What Does UL924 Relay Mean?

In modern building safety, emergency lighting systems are not just a convenience—they are a critical component of life safety. These systems guide occupants to safety during power outages, fires, or other emergencies. One of the key technologies that ensure these systems work reliably is the UL924 relay.

So, what is a UL924 relay, and why is it so important for your facility’s emergency lighting compliance? This article breaks down its function, applications, and the core UL924 requirements you need to understand.

What Is a UL924 Relay?

A UL924 relay is a control device specifically designed and certified to comply with UL 924, the standard for emergency lighting and power equipment developed by Underwriters Laboratories (UL). This relay plays a crucial role in enabling emergency lighting to function properly when normal power is lost.

Unlike standard relays, a UL924 relay ensures that emergency lighting circuits are energized during a utility power failure—regardless of the position of the normal lighting controls. It does so by overriding local switches or dimming systems, ensuring that life-saving illumination activates automatically.

How Does a UL924 Relay Work?

A UL924 relay is typically installed between the normal lighting controls and the emergency lighting circuits. Here’s how it works:

• Under normal conditions, the lighting system operates as usual—controlled by switches, occupancy sensors, or dimmers.

• When normal power is lost, the UL924 relay senses the outage.

• It then automatically closes its contacts, bypassing the regular lighting control and delivering full power from an emergency source (battery pack or generator) to the emergency lights.

• If the emergency lights support dimming (like 0-10V systems), the relay also forces them to full brightness.

• Some models support remote test buttons and fire alarm system interfaces, allowing manual or automated testing.

By functioning this way, the UL924 relay guarantees code-compliant emergency lighting performance, even in complex lighting control environments.

3 Applications of UL924 Relays

Application	Description
Emergency Lighting	Activates emergency lights during power loss
Fire Alarm Systems	Triggers emergency lighting when fire alarm is activated
Elevator & Egress Control	Maintains lighting in elevators and escape routes

Emergency Lighting

UL924 relays are most commonly used to activate emergency lighting in corridors, stairwells, lobbies, and other egress paths. They override normal controls and guarantee that emergency fixtures illuminate immediately during power loss.

Fire Alarm Systems

In integrated safety systems, a UL924 relay can interface with a fire alarm control panel (FACP). When the alarm is triggered—even before a full power outage—it can signal the relay to activate emergency lighting preemptively.

Elevator and Egress Path Control

Elevators and designated escape routes require reliable illumination. UL924 relays help ensure elevator cabs, machine rooms, and exit passageways maintain lighting to assist with occupant evacuation or firefighter access during an emergency.

UL924 Requirements: 3 Things You Should Know

Basic Compliance Requirements

To meet UL924 certification, relays must demonstrate that they can:

• Detect loss of normal power

• Automatically bypass control devices

• Maintain illumination for a minimum of 90 minutes

Referenced Standards

UL924 is not an isolated rule—it integrates with major safety codes such as:

• NFPA 70 (National Electrical Code, Articles 700 and 701)

• NFPA 101 (Life Safety Code)

• IFC (International Fire Code)

• IBC (International Building Code)

These standards collectively require that emergency systems restore lighting within 10 seconds of power loss.

Illumination Performance Criteria

Emergency lighting must meet specific performance:

• 1 foot-candle (fc) average illumination

• 0.1 fc minimum at the start of the event

• After 90 minutes, lighting may decline to 0.6 fc average and 0.06 fc minimum
  These values ensure that occupants can safely navigate the premises during an evacuation.

Conclusion

The UL924 relay is a vital piece of technology that bridges the gap between building automation and life safety. Whether integrated into emergency lighting, fire alarm systems, or egress control, it ensures compliance with the most stringent safety standards.

By understanding what a UL924 relay is, how it works, and the UL924 requirements it must meet, facility managers, contractors, and engineers can design safer and more reliable emergency systems.

If you’re building or upgrading emergency lighting systems, be sure to select UL924-certified devices for dependable performance—because when the lights go out, compliance saves lives.

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