Shunt/Bypass UL 924 Emergency Lighting Relays

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Functional Devices offers UL 924 Emergency Lighting Relays listed for controlling lights on an Emergency lighting circuit. The Automatic Load Control Relay (ALCR) versions are very popular (see the ESRN device). However, what if a simpler UL 924 device could be used to shunt or bypass a local electrical switch for Emergency lighting control? For such applications, we offer the slimmed-down, 20A contact rated, ESR2401B or ESR2402B as a good starting point.


When Normal Power is present, the ESR Bypass/Shunt relay coil is activated (Contacts N/O), and the emergency panel is fed from Normal Power. The lighting load can be switched on/off using an individual wall switch. When normal power drops out, the ESR coil is deactivated and N/C contact falls closed.

Shunt/Bypass Application

If there is already a local wall switch or other means of controlling power to an Emergency light, then the ESR2401B could be used to force the light on if Normal power drops out. The ESR2401B is for a feed of 120Vac Normal power. If there is a feed of 277Vac Normal power, use the ESR2402B model.

Now, what about applications where there is 0-10Vdc dimming and the light needs to be forced to full brightness if Normal power drops? The ESR2401B or ESR2402B may still be used and wiring it up would look like the diagram below.


When Normal Power is present, the ESR coil is activated and the N/O contacts are closed, allowing for the 0-10 Vdc to control the dimming of the load. When Normal Power is lost, the N/O contacts open, breaking the 0-10 Vdc dimming control, bringing the light load to full brightness.

Dimming Override Low Voltage Application

We also offer double-pole, double-throw (DPDT) versions in both 10A contact rating and 20A contact rating. Please see those options in the table below:


When it comes to UL 924 applications of controlling Emergency lighting, you may not need a full ALCR device. Using the shunt/bypass options detailed in this blog may be wise, as the installation will be simpler and more cost effective. The results are savings in time and money. Be sure, though, that a UL 924 device for shunt/bypass is appropriate for your application.

Do you have any technical questions? Give us a call and ask for Tech Support or send in a support email – we’re happy to help!

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Sam Klennert
Sam Klennert

Meet Samuel Klennert – you can call him Sam. He was born and raised amongst the farmland of Indiana, which includes corn, soybeans, and sometimes wheat. Sam graduated from Purdue University with a Bachelors degree in Electrical and Computer Engineering Technology in 2015.

Outside of work, Sam enjoys spending time with his wife and dog – whether it’s inside or outside their home. Sam can also be seen serving at his local church and glorifying God with other believers, which is by far his favorite way to spend time.

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