Shunt/Bypass UL 924 Emergency Lighting Relays

Emergency lighting is a vital part of building safety, providing reliable illumination during power outages to help occupants navigate safely. Ensuring compliance with UL 924 standards is critical when selecting controls for emergency lighting circuits.

This blog will explore how these shunt and bypass UL 924 relays work, highlight key Functional Devices models, and explain practical applications to help engineers and facility managers implement reliable emergency lighting solutions efficiently.

Here are Functional Devices, we’re dedicated to producing safe, reliable, and efficient components for your emergency lighting needs. Explore our full range of UL 924 products and contact us with any questions about application or installation.

What Are Shunt/Bypass Relays & How Do They Work?

Shunt and bypass relays are specialized devices used to ensure emergency lighting circuits remain illuminated during power interruptions. Their primary function is to maintain or restore power to designated lights regardless of the state of local switches.

Here are more specifics and some of their differences:

• Shunt relays override a local wall switch, allowing emergency lights to turn ON automatically during a power failure even if the switch is in the OFF position. This ensures continuous illumination in critical areas without requiring manual intervention.
• Bypass relays temporarily bypass the local switch entirely during an emergency event, keeping the lighting circuit energized until normal operation resumes. This method is particularly useful in spaces where multiple switches control the same circuit.

Compact relays are well-suited for smaller circuits or areas with space constraints, providing reliable shunt or bypass functionality. Larger relays with higher contact ratings are ideal for circuits with greater load requirements. These devices are designed to simplify installation while ensuring dependable emergency lighting control.

Understanding UL 924 Compliance

UL 924 is the nationally recognized standard that governs emergency lighting and power equipment, ensuring reliability and safety during power outages. Key requirements include:

• Automatic activation of emergency lights when primary power fails
• Reliable operation under specified environmental and electrical conditions
• Verification that relays and circuits meet defined load capacities and switching performance

Functional Devices’ UL 924-listed relays meet or exceed these standards. Each relay is tested for consistent activation, contact integrity, and durability, giving engineers and facility managers confidence that their emergency lighting systems will perform when needed. By combining UL 924 compliance with flexible shunt and bypass functionality, these relays deliver safety, reliability, and ease of integration. 

Now let’s dive into some specific models and their shunt or bypass applications.

Key Functional Devices’ Shunt/Bypass UL 924 Emergency Lighting Relays

Functional Devices offers a range of UL 924-listed relays designed specifically for emergency lighting applications. Our Automatic Load Control Relays (ALCR), such as the ESRN, are widely used for comprehensive emergency control. For applications that require a simpler approach — shunting or bypassing a local electrical switch — our compact ESR2401B and ESR2402B relays provide 20A contact ratings and UL 924 compliance.

Shunt & Bypass Application

Here’s how our relays shunt or bypass during emergency situations:

When Normal Power is present, the ESR Bypass/Shunt relay coil is activated (Contacts N/O), and the emergency panel is fed from Normal Power. The lighting load can be switched on/off using an individual wall switch. When normal power drops out, the ESR coil is deactivated and N/C contact falls closed.

If there is already a local wall switch or other means of controlling power to an Emergency light, then the ESR2401B could be used to force the light on if Normal power drops out. The ESR2401B is for a feed of 120Vac Normal power. If there is a feed of 277Vac Normal power, use the ESR2402B model.

Now, what about applications where there is 0-10Vdc dimming and the light needs to be forced to full brightness if normal power drops? The ESR2401B or ESR2402B may still be used and wiring it up would look like the diagram below.

Dimming Override Low Voltage Application

Here’s how our relays would force light to full brightness in emergency scenarios:

When Normal Power is present, the ESR coil is activated and the N/O contacts are closed, allowing for the 0-10 Vdc to control the dimming of the load. When Normal Power is lost, the N/O contacts open, breaking the 0-10 Vdc dimming control, bringing the light load to full brightness.

We also offer double-pole, double-throw (DPDT) versions in both 10A contact rating and 20A contact rating.

Functional Devices’ UL 924 shunt/bypass relays provide a reliable, code-compliant solution for emergency lighting control, ensuring circuits remain operational during power interruptions. Their compact design and straightforward functionality make them an ideal choice for both new installations and retrofit projects.

Emergency Lighting Solutions from Functional Devices

When controlling emergency lighting in UL 924 applications, a full Automatic Load Control Relay (ALCR) isn’t always necessary. Functional Devices’ shunt/bypass relays, like the compact ESR2401B and ESR2402B, provide 20A contact ratings and UL 924 compliance, offering a simpler, cost-effective solution that saves time and effort while maintaining reliability. Just make sure a shunt/bypass relay is appropriate for your specific setup.

Have technical questions? Contact our Tech Support team! We’re happy to help you determine if the ESR2401B, ESR2402B, or other UL 924 relays are the right choice for your emergency lighting project.

About Functional Devices, Inc.

Confidence and Peace of Mind in Every Box

At Functional Devices, Inc., we have been designing and manufacturing high-quality electronic components in the USA since 1969. Our mission is simple: to enhance lives in buildings and beyond by delivering reliable, innovative solutions for the building automation industry. Our product lineup includes RIB® relays, current sensors, power controls, power supplies, transformers, lighting controls, and more—each designed to meet the highest industry standards.

We test 100% of our assembled products, ensuring an exceptionally low failure rate—less than 1 in 16,000 in the field. This commitment to excellence means you get products that perform reliably, reducing downtime and increasing efficiency. With every Functional Devices product, you receive more than just a component—you gain confidence and peace of mind knowing your systems are built on trusted, field-proven technology.