MH Controls Corner: Controlled Emergency Lighting — UL924 vs. UL1008 Explained

All commercial facilities require emergency lighting with a secondary power source, to allow building occupants to exit the building safely in the case of an emergency. It is permissible to have these fixtures controlled while under normal power for space functionality, and to save energy. Correct design and installation of these systems is crucial for occupant safety and code compliance.

NFPA 101 includes the requirements for illumination of the path of egress, which is, in general, one foot-candle minimum at the floor level. Article 700 of the National Electrical Code covers emergency systems, lays out the requirements for the secondary power source, and requires that any controlled emergency fixtures have a mechanism that overrides all control under the loss of normal power. It is important to note that this requirement is for all controls ranging from simple manual wall switches to automatic occupancy sensor, or time based control systems. Night light or always-on fixtures, powered by an emergency source, do not require any additional equipment to operate appropriately when normal power is lost.

For most facilities, the normal power source is the incoming electrical feed from the utility. Secondary power sources generally include:

1. Per-fixture battery packs
2. Emergency generators
3. Central inverters

Most fixture battery packs offer the ability to have a control input, and are UL924 listed. The battery pack automatically overrides this control when normal power is lost, and the battery illuminates the fixture. When generators and central inverters are used, additional control devices are required to ensure that the fixture turns on to full brightness under the loss of normal power.

For facilities that use a central system (generator or inverter), there are two types of devices used to override lighting control. These devices produce the same ultimate functionality, but their application and operation are significantly different. The national electrical code refers to these devices by their Underwriters Laboratories’ (UL) listing number:

1. UL924 – Automatic Load Control Relays (ALCRs), also known as “shunt” devices.
2. UL1008 – Automatic Transfer Switches, also known as “bypass” devices.

Facilities using a central inverter, or generator, will contain a UL1008 listed automatic transfer switch as a part of the power distribution system. These are large switches, designed to carry the full electrical load of all emergency power and lighting for the building. These devices have the same UL listing as the lighting branch circuit transfer switch that is discussed in this article, because the functionality is the same. Systems using UL924 devices will be on branch circuits, downstream of a UL1008 listed switch.

UL924 Device Application

UL924 devices are load following relays. The emergency circuit provides power to the device, which includes a relay that switches the emergency fixtures on and off, following the operation of a switched input. There is also a normal sensing input which is a constantly energized circuit coming from the normal power distribution. Typically, the switched input comes from the switched side of the normal circuit in the same area. This wiring configuration allows a single switch (or automatic controller) to control both the normal and emergency fixtures together under normal power. When the normal power sensing input loses power, the relay automatically closes and turns the emergency fixtures on, overriding the switched input. In this configuration, the emergency circuit always powers the emergency fixtures, and the UL924 device is where the switching of the emergency circuit occurs. Examples of high quality UL924 devices are the ELCU series from Wattstopper, UL924SR series by Hubbell Control Solutions.

UL1008 Device Application

UL1008 devices transfer the power from the normal circuit to the emergency circuit under loss of normal power. Under normal operation, the normal circuit powers the emergency fixtures. Once normal power is lost, the power feed transfers to the emergency circuit. Most UL1008 devices have a normal power sensing input and a controlled input. The control device is active under normal power, but when the sensing input loses power, the transfer occurs and power to the fixtures comes from the emergency circuit with no control device. It is important to take circuit loading into account when using UL1008 devices. Since this device transfers the load, both the normal and emergency circuits require capacity to accommodate the load of the emergency fixtures. There are a variety of different options for branch circuit UL1008 devices. The Hubbell Control Solutions UL1008BCELTS1277 is a great solution for applications where an easily accessible test switch is required. The Dual-Lite ALCR20 provides another solution with expanded wiring options to fit particular applications.

To summarize the significant difference: For UL924 devices, the emergency circuit always powers the emergency fixtures and the switching occurs within the device. For UL1008 devices, the normal circuit powers the fixtures under normal operation and the emergency circuit powers the fixtures under emergency operation. The control of the emergency fixtures under normal operation occurs with a separate control device.

Dimming Considerations

When the emergency fixtures are under dimming control, there are additional considerations necessary to bring the fixtures to full brightness under loss of normal power, as required by code. The design approach will vary depending on the type of dimming used.

Override of 0-10V Dimming

With 0-10V, or four wire dimming, the dimming conductors are separate from the line voltage conductors. For this configuration, UL924 devices are the most common way to override the switching. However, to bring the fixtures to full light output, an additional device is required to override the 0-10V dimming conductors. Since 0-10V is simply DC voltage where 0 volts (short circuit) between the conductors is fully dimmed and 10V (open circuit) between the conductors is fully bright, a secondary device is needed to open this circuit. A second, normally open relay held closed by a normal circuit, accomplishes the necessary functionality. Some lighting control systems also have this feature incorporated into the design of the system. For instance, Wattstopper DLM room controllers are UL924 listed to open the 0-10V conductors when used in conjunction with an ELCU (Watttstopper’s UL924 device).

Override of Line Voltage Dimming

UL1008 devices are appropriate for fixtures with line voltage or two wire dimming. Since the UL924 device contains a separate relay that follows the switched load, it cannot accomplish line voltage dimming. The transfer devices will allow dimmed signals through the device, under normal operation. Once normal power is lost, the transfer to the emergency circuit brings the lights to full bright. Some of these devices allow wiring in a configuration that operates as a UL924 device and allows two wire dimming. To be suitable for the particular application, the device label is required to be either UL924 or UL1008. This is accomplished by feeding both inputs to the transfer switch from the same emergency circuit. The normal operation input will be downstream of the dimmer, while the emergency operation input is upstream of the dimmer. The device will also need a normal sensing input, which will initiate the transfer after normal power is lost. It is important to note that most jurisdictions will require that the device be listed as either UL924 or UL1008, based on how it is wired and installed. If the device is wired in this configuration (on the same circuit), it will need to be UL924 listed. Unfortunately, most jurisdictions do not allow devices that have both listings, as the functionality is determined by how the device is wired.

Designing spaces with controlled emergency loads will offer improved functionality for the occupants of the space and will save energy over using constantly illuminated emergency fixtures. Current versions of the International Energy Conservation Code (IECC) do not require the control of emergency circuits, but other codes used elsewhere do. Future versions of the IECC will undoubtedly incorporate this requirement as well.

Using the above guidelines, control systems will offer flexibility in space functionality and energy savings while still allowing safe exit from facilities in emergencies.

As always, don’t hesitate to contact the controls team at MH for further information or to review specific applications. And remember, whenever you have a technical controls question, on any project or application, simply email controls@mhlighting.com. This email address is monitored by our entire controls team. During our normal business hours, we are committed to a four-hour response time for all emails sent to this email address!

This article was written by Clint Conley, our Lighting Controls Department Manager. Clint can be reached at 720.904.8554 or clint@mhlighting.com.