Emergency Lighting Transfer System. A Listed device that transfers a branch circuit from the output of a dimmer to a constant emergency power source in the event of an emergency power outage. Sometimes referred to as a BATS or Branch Circuit Automatic Transfer Switch.
An ELTS must be a true "Automatic Transfer Switch" (ATS) and must meet the stringent requirements of the UL1008 standard. Sometimes confused with the ATS is the Automatic Load Control Relay (ALCR), another device used in Emergency Systems. The Automatic Load Control Relay is used for bypassing a dimmer or wall-switch, and cannot be used for transferring a load between normal and emergency power sources. It is listed to the UL924 standard, a much less stringent document.
For more details, see attached article "Understanding Control of Emergency Lighting Circuits" by Steve Terry.
One such system:
Union Connector: Emergency Transfer Systems