I was installing new LED lamps on my kitchen, which seemed to be functioning well. After turning off the lights, the newly installed lamps still had a dim light, that kept itself for quite long.

I've measured around 60VAC on the OFF switch for these lamps and 110VAC for the OFF switch attached to another lamp.

After lots of head scratching, I've tried to plug just one leg of the LED lamp into a plug hole, result on the picture below.

One leg connected, but partially ON

Why does this happen? How can I prevent it? Does it take a toll on my electrical bill?

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    What kind of switches do you have? – manassehkatz Mar 19 at 17:38
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    @EricF, you have a citation for that? I can't imagine an ordinary switch that isn't totally on or off. I'm not even sure how something can physically "leak a little voltage". – JPhi1618 Mar 19 at 18:42
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    no, it doesn't cost you anything. the power comes from EMI leaking into the wires. This is what causes false positives on non-contact voltage detection sticks. Even a few microamps can produce (faintly) visible light from LEDs. you could use a bleeder resistor or dummy load to snub the glow, but does it really hurt anything? – dandavis Mar 19 at 18:45
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    @EricF: no, they don't. You might be thinking of active switches like motion detectors, smart switches, solid-state dimmers, or digital timers, but regular 3-way switches physically break the connection. – dandavis Mar 19 at 18:47
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    @EricF, I think this article should clear it up for you. It's not a cheap switch "leaking", it's a byproduct of AC wiring and the tools used to measure "voltage". – JPhi1618 Mar 19 at 19:00

What you are seeing is the effect of Stray Voltage caused by capacitive coupling. Electricians refer to this sometimes as Phantom Voltage.

Ungrounded metal objects close to electric field sources such as neon signs or conductors carrying alternating currents can have measurable voltage levels caused by capacitive coupling. Since voltages detected by high-impedance instruments disappear or become greatly reduced when a low impedance is substituted, the effect is sometimes called phantom voltage (or ghost voltage). The term is often used by electricians, and might be seen, for example, when measuring the voltage at a lighting fixture after removing the bulb. It is not unusual to measure phantom voltages of 50–90 volts when testing the wiring of ordinary 120 V circuits with a high-impedance instrument.

Sections bolded by me

The voltage you read with a common multimeter can change depending on how the wire is run, how long it is, etc, but you will usually be able to measure some voltage even when a light switch is turned off. If you connect even a tiny resistor between the phantom hot and neutral, the voltage will disappear. It is very, very low amperage and can't be felt or do any work.

Very simple LED bulbs that have the bare minimum of driver components can be illuminated by this voltage because they are very low power devices. LED bulbs that look more like regular light bulbs from major manufacturers have complex circuits to drive them and they typically will not show any glow.

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