What are Dry Contact and Wet Contact and their differences?

What is Dry Contact?

  • Dry contacts are known as voltage-free contacts or potential-free contacts.
  • These contacts don’t provide any power or voltage source from the switch.
  • But power or voltage source is provided from an external source.
  • The contact current for these dry contact is less than one milli-ampere.
  • The power supply must be provided because these connectors are disrobed and don’t have energy.
  • They are known as dry contact because the relay required for dry contact can be energized or de-energized for change in input.
  • This energizing and de-energizing condition helps us to know whether the input is switched on or off.
  • The power supply required to energize the relay is connected using separate cables or we can apply a local power supply located near the relay.
  • The flow of current through dry contact is possible only during closed or shorted. But no current flows through dry contacts during the open conditions.
  • Dry contact functions like an ordinary switch to open or close the circuit.
  • In other words, these dry contacts are called passive contacts because energy is not applied to contacts.
What are Dry Contact

How to connect a dry contact relay?

  • Dry contact relay installation and connection must be done professionally by a skilled electrician or technician considering the space may be far away from the central hub or the ubiquity of different items that may interpose with the signal.

What are the Examples of Dry Contact?

Examples of dry contact are

Dry contacts are commonly used in all types of relays, including solid-state relays. One key advantage of using dry contacts in a relay is their flexibility in handling a wide range of output voltage levels. For instance, a relay with a 24 V coil can control loads at different voltage levels using a dry contact. This is not feasible with wet contacts, as they use the same voltage level to control the load.

Another example of a dry contact application is in compressor contactors. A compressor contactor typically has a separate 24 V coil, and the power supply does not directly power the compressor contactor. Dry contacts are favored in such setups because they provide complete electrical isolation between devices, enhancing safety and reliability.

Dry contacts are also utilized in Programmable Logic Controller (PLC) modules. In such modules, a 24 V input voltage is provided to the PLC’s input side, while the output side uses a separate control voltage of 5 V from the processor. This separation allows for better control and isolation.

Important Guidelines for installing a Relay:

The relay must not be installed in the following places.

  • Do not install in the open air or in an outdoor location
  • Do not install with electric breaker panels inside metallic junctions.

What is Wet Contact?

  • A contact that can be activated or energized by the same power source applied by the control circuit to change the contact is known as wet contact.
  • Wet contact is also known as the primary contact.
  • Wet contacts are commonly observed in solid-state switching devices such as sensors.
  • When power is provided to wet contact, the simple switching action will release the same supply to turn off the load device without consuming any extra power supply.
  • This wet contact works with the same potential as the potential given to energize the sensor.
  • The wet contact is turned on only when the sensor detects an object, and the output of the contact is turned off if the sensor doesn’t sense any object.

What are examples of Wet Contact?

Examples of wet contact

A common example of a wet contact is found in thermostats, where the same power supply is used for both the control circuit and the contact circuit, directly powering the load.

Wet contacts are widely used in solid-state switching devices, such as proximity sensors, temperature sensors, and airflow sensors. In these devices, the same voltage level is provided to both the sensor and the load, eliminating the need for extra common power wires. Additionally, both the sensor and the load consume minimal power, making wet contacts ideal.

In a Ground Fault Interrupter (GFI) circuit, the same wire is used to provide power to the internal circuit and the output terminals. This setup employs wet contacts to simplify the circuit design by using a single power source for both control and load.

Click here for Understanding Wet Contacts in PLC Wiring

Differentiate between Dry Contact & Wet Contact:

The below table shows the differences between dry contact and wet contact:

Dry ContactWet Contact
Here the power is supplied by another source.Here the power is supplied by the same power source. The control circuit is used to energize the contact.
It operates as an ordinary single pole On and Off switch.It operates like a controlled switch.
It can be considered secondary contact of the relay circuit.It can be considered the primary contact of the relay circuit.
Dry contacts provide isolation between two devices.Wet contacts will not provide any isolation, but gives the same potential to control the device.
Dry contacts are called Passive contacts.Wet contacts are known as Active contacts or Hot contacts
These are available in relay circuits because these relays will not provide any intrinsic power supply to the contact.Wet contacts are utilized in the control circuits where the power is intrinsic to energize the contacts. Such as Control Panels, and Temperature Sensors.
Dry contacts will not use mercury-wetted contacts.  Wet contacts use mercury-wetted contacts.
Dry contacts provide complete isolation between two devices.Wet contact makes troubleshooting much easier because of the simplicity of wiring and the same voltage level.
Differences between dry contact and wet contact

What are the advantages and disadvantages of Dry Contact?

Advantages of Dry Contact:

Disadvantages of Dry Contact:

  • Requires external source to power up.
  • Not applicable for High voltage

What are the advantages and disadvantages of Wet Contact?

Advantages of Wet Contact:

  • Cabling is simple
  • Requires a minimum number of cables.
  • Consumes less power.   
  • Cabling comes with the same potential.

