0

Just bought new flush mount ceiling LED lights. the light in the hallway has 2 switches to it and this light does not completely turn off when flipping the switch off. The tabs are always dimly lit. Is this normal? Should we be concerned about fire? Why does this happen? Is the light defective? Thanks

isherwood
  • 137,324
  • 8
  • 170
  • 404
Mark
  • 1
  • 1
  • 1
  • 1
  • 2
    It's floating/induced voltage that exists due to the travellers between the switches run in parallel. It's NOT dangerous, but annoying. One answer is buy higher quality bulbs, another is just ignore it. You can prove this by pulling just one travellers off a switch on either end of the switch circuit (or putting one switch in the non-existent yet functional 'middle' position) which breaks the connection and doesn't keep one traveller wire energized. – Tyson Oct 27 '16 at 01:01
  • 4
    Do the switches have a light in them (to help you find the switch in the dark)? If so, you'll have to change them. Those work by letting a small amount of current through the actual bulb: not enough to turn an incandescent filament on, but enough to power the electronics in a modern LED (see also: http://diy.stackexchange.com/a/54111/157) – gregmac Oct 27 '16 at 03:58
  • This is a duplicate -- can someone find the previous question? – Carl Witthoft Oct 27 '16 at 14:57
  • There's something you're not telling us. The light switch is a dimmer, self-lighting, it has some sort of special feature in it that "leaks" power through the circuit to function. @Tyson I am skeptical that inductive or capacitive coupling can leak enough current to light a bulb even a little. That's also enough energy to shock, so to service any circuit you'd have to shut the whole panel off. – Harper - Reinstate Monica Oct 28 '16 at 04:57
  • 1
    @Harper we duplicated this effect on the shop floor using 14/3, leviton 1453 switches and rubber pigtail sockets and Feit Electric BR30 LED bulbs. Another brand bulb (I don't remember which) did not exhibit the issue. After Home Depot in our are sold the Feit BR30s in a 6 pack for $38 our service departments phone rang a lot for this exact issue. – Tyson Oct 28 '16 at 13:13
  • CRAZY. I'm gonna have to play with that myself. I had no idea usable power could be harnessed from capacitive coupling. – Harper - Reinstate Monica Oct 29 '16 at 00:54
  • @Harper there is not enough lumen output for the light to be usable, just enough glow to be noticeable in complete darkness. – Tyson Oct 29 '16 at 00:57
  • Well sure, but maybe enough to operate an LED "night light", reed relay or other sensor to tell me power is on. – Harper - Reinstate Monica Oct 29 '16 at 01:14

3 Answers3

3

It's NOT dangerous, but annoying.

It's floating/induced voltage that exists due to the travelers between the switches run in parallel. You can prove this by pulling just one travelers off a switch on either end of the switch circuit (or if standard toggle light switch putting one switch in the non-existent yet functional 'middle' position which breaks the connection and doesn't keep one traveler wire energized).

One answer is buy higher quality bulbs, another is just ignore it.

Tyson
  • 4,230
  • 1
  • 18
  • 27
1

This is a problem I experienced when I replaced an older style of light fixture with a new LED one. Even with the switch off, one row of LEDs in the fixture continued to glow in daylight.

My immediate thought was that there could be a wiring fault somewhere. I checked and found nothing wrong so I replaced the switch and wiring to no avail. So, it was time to get out the multimeter and trace what was happening.

Our wiring was set up in a standard fashion for when the house was built. Power cables (three, for live, neutral, and ground) come directly to the light fixture. Then a twin core cable runs from the fixture to the switch, so that the live wire can be switched on and off. I had already replaced the twin core cable going to the switch, along with the switch itself, when I encountered this problem.

The multimeter showed a voltage of about 130V on the return from the switch when not connected to the light (This is not normal, because our supply is 240V 50Hz) When connected to a load, the voltage dropped to about 65V, which was still enough to power one row of LEDs. Disconnecting the live wire from the switch had no effect. The wire could still provide enough power to light the LED even when not connected to anything. There was enough inductance over the 8 ft or so of the twin wire to partially power the light.

I could have just let the LED light stay partially on when the switch was off, as it would not be much of a power loss. But it occurred to me that it could reduce the life of the LED, so I decided to change the wiring. I set it up so that the source power first went to the switch (the wire was exactly the correct length for this!), and then run a cable for neutral, ground and the switched active from there to the light. It seems to work perfectly now. Theoretically, there could still be a little inductance as the two cables are still in the same wall cavity, but they are inches apart and only run parallel for a couple of feet.

It is interesting that there is enough inductance over 8 ft of twin cable to partially power a LED light. This shows that:

  • LED lights are incredibly efficient compared to any other lighting
  • There must be a lot of power loss in a house because of inductance (we have dozens of similar switch arrangements)

While talking to an electrician later that night he confirmed that inductance is a problem. By his estimate (verbal source only!) more than 5% of a typical homes electricity bill is due to inductance.

JACK
  • 80,300
  • 19
  • 72
  • 190
Paul
  • 11
  • 1
0

Another possible cause: Some bulbs use a phosphor to smooth out pulsation in the light at lower dimmer levels. That can result in a low glow for some time after being powered off -- atoms discharging rather than electric fields discharging.

keshlam
  • 28,994
  • 5
  • 48
  • 98