An emergency lighting relay forces designated lights to turn on at full output the moment normal power fails, overriding any switch, dimmer, or controller that would otherwise keep them off. The right choice depends on how the relay must act on the circuit and which UL924-listed model suits the load, voltage, and enclosure. This guide walks specifiers and installers through the selection and names the models offered by Functional Devices, Inc.
What Is an Emergency Lighting Control Relay?
An emergency lighting control relay sits between the normal lighting controls and an emergency lighting load. Under normal conditions it lets the lights behave like any other circuit, responding to wall switches, occupancy sensors, and dimmers. When utility power drops, the relay ignores those controls, connects the emergency source, and drives the designated lights to full output.
Two behaviors cover most jobs:
- Automatic load control relays (ALCRs) sense the loss of normal power and hold the emergency load on through the transfer to backup power, regardless of the wall switch or control scheme. Use one for a dedicated emergency circuit that must stay on to meet code requirements.
- Bypass/shunt relays force a light on by bypassing a local switch when normal power fails. Use one for simpler applications, like an exit light or fixture already on a local switch that must come on during an outage.
How to Select the Right Relay for Emergency Circuits
A few factors drive this choice:
- Control Behavior: Decide whether the circuit needs a full ALCR that holds the load on through a transfer to backup power, or a simpler shunt/bypass relay that forces a switched light on when power fails. The ALCR suits dedicated emergency panels; the bypass suits a single fixture on a local switch.
- Voltage: Match the relay's input and coil voltage to both the normal and emergency sources. Mixed-voltage facilities benefit from a model that accepts both 120 V and 277 V.
- Load and Contacts: Size the contacts to the emergency load. Ratings of 10 A and 20 A are common, in SPDT and DPDT configurations.
- Dimming: If the normal circuit dims over 0–10 V or DALI (Digital Addressable Lighting Interface), choose a relay that overrides the dimming to full brightness on power loss, or one with an isolated relay built for dimmed and linear fixtures.
- Mounting and Testing: Confirm the enclosure fits the space—prepackaged NEMA 1, panel-mount, or ballast-channel—and specify an on/auto override switch wherever periodic testing is required.
Functional Devices UL924 Product Line
Functional Devices offers a UL924-listed line, part of its RIB® (Relay-In-a-Box®) family, that covers both ALCR and bypass duty:
- For full automatic load control, the ESRN accepts both 120 V and 277 V inputs and energizes the emergency load on loss of power regardless of the local control, overriding a connected dimmer to full output.
- Where fixture design leaves no panel space, the ESRB offers the same emergency function as the ESRN but mounts in the ballast channel—suited to retrofits and shallow enclosures.
- For linear and dimmed fixtures, the ESRLBC uses a slim enclosure and adds an isolated relay for 0–10 Vdc and DALI dimming.
- For a simple shunt or bypass, the ESR2401B (20 A SPDT, 24 Vac/dc or 120 Vac coil, NEMA 1) forces a switched light on when normal power drops, with DPDT versions (the ESR2401D and ESR2402D, 10 A) and 277 V options across the series.
Most ESR models come with a pushbutton test switch as well as remote test button input for testing the emergency load after installation.
Typical Wiring Configurations
In each configuration the relay monitors normal power on the line side and acts the moment that power drops. Three patterns cover most installations:
- Bypass of a Local Switch: With the ESR2401B, the wall switch controls the light as usual while normal power is present. On power loss, the relay's contacts transfer and force the light to full output, bypassing the switch position.
- Override of a 0–10 V Dimmer: While normal power is present, the relay passes the 0–10 Vdc dimming signal through, so the fixture dims as programmed. When power drops, the relay opens that path, breaking the dimming control and driving the load to full brightness.
- Full Load Control to Backup Power: In an ALCR setup, the ESRN will activate an emergency lighting load automatically upon the loss of normal power. If normal power is absent, the status of the load cannot be overridden. However, if the normal power is present, the emergency lighting load can be controlled by other means in order to use it as a normal lighting load, depending on the wiring setup.
Frequently Asked Questions
What is the difference between an ALCR and a shunt or bypass relay?
An automatic load control relay (ALCR) holds an emergency load on, ignoring the local controls, so it suits dedicated emergency circuits. A shunt or bypass relay forces a switched fixture on when normal power fails, which fits a single light already on a local switch.
Which Functional Devices relay should I order for a dimmed circuit?
For a 0–10 V or DALI dimmed circuit, choose a model that overrides the dimming to full on power loss. The ESRN does this, and the ESRLBC adds an isolated relay built for dimmed and linear fixtures.
Can I test an emergency lighting relay after it's installed?
Yes. Order the relay with the pushbutton test switch, which lets you simulate a loss of normal power and confirm the emergency load energizes, as the testing rules expect.
Specify the Right Emergency Lighting Relay
Need to specify a UL924 relay or order one for a job site? Browse the emergency lighting relay line from Functional Devices, find an authorized Functional Devices distributor in your region, or call the technical team at 800-888-5538 to confirm the right model for your circuit.