We have an outlet that is controlled by a switch, but we haven't had anything plugged into it since we moved in. This year we thought it would be a great idea to plug in the Christmas tree LED lights to the outlet so we can just turn them off and on easily. The problem is that when the switch is off, the LED lights still have a very dim light coming from them. Does anyone know why the outlet is still getting some power when the switch is off? Is this dangerous?
IMHO, it could be one of three things:
1) A feature of the lights. Some lights might have a "nightlight" feature.
2) Electromagnetic interference (EMI). Depending on how much electricity the lights require, and how dim they are, this might be possible. Especially if it's a half switched receptacle that was wired with a single 14-3 wire and the constant hot is powering the other half of the plug and something is plugged into it and drawing power while the switch is off... but I guess it could feasibly happen if the wires were just run next to other wires that are constantly powering something else. If this is the case, there's nothing wrong - it's just annoying
3) There's a small arc fault. I don't know how to verify this for sure because it might look just like EMI. These can go undetected for years and have the potential to start a fire. I've opened up walls before and found that half the 2x4 was burned away. I suggest doing whatever is cheapest and easiest first just in case the fault is in something easily replaceable (like replacing the switch and/or receptacle, or checking the receptacle with a volt meter). Or you could start with this just to make sure it's wired correctly (although I don't see how that could be the problem). But if you can't find it easily right away with those things, I'd install an AFCI on that circuit... and if that continues to trip, you know you have a serious problem, and it's time to call a professional electrician with equipment that can trace the wire and/or find the location of the fault.
How much voltage do you measure at the outlet with the switch on, and with it off?
You should inspect/replace the switch.
Turn switch on, Shut off breakers until the lights go off, or go dim. Note which breakers did what. Once they're completely off, Remove the switch from the wall, disconnect its wires, and test with multimeter to make sure its essentially infinite resistance when off, and 0 ohms when on. Buy a replacement if it doesn't look clean and orderly. They're inexpensive.
With the switch still removed, and the wires positioned so that they aren't touching anything metal, power all breakers back on. Measure voltage between the two wires that were going to the switch, with and without the lights plugged in, and take note of whether the lights glow dim.
Returning to us with the measurements requested above will help us give an educated guess as to the cause.
Whatever it is, it is probably NOT a nightlight feature unless these things have some big battery you didn't notice. Very unlikely to be EMI, and Arc Faults would not be capable of this.