power supply to power the control and its contact, it means that the power supply directly provides power to the control circuit and its contact.
Wet contacts are commonly found in solid-state switching’s, such as proximity sensors, temperature sensors, and air-flow sensors, in which the same voltage levels are provided to the sensor and the load, no extra common power wires are required, also the power consumption of the sensor and the load is very small.
In a Ground Fault Interrupter (GFI) circuit the same wire is used to provide the power to the internal circuit and the output terminals. Hence wet contacts are used in the GFI circuit.