Let's start with the more common one: UL 924 applications. UL 924 devices are meant to be used in applications that take place downstream of a UL 1008 device. They are often used to bypass a wall switch or override a dimming signal on your emergency circuit in the event of normal power loss. Devices like this include models ESR2401B and ESR2401D. Other more complicated versions of UL 924 devices, such as models ESRN or ESRB, have automatic load control, which allows you to control the lights on your emergency circuit with the same wall switch and dimming signal as your normal circuit, in addition to overriding both so that the emergency lights always come on at full brightness in an emergency situation.
If you are installing lights, this is where you will be spending most of your time, and so in most situations a UL 924 device is what you will need. UL 924 devices do not, however, transfer power on the emergency circuit from normal to back-up. This is where UL 1008 devices come in.
UL 1008 devices are often called transfer switches. Transfer switches are used to transfer the power source of the emergency circuit from normal power to back-up power in the event of normal power loss. Typically, these are large and used to switch over entire buildings to back-up power. Sometimes a smaller transfer switch will be used to switch just a branch circuit of lights. These are called Branch Circuit Emergency Lighting Transfer Switches, or BCELTS for short. It's these BCELTS that UL 924 devices often get confused for as they are closer to the same size. Some BCELTS on the market today even include a UL 924 rating. This means they both transfer power and bring emergency lights on at full brightness. BCELTS are usually only used in a small amount of retrofit applications. It's important that a UL 924 device isn't used in its place.
If you have two power sources and one circuit that needs to switch between them, use a UL 1008 device. Use a UL 924 device if you're just looking to override a switch or dimming signal so that an emergency light will come on full brightness.
If you have questions, contact us and we will do our best to point you in the right direction.
Functional Devices, Inc., located in the United States of America, has been designing and manufacturing quality electronic devices since 1969. Our mission is to enhance lives in buildings and beyond. We do so by designing and manufacturing reliable, high-quality products for the building automation industry. Our suite of product offerings include RIB relays, current sensors, power controls, power supplies, transformers, lighting controls, and more.
We test 100% of our products, which leads to less than 1 out of every 16,000 products experiencing a failure in the field.
Simply put, we provide users of our various products confidence and peace of mind in every box.