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In my home, electricity passes through a bulb holder even when the switch is off. I heard that happens due to 3-way switches. Could anybody tell me if it is okay or worth worrying?

Update

Let me explain it in detailed:

I use that holder for LED bulb which glows (no issue with plastic frame of the holder) even when the switch is off so I ran the following tests by taking out the bulb.

1) Normal Line Tester - When I test with line tester, one of the two pins, i.e., line pin indicates that the electricity passes through it slightly when switched off.

2) Digital Teser - Digital tester indicates as following:

  • Line test - 12v 36v 55v 110v
  • Inductance break point test - 12v 36v

Here is the screenshot of the switch:

Here

One of them located at the beginning of staircase (in the main hall) and another at the end of the staircase on the first floor​'s hall. It's a duplex building.

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    When you write "electricity passes through a bulb holder even when the switch is off", do you mean that you measure a voltage at the holder somehow? Do you have a bulb in the holder? Is the switch just a plain simple switch? – brhans May 26 '16 at 14:32
  • How old is the house? – Tyson May 26 '16 at 14:46
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    A long long time ago, there were things like "switched neutrals" and "California 3-ways" that would result in bulb holders having voltage when they wouldn't by today's standards. – Tyson May 26 '16 at 15:23
  • @Tyson -- switched neutrals were a thing -- but a CORRECT California/Coast 3-way does NOT switch the neutral. The problem 3-way system is the Carter system. – ThreePhaseEel May 26 '16 at 22:23
  • @brhans Yes, I've updated the question with details about the test and screenshot fo the switch. I use an LED bulb there. Thank you! – Netizen May 27 '16 at 6:48
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Look closely at that light switch. See the two switch blocks (one blank) and the almost-square translucent fringe around both of them? That lights up. That is a backlight designed to help you find it in the dark. You may have known this and forgot to mention it, or maybe it doesn't work for a particular reason. Here is how those switches are wired internally.

enter image description here

As you can see, power flows through this circuit at all times. If it's not going through the switch, it's going through the backlight. Incandescent bulbs are quite low resistance when they are not burning (their resistance increases greatly as they heat up). These backlights exploit that by leaking a tiny amount of current through the incandescent bulb, too little to make it glow, but enough to light the backlight.

Well, LED bulbs monkeywrench this arrangement. They aren't low resistance when off. If current will go through the LED bulb with the backlight impeding flow, it may make the LED bulb function. It might even make the LED bulb work, and the backlight not. And that kind of thing is what you're running into.

Solution:

  • Go back to incandescent bulbs. If it's a multi-bulb lamp, only one needs to be incandescent.
  • Go to a plain switch with no backlight.
  • Get a more modern switch that works the new way (not depending on leakage current, but requiring a neutral to the switch).

enter image description here

  • Thanks for your detailed explanation. Lighting from the LED bulb even after switching it off wouldn't be a problem for me. I was just worried if it is a defect within the line / switches and curious to know if there will be a danger. Could you clarify me about this? – Netizen May 27 '16 at 8:49
  • Harper, by the way, I have the same switchboards throughout the house and never saw any of them (backlights) lighting in the dark. Also, other bulb holders without 2-way switch don't have that problem. – Netizen May 27 '16 at 9:03
  • OK, I see where I may be confused. What you call "2-way" we call "3-way". Certain ways of wiring a 3-way configuration can cause a lamp socket to be energized hard, but they wouldn't be energized just a little and allow an LED to glow slightly. Backlights and other "smart switches" which power themselves in the same way, would cause that. If they are indeed plain old simple 2-way (your term) then you may have a bad switch. – Harper May 27 '16 at 9:09
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    It's not a typo, I meant what Americans call "3-way" is commonly called "2-way" in most other countries, and that is correct there. What I am saying is that 2-way/3-way switches would not explain why the LED would come on "a little". It should either be fully on, or fully off. The only thing that explains coming on "a little" is a backlit, smart or defective switch. – Harper May 27 '16 at 9:22
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    There's another possibility: if he's got paired 3-ways which communicate with each other, then there's current here and there due to the fourth wire. I have some of these: both switches (Lutron) are touch-sensitive dimmers, but the actual dimmer control is only in the master. The slave switch sends the touch signal via that 4th wire. – Carl Witthoft May 27 '16 at 13:11
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This wouldn't happen with "two way switches". The only time you would have voltage while the switch was off is if the switch has a pilot light or has a motion detector or electronic timer, etc. If the switch is powered at all, it could be pulling the small trickle of power it needs through the bulb circuit.

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    This will happen any time some genius wires the wrong side of the power (return) to the switch. – Carl Witthoft May 26 '16 at 18:54
  • @CarlWitthoft, I guess I'm going to need more of an explanation that that. I'm imagining touching the center contact of the bulb holder with one probe from a voltmeter, and touching the threads with the other. How is any combination of normal switches going to result in voltage when the switches are off? You might get voltage to ground on a switched neutral, but (and I could be missing something easy) I don't see how you would have voltage through the bulb holder. – JPhi1618 May 26 '16 at 19:06
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    you shouldn't, but if anything's grounded and you touch that, you'll get voltage. It's also possible that the OP saw a little voltage, which you can see when comparing an AC hot to a floating return. – Carl Witthoft May 26 '16 at 20:43

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