Disadvantages of Wet Contact:

  • Wet contacts can’t provide any isolation between two circuits.
  • Both input circuits and output circuits are dependent on each other.
  • Difficult to isolate.
  • Used to integrate and control HVAC (Heating, Ventilation, and Air Conditioning) systems, lighting controls, and security systems. Dry contacts allow interfacing between different systems without worrying about voltage compatibility.
  • Dry contacts are used in alarm panels to monitor the status of doors, windows, motion detectors, smoke detectors, and other sensors. The “normally open” (NO) or “normally closed” (NC) contacts will trigger alarms based on the circuit condition.
  • Employed in Programmable Logic Controllers (PLCs) to read the status of devices (e.g., limit switches, push buttons) and to control other devices such as motors and solenoids.
  • Dry contacts are often used to indicate the status of equipment (like servers or communication racks) to remote monitoring systems.
  • Used in process plants (such as chemical, petrochemical, and water treatment plants) to signal discrete states such as the opening/closing of a valve or the status of equipment like pumps and compressors.
  • Dry contacts are widely used in elevator control systems to indicate door status, floor levels, and emergency stop functions.
  • Wet contacts are used to provide real-time status updates by directly sending voltage signals. This is common in electric substations for breaker status indication and in power distribution networks for controlling and monitoring operations.
  • Wet contacts are used to activate alarm horns, strobes, and other notification devices that require a specific voltage. They also interface with fire alarm control panels to trigger emergency protocols.
  • Wet contacts can be used to control electromagnetic locks, door strikes, and other devices requiring direct current (DC) voltage to operate.
  • Wet contacts are often used in motor starters to provide control power for indicating lights, control relays, and other auxiliary devices.
  • In intelligent lighting systems, wet contacts are used to provide a voltage output to control the lights directly, such as dimming or turning them on/off.
  • Wet contacts are used in battery monitoring systems to provide output signals corresponding to the charge level, temperature, or fault conditions.
  • Wet contacts are used in uninterruptible power supply (UPS) systems to provide alarms and status information (e.g., low battery, loss of power).

Dry and wet contacts are used in various electrical devices and systems across industries. Here are some important standards and regulations to consider:

  • IEC 60947 Series: Covers safety and performance of low-voltage switchgear, relays, and contactors.
  • IEC 60664: Focuses on insulation requirements for equipment using dry and wet contacts.
  • Article 430: Guidelines for motors and motor controllers, including those with dry/wet contacts.
  • Article 409: Covers safety for industrial control panels.
  • UL 508A: Safety for industrial control panels using dry and wet contacts.
  • UL 60947-4-1: Focuses on contactors and motor starters.
  • EN 60947 Series: Similar to IEC 60947, covers low-voltage controlgear.
  • EN 60204-1: Safety of machinery electrical equipment.
  • 29 CFR 1910.303: General electrical safety requirements.

These standards ensure safety, reliability, and compliance for devices using dry and wet contacts in various industries.

FeatureDry ContactWet Contact
Power SourceExternalInternal (same as control)
IsolationYesNo
Typical UsePLCs, alarms, BMSSensors, fire panels, door locks
Wiring ComplexityHighLow
Voltage FlexibilityHighLow
Best ForInterfacing & safetySimple DC control circuits

Here are the steps to test a dry contact with a multimeter:

  1. Set the multimeter to either continuity mode or resistance (ohms).
  2. To be safe, unplug the circuit.
  3. Put the probes of the multimeter on the two ends of the dry contact.
  4. To alter the state of the contact, turn on the relay or switch.
  5. The multimeter will beep or show a low resistance value (usually close to 0 ohms) if the contact is closed.
  6. The meter will show infinite resistance or “OL” if the contact is open.
  7. This method helps check that the dry contact is working as a switch.

When you need to keep control and load circuits apart from one other, dry contacts are the ideal choice. These are often used in PLCs, alarm systems, and building management systems where the load and control signal need to be kept apart.

If the same power source is utilized for both control and load, wet contacts work well. They are utilized in simple automation circuits, thermostats, sensors, and fire alarm panels.

Use dry contacts when you need to keep the voltage separate, be flexible, or be safe. Use wet contacts when you need simpler wiring and a smaller circuit design.

To connect a dry contact relay:

  • Use the right voltage (such 24 V DC) to connect the coil terminals to the control circuit.
  • Based on how you want the switch to work, connect the load circuit to the NO (normally open) or NC (normally closed) terminals.
  • Make sure that the load’s power source is independent from the coil’s power supply.
  • To cut down on noise or interference, use control wires that are properly insulated and shielded.
  • Mark all wires so that they can be found during maintenance or troubleshooting.

Make sure the system is off when you wire it up and follow all regular electrical safety rules.

  • Connecting the relay without an outside power source for the contact circuit.
  • Not comprehending NO and NC configurations, which causes logic to act in a wrong way.
  • Putting moist and dry contacts in the same circuit might produce electrical problems.
  • Not giving circuits enough electrical isolation from each other.
  • Running dry contact wires near high-voltage lines, which could cause interference.

Avoiding these mistakes ensures the dry contact circuit performs reliably in all applications.

Before purchasing or installing a dry contact relay, check the following:

  • Check the following things before you buy or install a dry contact relay:
  • The voltage rating of the coil should match the voltage of your control circuit (for example, 12 V, 24 V DC, or AC).
  • The contact rating is right for the load, such 5 A at 250 V AC.
  • Relay has the right contact configuration, whether it’s NO, NC, or changeover.
  • Relay keeps the input and output separate enough.
  • It can be mounted on a DIN rail, a socket, or a panel.
  • You can get UL, CE, or IEC certifications to show that you are following the rules and are safe.
  • Datasheets from the manufacturer confirm the electrical and mechanical requirements.

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