How to Select the Right Relay for Your Application

Relays are among the most versatile components in building automation; yet specifying the right one for a given application can feel surprisingly complex. With choices in coil voltage, contact configuration, form factor, and load type, the “one-size-fits-all” approach rarely applies. Choosing the correct relay not only ensures reliable switching and long service life, it also makes installation smoother and reduces maintenance headaches down the road.

Understanding how different relay designs match specific use cases—from resistive lighting circuits to inductive motor loads, and from dry contact signaling to BACnet® integrations—helps contractors, designers, and technicians make smarter decisions in the field. This guide breaks down key application types and the relays that work best with them so you can confidently select solutions that deliver dependable performance every time.

At Functional Devices, relays are our bread and butter. We offer various options for building automation and lighting controls, including our handy “Relay in a Box®.” While diving into relay types, we can’t outline all the nuances between relays in one post. Call us to ask your questions and determine which makes sense for your specific needs!

Types of Relays and Their Applications

There are many ways to categorize relays. When we talk about types in this post, we mean the three main classifications based on operation principles. Each design type suits different needs and applications, and understanding each type’s characteristics can help you make the right choice.

When choosing a relay, let its characteristics guide your decision, but remember to consider other factors, such as load requirements, contact configuration, voltage, switching speed and frequency, and environmental conditions.

Here are the three most commonly used relays based on operation principles.

Electromechanical Relays

The grandfather of all relays, electromechanical relays (EMRs) are electrically operated switches that use electromagnets to mechanically switch contacts. Robust and capable of handling a wide variety of voltages and currents, EMRs are most often used in high-power situations. An EMR’s ability to operate high-power circuits with low-power signals is where it shines. 

Advantages of an EMR include:

• Simple design
• Affordable
• Versatile
• Good thermal management (doesn’t produce a lot of heat)
• Reliable for general use

Appropriate applications for EMRs could include:

Of course, an EMR won’t work for every application. Here are some of the characteristics that might limit their use:

• Mechanical wear over time that requires regular maintenance
• Possible mechanical failures and contact erosion
• Larger and heavier design
• Slower switching than other types
• Audible clicking noises when switching
• Require constant power to maintain coil activation
• Can’t perform complex logic or communication tasks

With their cost-effectiveness and ability to manage high loads, EMRs make sense in many basic applications. 

Solid-State Relays

A newer technology than EMRs, solid-state relays (SSRs) have no moving parts and rely on semiconductors for completing electronic switching. They perform well in conditions that require precision, quick switching, and quiet operation.

Advantages of an SSR include: 

• Almost silent operation
• Fast switching speeds
• Longevity
• Durable and reliable with no mechanical parts
• Low power consumption
• Small and compact size
• Efficiency
• Precision
• Low maintenance (possible cost savings in the long run)

Appropriate applications for SSRs could include:

Here are some characteristics of SSRs that might limit their use:

• More expensive upfront than other types
• Generate heat when switching large loads, which can require additional cooling systems or heat sinks
• Limited current capacity
• May require a minimum load to operate (not suitable for small output signals)
• Only a single contact

As you can see, there are many places where EMRs and SSRs can overlap in their applications. Choosing one over the other might depend on desired switching speeds, operational noise, budget, load requirements, etc. As the newer, quieter, and faster technology, SSRs are slowly replacing EMRs in many industries.

Reed Relays

The last type of relay we want to cover is technically an EMR, relying on an electromagnet to mechanically switch circuits on or off; however, a reed relay’s unique, compact construction deserves some focus. The mechanism at play is the reed switch, consisting of two metal ferromagnetic blades hermetically enclosed in a glass tube that touch/switch when enough of a magnetic field is applied.

Advantages of a reed relay include:

• Small and lightweight design
• Fast switching speeds
• Low power consumption
• Longevity
• Protected from the environment by hermetical seal
• Low contact resistance
• High insulation resistance
• Resistant to shock and vibration
• Highly reliable for low-power switching

Appropriate applications for reed relays could include:

With their small size and ability to detect extremely low voltages, reed relays aren’t as interchangeable as EMRs and SSRs are with each other. Reed relays were developed in the 1930s and, despite being an older technology still find a home in many industries, especially where reliable, quick switching of low-power devices is required. 

A Relay Cheat Sheet

General Purpose Relays

Versatile relays used for switching moderate loads in HVAC, lighting, and control circuits across a wide range of building automation applications.

Power Relays

Designed to handle higher current loads such as motors, compressors, and larger lighting circuits where greater switching capacity is required.

Low Inrush Coil Relays

Engineered to reduce control-side current draw at energization, making them ideal for panels with limited or sensitive control power.

Time Delay Relays

Incorporate adjustable timing functions that delay energizing or de-energizing contacts to sequence equipment and prevent short cycling.

Latching Relays

Maintain their contact position after the control signal is removed, reducing continuous coil power consumption.

Solid-State Relays (SSRs)

Use electronic switching instead of mechanical contacts, offering silent operation and long life in applications with frequent cycling.

Enclosed Relays (RIB® Relays)

Factory-packaged relays in protective enclosures that simplify installation and provide ready-to-wire solutions for BAS environments.

Pilot Relays

Compact relays used to isolate and amplify control signals when interfacing low-voltage controllers with higher-voltage equipment.

Make Relay Selection a Design Advantage

Choosing the right relay is more than a box to check on a parts list. The correct coil voltage, contact rating, and configuration can prevent nuisance failures, simplify installation, and extend the life of a system. When relays are properly matched to their applications—whether switching lighting, driving HVAC equipment, or isolating control signals—the entire automation system performs more reliably.

Functional Devices designs RIB® relays with real-world applications in mind, offering a wide range of coil voltages, contact types, and enclosure options to fit diverse building automation needs. Instead of forcing a universal solution into every scenario, selecting a relay tailored to the load and environment helps reduce callbacks and protect margins.

If you’re evaluating relay options for a new project or retrofit, the team at Functional Devices can help you identify the right model for your application. Contact us to discuss your requirements and ensure your next installation is built on a dependable switching foundation.

About Functional Devices, Inc.

Functional Devices, Inc., located in the United States of America, has been designing and manufacturing quality electronic devices since 1969. Our mission is to enhance lives in buildings and beyond. We do so by designing and manufacturing reliable, high-quality products for the building automation industry.  Our suite of product offerings include RIB relays, current sensors, power controls, power supplies, transformers, lighting controls, and more.

We test 100% of our products, which leads to less than 1 out of every 16,000 products experiencing a failure in the field.

Simply put, we provide users of our various products confidence and peace of mind in every box.