1- Dry Contacts: What is it?

   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.

2- Dry Contact Vs. Wet Contact

The differences between dry contact and wet contact are discussed in the table below.

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.

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.

Source : Electrical 4u

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