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On one circuit breaker I have a manual switch. On the second circuit breaker I have a motion sensing switch (using a mechanical relay). I have a number of lights wired in parallel as the load. The lights should turn on as a result of motion or manually turning them on. Unfortunately I can't just wire the two switches in parallel on the same breaker because of physical constraints; I'd have to perform a major renovation to rewire.

It seems I need a sort of DPDT relay like this, which is normally open, has two magnetic coils, and keeps both circuits separate:

schematic

simulate this circuit – Schematic created using CircuitLab

Does a relay like this exist? Is this the correct way to solve this problem?

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  • \$\begingroup\$ It's not clear enough: a) are L1 and L2 part of three phase network or they are the same phases? b) what are 1H inductances? c) draw in the schematics editor used in this forum d) where are the circuit breakers? \$\endgroup\$ – Marko Buršič Sep 3 '17 at 11:57
  • \$\begingroup\$ a) L1 and L2 are residential European 240V single phase. b) The 1H inductance was just added by the schematic editor I used, I couldn't get rid of them, I was looking for a coil symbol. I will redraw in the editor you suggest. c) I left them out of the drawing. One breaker controls circuit 1, the other break controls circuit 2. \$\endgroup\$ – rcampbell Sep 3 '17 at 12:02
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Your original circuit was very foolish: you back-fed both breakers so that someone working on what they thought was an isolated circuit (having switched off that breaker) would be exposed to live wiring if the switch on the other phase turned on.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Dual supply lamp circuit.

You can achieve isolation using one relay as shown in Figure 1. If SW2 is closed the load will be supplied by L2 regardless of SW1 status.

You need to clearly label this control box and light fitting to state that it has dual supply - ideally giving the circuit breaker location and numbers that need to be isolated to fully isolate the circuit for maintenance.

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  • \$\begingroup\$ "Your original circuit was very foolish" -- definitely. I understand how your version works and, as a bonus, uses a simpler relay! Thank you! \$\endgroup\$ – rcampbell Sep 3 '17 at 16:20

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