# How to connect a mains 24Vdc and an Emergency 24Vdc supply to lights that should switch the mains supply off when the emergency feed is switched on

I am looking for a solution for some lighting that was installed recently. The lights are single LED downlights that are connected from a direct connection to 24Vdc through a constant current driver to the LED.

The problem is, there was supposed to be an emergency lighting supply connected to some of these lights as well, and there is a test button that will turn this supply on. This obviously would be problematic without some additional electronics that would allow the presence of the emergency supply to cut off the mains supply from an individual light (lights on a given circuit are not all fed with an emergency supply) and allow the emergency feed to take over, and of course, when the emergency feed is switched off, the mains would take over again.

I have been looking for a commercial product for such applications, but I have not yet come across any.

Does anyone know where I could look, or know how to do a DIY resolution with electronics?

Thanks.

• A DPDT (or SPDT, but DPDT is more common) relay? – Hearth Dec 16 '20 at 4:52
• you have it backward ... it should be switch the battery supply off when the main feed is switched on because emergency lights are controlled by the main feed ... the test button disconnects the main feed – jsotola Dec 16 '20 at 5:27
• Hi jsotola, not in this system. This was a pre existing system where the emergency 24V fed separate lamps within a light fixture. Now the lights are single spot lights and another lamp cannot be accommodated. I understand your point though. – Benjamin Angel Dec 16 '20 at 7:22
• So, to clarify further, when the test button is pressed the main lighting supply remains on. This is why I need to find a way to control the lights supply individuially. – Benjamin Angel Dec 16 '20 at 7:24
• a 24V SPDT or DPDT relay seems like a good solution.but without a full description of the supplies and test procedure involved I can't say exactly how to connect it – Jasen Dec 16 '20 at 11:00