When emergency lighting is involved, full brightness isn’t optional; it’s code. Many facilities now use fixtures with integral emergency (EM) batteries and 0-10 V dimming drivers. While this combination saves space and simplifies installation, it introduces a key challenge: during a power outage, the dimming signal can still hold the fixture at a reduced level. That’s a problem if your inspector expects the lights to go full-bright during an emergency.
This blog explains how to solve that problem using Functional Devices’ ESR relays and why it matters for code compliance. Functional Devices has been helping contractors, integrators, and engineers solve these kinds of field headaches for decades, and we’re always happy to help you choose the right device. Contact us or browse our full ESR line for more information.
The Challenge: Dimming + Emergency Lighting
In most UL924 applications, the building has two circuits involved. One circuit is Normal Power being supplied from the Utility feed, and the other is the Emergency Power being supplied by either a generator or inverter.
During normal operation, both circuits draw power from the Utility feed, allowing lights on both circuits to be controlled as the user sees fit. 0-10 V dimming works by applying a low-voltage signal to the lighting driver to set the output level. If normal power drops and the fixture switches to its internal battery, the dimming signal can remain active, leaving the light dimmed at the exact moment you need full output.
What About an Internal Battery?
As batteries increase their storage capacity and light fixtures become more efficient, it has become more and more popular to include an internal battery in the fixture as the emergency source of power. This typically eliminates the need for emergency circuits, inverters, generators, and transfer switches altogether. This method works great as a simple solution for on/off fixtures, but it gets a bit more complicated the moment dimming gets involved.
Most 0-10Vdc dimming drivers supply a constant current to a separate dimming switch or rheostat, which acts as a variable resistor. As the user adjusts the resistance, the voltage changes with it, giving the driver the signal it needs.
The problem with this is that the driver will supply the constant current to the dimmer switches as long as it is receiving power, which includes when it is drawing from the EM battery. Unless the dimming signal line is broken, the light will remain at the brightness set by the user before a loss of power, and not full brightness. In other words, even with an integral EM battery in your fixture, a UL924 ESR device is still needed to force your emergency lights to full brightness upon loss of power and comply with code.
The Solution: ESR Relays to Break the Dimming Line
Our Emergency Switching Relays (ESR2401B for 120 Vac, ESR2402B for 208–277 Vac) automatically detect the loss of normal power and open the dimming circuit. Breaking the 0-10 V signal forces most drivers to full brightness, satisfying UL 924 and NFPA life-safety requirements.
When Normal Power drops, an Automatic Transfer Switch upstream directs power from the emergency source to only the Emergency Power circuit, and an ESR device bypasses any switch in the circuit to force the emergency lights on.
The ESR is wired so that when utility power is present, the dimming line passes normally. When utility power is lost, the ESR opens that circuit, isolating the dimming wires and letting the fixture’s emergency driver output at full brightness.
Why It Matters
When pairing 0–10 V dimming with integral emergency batteries, it’s important to understand why the right override solution isn’t just convenient; it’s essential. Here’s why it matters:
- Code Compliance. Life-safety codes require emergency lighting to achieve full output on loss of power.
- Driver Compatibility. Some drivers can dim-to-off but revert to full brightness when their dimming circuit is broken.
- Peace of Mind. With the right ESR, inspectors can verify full brightness at the push of a test switch.
Together, these factors show that using an ESR relay isn’t just about wiring, but also compliance, compatibility, and confidence when it matters most.
Wiring Tips
Luckily, the setup is very simple and similar to just about any UL924 application despite the difference in Emergency Power source. Here's the diagram:
Like in most ESR applications, the ESR monitors the incoming Normal Power. When power drops out, the ESR's N/O contact opens up, breaking the 0-10Vdc dimming signal. A great device for this application would be either the ESR2401B for 120Vac or ESR2402B for 208-277Vac.
Remember these tips for setup:
- Keep 0-10 V wiring separate from line voltage to avoid noise or leakage currents.
- Test both normal and emergency operation after installation to confirm full brightness.
- For longer wire runs, use properly rated cable and confirm driver behavior before final sign-off.
FAQ: Overriding 0-10 V Fixtures with EM Batteries
Q: Why can’t I just send 0 V on the dimming line?
A: Many drivers will stay dim or off at 0 V. Physically breaking the circuit with an ESR ensures full brightness and meets UL 924.
Q: Will this work with any 0-10 V driver?
A: Most standard drivers revert to full brightness when the dimming circuit is opened, but always verify with the driver manufacturer or test in the field.
Q: Will the battery drain faster if the fixture is forced to full brightness?
A: Yes; full brightness uses more power. Make sure the EM battery capacity meets the desired run time at full output.
Q: What’s the difference between ESR2401B and ESR2402B?
A: Voltage range. ESR2401B is for 120 Vac circuits; ESR2402B is for 208–277 Vac circuits.
Q: Any tips for commissioning?
A: After wiring, simulate a power loss and verify the light goes to full brightness. Check your connections to ensure the dimming circuit truly opens.
Ensure Full Brightness with Functional Devices
Integrating emergency lighting with 0-10 V dimming can be tricky, but it’s easy to get right with the proper relay. ESR relays from Functional Devices are designed to meet this exact need, giving contractors a tested, code-friendly solution. If you’re ready to ensure your emergency lights shine bright when it matters most, browse our ESR products or contact us directly for help selecting the right model for your project.
This article is one of our first customer-submitted suggestions for an application. Thanks to Michael Merchut from Graybar Phoenix for the idea!