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.

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

Relays:

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.

Compressor Contactors:

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.

PLC Modules:

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

Thermostats:

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.

Solid-State Switches:

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.

Ground Fault Interrupter (GFI) Circuits:

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:

What are the advantages and disadvantages of Dry Contact?

Advantages of Dry Contact:

• Operates in any orientation without draining.
• These are well applicable to portable equipment.
• Less Susceptible to Leakage
• It gives complete isolation.

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.

Common Applications of Dry Contacts

Building Automation Systems:

• 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.

Alarm and Security Systems:

• 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.

Industrial Control Panels:

• 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.

Telecommunications:

• Dry contacts are often used to indicate the status of equipment (like servers or communication racks) to remote monitoring systems.

Process Automation:

• 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.

Elevator Systems:

• Dry contacts are widely used in elevator control systems to indicate door status, floor levels, and emergency stop functions.

Common Applications of Wet Contacts

SCADA (Supervisory Control and Data Acquisition) Systems:

• 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.

Fire Alarm Systems:

• 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.

Access Control Systems:

• Wet contacts can be used to control electromagnetic locks, door strikes, and other devices requiring direct current (DC) voltage to operate.

Motor Control Centers (MCCs):

• Wet contacts are often used in motor starters to provide control power for indicating lights, control relays, and other auxiliary devices.

Lighting Control Systems:

• 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.

Battery Management Systems (BMS):

• Wet contacts are used in battery monitoring systems to provide output signals corresponding to the charge level, temperature, or fault conditions.

Telecommunications and Data Centers:

• Wet contacts are used in uninterruptible power supply (UPS) systems to provide alarms and status information (e.g., low battery, loss of power).

Regulatory and Compliance Standards for Dry and Wet Contacts

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

1. IEC Standards:

• 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.

2. NEC (NFPA 70, U.S.):

• Article 430: Guidelines for motors and motor controllers, including those with dry/wet contacts.
• Article 409: Covers safety for industrial control panels.

3. UL Standards (U.S.):

• UL 508A: Safety for industrial control panels using dry and wet contacts.
• UL 60947-4-1: Focuses on contactors and motor starters.

4. European EN Standards:

• EN 60947 Series: Similar to IEC 60947, covers low-voltage controlgear.
• EN 60204-1: Safety of machinery electrical equipment.

5. OSHA Regulations (U.S.):

• 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.