6 Best Interface Relays For PLC Connectivity To Use

Connecting a PLC to field devices is much like selecting the right fastener for a specific deck substrate; choose the wrong one, and the entire assembly fails under pressure. Interface relays serve as the critical bridge between low-voltage controller logic and high-voltage power components. Selecting the correct relay ensures the control system handles switching loads without burning out prematurely. This guide identifies the top six interface relays that hold up under the demands of industrial and commercial control panels.

Disclosure: As an Amazon Associate, this site earns from qualifying purchases. Thank you!

Phoenix Contact PLC-RSC: The Industry Workhorse

The PLC-RSC series functions like the standardized architectural shingle of the control world. It is the go-to component for most standard installations because it provides a reliable, modular base that integrates seamlessly with most PLC platforms.

These relays are built to withstand heavy-duty operation in challenging environments. The screw-clamp connections are robust, providing the same peace of mind that a heavy-duty cap nail offers when securing underlayment against high-wind uplift.

Consider this the primary choice for standard panel builds where accessibility and long-term serviceability remain the top priorities. When a failure occurs in five years, the ability to swap the relay module without disturbing the panel wiring is a massive time-saver.

Weidmüller RCIKIT: Best for General Purpose Use

Weidmüller is known for precision, and the RCIKIT series brings that engineering focus to the relay market. These units offer a compact footprint without sacrificing the ease of terminal access needed during a panel troubleshooting session.

The RCIKIT shines in general-purpose applications where space is at a moderate premium but electrical reliability cannot be compromised. They handle standard load switching with high consistency, similar to using high-grade synthetic underlayment that provides consistent protection across varying roof pitches.

Use these units if the goal is to standardize a panel build across a project with mixed voltage requirements. The clear labeling and intuitive status indicators make them an excellent choice for maintenance teams who need to diagnose faults quickly.

Finder 39 Series: Top Pick for High-Density Panels

When panel space is tight, the Finder 39 series acts like a low-profile ridge vent, providing necessary functionality without adding bulk. Its slim, modular design is engineered specifically for control panels where every square inch of DIN rail space costs money.

These relays feature an extremely thin profile, which allows for significantly higher component density within the same enclosure. This is essential for retrofitting older, smaller panels where moving the enclosure is not an option.

Despite the slim design, they maintain impressive thermal dissipation capabilities. Expect reliable performance as long as the panel is properly ventilated to prevent heat buildup, much like ensuring adequate intake air for a balanced attic system.

Allen-Bradley 700-HL: For Allen-Bradley Systems

Sticking with the same manufacturer for the relay and the PLC is often the safest bet for high-end industrial systems. The Allen-Bradley 700-HL series is built to match the longevity of the controllers themselves, ensuring that the interface never becomes the weakest link in the chain.

The primary advantage here is the diagnostic feedback and total system integration. These relays often feature built-in surge protection, which acts as a literal lightning rod for sensitive electronics, preventing voltage spikes from traveling back to the CPU.

If the project involves a large-scale Allen-Bradley installation, do not experiment with third-party budget alternatives. Use the 700-HL to ensure the entire system communicates with the reliability expected of a professional-grade industrial setup.

Omron G2RV-SL: Best Slim Relay for Fast Installs

The Omron G2RV-SL series is designed for the contractor who needs to populate a panel quickly without sacrificing build quality. These relays feature a push-in terminal design that eliminates the need to tighten individual screws, drastically reducing total labor time.

The push-in technology provides a secure connection that resists vibration—an essential feature for installations near heavy machinery or in industrial facilities with constant foot traffic. It is the electrical equivalent of using a pneumatic nailer for consistent fastener depth across a large roof deck.

Choose this unit when labor hours are the biggest constraint on the project budget. The time saved during the assembly stage usually pays for the slightly higher unit cost of the relay itself.

Schneider Zelio RXG: Most Versatile Plug-in Relay

The Schneider Zelio RXG is the Swiss Army knife of the relay world. It offers a broad range of coil voltages and contact configurations, making it a highly adaptable solution for bespoke control panels or older, non-standard systems.

The lockable test button is a standout feature, allowing for manual operation during the commissioning phase of a job. It allows for testing the circuit logic without needing to trigger the PLC, saving hours of back-and-forth communication between the panel and the field.

This flexibility makes it ideal for contractors who maintain a wide variety of systems. Keep a few of these in the service truck for emergency repairs, as their versatility covers most common control voltage needs.

Solid-State vs. Electromechanical: Which to Pick?

The debate between solid-state and electromechanical relays mirrors the choice between metal panels and shingles. Electromechanical relays are cost-effective and handle high current surges well, but they possess moving parts that eventually wear out.

Solid-state relays use semiconductors, meaning they offer silent, high-speed switching with no moving parts to break. However, they are sensitive to temperature and voltage spikes, requiring heat sinking and careful protection against transients.

Use electromechanical relays for standard logic circuits where high-speed switching is not required. Reserve solid-state units for high-cycle applications where the physical contact wear of a traditional relay would cause premature failure.

Understanding Coil Voltage Contacts and Current

Every relay purchase must be grounded in the electrical specs of the design. A relay with the wrong coil voltage will fail to trigger, and one with a lower contact current rating will weld shut under a heavy inductive load.

Always check the peak current, not just the continuous current rating. Just as a roof must be rated for peak uplift rather than average wind speeds, the relay must handle the inrush current of a motor or solenoid startup.

Verify the voltage type (AC vs. DC) at the design phase. Installing a DC coil relay in an AC circuit results in immediate failure, causing humming, overheating, and potential damage to the PLC output card.

Tips for Clean Wiring in a Crowded Control Panel

Panel organization determines how quickly a technician can troubleshoot a system during a breakdown. Use wire ferrules for all connections to ensure a high-integrity contact that does not fray or loosen over time.

Maintain clear wire separation between control signals and power lines. Following this practice prevents electromagnetic interference, similar to how proper flashing prevents water infiltration at a chimney base.

Label every wire at both ends with heat-shrink markers. A panel with unmarked wires is a liability that turns a simple ten-minute fix into a three-hour guessing game.

When and Why Do Interface Relays Fail on the Job?

Failure is rarely a product defect; it is almost always an environmental or application error. The most common cause is exceeding the current rating, which creates an arc that pits the contacts and eventually causes them to fuse together.

Excessive heat is the secondary killer. If a control panel is not properly ventilated or is mounted in an area with high ambient temperatures, the insulation inside the relay will degrade, leading to a short circuit.

Voltage transients from inductive loads, such as large motor starters or heavy-duty solenoids, can also destroy relay electronics. Always install a flyback diode or an RC snubber circuit if the relay is switching an inductive load to protect the life of the relay.

Choosing the right interface relay requires balancing cost, space, and the specific electrical demands of the system. While many products look identical on the shelf, the differences in durability and ease of installation directly impact the long-term reliability of the control panel. By carefully matching the relay to the load and maintaining a clean, organized installation, the life of the control system is extended significantly.