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I have access to a power cable that powers devices through a 2-way switch, basically:

Situation

Is it possible to add an always-on socket in between using L1, L2 and N? The problem is that either L1 or L2 is live at a given time and, obviously, they cannot be connected together as that would invalidate the 2-way switch.

Does there exist a device that would pass through L1 and L2 without connecting them together?

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    \$\begingroup\$ I don't see how would that be possible \$\endgroup\$ – Eugene Sh. Mar 15 at 19:52
  • \$\begingroup\$ I don't even understand the question. \$\endgroup\$ – Marko Buršič Mar 15 at 19:57
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    \$\begingroup\$ It is unlikely that addition of a socket to a lighting circuit would meet local electrical regulations. \$\endgroup\$ – Transistor Mar 15 at 21:06
  • \$\begingroup\$ Add another cable. \$\endgroup\$ – user_1818839 Mar 15 at 21:08
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    \$\begingroup\$ If you're looking for answers that address how to do this legally and safely (or even if it can be done) to an electric circuit in a house, instead of from the circuit-theoretical point of view that you got here, please post this on Home Improvement. \$\endgroup\$ – Niall C. Mar 16 at 2:29
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If you have the neutral then yes. Use a SPDT relay with the common contact on the socket, NO on L2, NC on L1 and control coils connected to L2&N. When L2 is energized, the contact will switch the socket to L2. The light will flash when the switch transitions but will essentially be "always" on. I'll try to post a schematic when the circuits tools starts working again... Anyone else having trouble with CircuitLab? Until I figure out why CircuitLab isn't working... Digikey Scheme-it will have to do.

Important Edit: Just because it is possible, doesn't mean it should be done. Tapping travelers is not considered best practice.

enter image description here

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  • \$\begingroup\$ the load can't be fed 0v, 0a in this setup, right? \$\endgroup\$ – dandavis Mar 15 at 23:32
  • \$\begingroup\$ That’s better now, AC coil relay on in one position only. Get 25A if you plan to run heavy motors \$\endgroup\$ – Tony Stewart EE75 Mar 16 at 1:17
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    \$\begingroup\$ In this situation, the operation of switching will always be visible on the said socket. It will only be a very short time (tenth of milliseconds or so), but it can disturb any sensible electronic devices connected there. \$\endgroup\$ – glglgl Mar 16 at 7:56
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    \$\begingroup\$ @glglgl: I think you mean "sensitive electronic devices" - sensible ones will have power smoothing that will reduce the effect of mains glitches. \$\endgroup\$ – psmears Mar 16 at 11:05
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    \$\begingroup\$ @psmears I think you are right. False friends on foreign languages sometimes can be confusing. \$\endgroup\$ – glglgl Mar 16 at 14:02
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When there is a will, there is a way.

AC Relays for line voltage use a shaded pole which acts somewhat like my simulated DC relay with a diode&cap.

This is an interactive simulation. You click on the centre of either switch. ( but don't drag it)

There are several options for pre-installed source and load. The above is one way, so that the source end controls the relay but not the other. This results in an interruption much like when the city switches grids with a transfer switch and the lights blink for a half cycle but PC PSU's have storage capacity for at least 1 cycle.

enter image description here

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  • \$\begingroup\$ nice sim! this seems to leak the coil current through the supposedly-off light, which likely would be enough to dimly drive an LED bulb... \$\endgroup\$ – dandavis Mar 15 at 23:30
  • \$\begingroup\$ 171 nA ! ??? ?.... \$\endgroup\$ – Tony Stewart EE75 Mar 16 at 6:51
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Unfortunately, the answer is no. There is no way for there to be any voltage potential between L1 and L2 without also introducing voltage to your load or otherwise invalidating how the switches operate today.

Also, these are 3-way switches; 2-way switches are your standard single on/off switches. Assuming these are 120v lines, new circuits with this wiring configuration are prohibited by NEC (if you're in the USA), and call for both live and neutral to pass through switch boxes.

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  • \$\begingroup\$ I had an old house where some DIY’er passed neutral only thru the switch and had neutral grounded to the receptacle box at one end !%&#@* and did the latter also in one of the 30 ceiling lights in the attic under insulation. !%@#$&*. Even the “grand-father “ clause would frown. I did not know about the NEC change , but certainly understand why. \$\endgroup\$ – Tony Stewart EE75 Mar 15 at 19:55
  • \$\begingroup\$ Ooph, I cringe when I hear stuff like that. Please upvote my answer! I'm trying to get improve my reputation on Electronics! \$\endgroup\$ – tgpaul Mar 15 at 20:07
  • \$\begingroup\$ The question notes that the socket can be installed using L1, L2, and N. So there is no reason to need a potential between L1 and L2, nor any reason to think that the neutral isn't passing through the 3-way switches. \$\endgroup\$ – Ben Voigt Mar 15 at 21:59
  • \$\begingroup\$ I'm looking at this question from a more practical standpoint than a theoretical one. If there is a neutral in the switch boxes, then the simplest answer is "connect an outlet to C and N from the switch box." \$\endgroup\$ – tgpaul Mar 15 at 22:20
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    \$\begingroup\$ These are called 3-way switches in the US. In other parts of the English-speaking world, they are called 2-way switches. Since the OP states they're in Europe, let's assume they're using UK English. \$\endgroup\$ – Niall C. Mar 16 at 2:25
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Adding another answer.

Assuming this is more than just a thought experiment, assuming you have access to the neutral, connect the load side of your receptacle to C instead of L1 or L2.

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  • \$\begingroup\$ Normally 3 wire is used \$\endgroup\$ – Tony Stewart EE75 Mar 15 at 22:53
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 yep \$\endgroup\$ – tgpaul Mar 16 at 2:55
  • \$\begingroup\$ @tgpaul this doesn't appear to necessitate a second answer, and it's worth noting it only works at the box closest to the source where you have a line wire present anyway. At the other end, common attaches to the outbound switch leg. It's probably better to spell out the word common in this case too. I'm an electrician and I had to think about it for a second ("Oh he means common on the panel side switch!"). \$\endgroup\$ – K H Mar 16 at 4:30
  • \$\begingroup\$ @KH I agree with you--but the question is too ambiguous that I wanted to make sure the OP wasn't overlooking something extremely obvious. \$\endgroup\$ – tgpaul Mar 16 at 16:16

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