Power failure and LEDs

[kelLOGg]

Yesterday we had a power failure and 2 of our led nights were still burning although slightly dimmer than normal. One was one of 6 recessed bulbs overhead in the kitchen wired in parallel and operated by three-way switches. The other was in a floor lamp. Amazed, I got my volt meter and measured 0 volts at the lamp receptacle into which the lamp was plugged.  20 minutes into the outage the lamp went off.  I unscrewed the lit overhead lamp and it turned off, screwed it in again and it lit again. Unscrewed it again and it went off again and screwed it in again and this time it stayed off.
I'm stunned and amazed. My wife is my witness. Any clues?

[aigheadish]

I'm pretty ignorant on such things so I'd suggest trusting someone else more but I think I read, within the past couple months, a similar story and the answer was that there are sometimes capacitors on the LED boards that would hold enough power to keep things lit for a bit. Pretty amazing they lasted 20 minutes.

[Ljohnsaw]

I have 12 can lights in my kitchen.  There is one that stays on for quite a bit (very dim) when the lights are turned off at night.  I also have 3 can hall lights.  If I bobble the switch (big, flat kind) on-off-on, it will lock out two of them while the third will light.  I have to turn them off for a few seconds to "reset" them.

[Ianab]

I'd suspect there was some residual voltage still on the mains wiring, maybe induced voltage from nearby wires that were still live?

The individual LEDs are very power efficient, and unlike old school lamps, will light up with any tiny amount of current though them. An incandescent lamp needs almost full current to actually get any light, where a LED will produce at least some light at any current. Put 1 milliamp though a filament, it may as well be off. Feed 1 mA through a LED, and you will get light. 

This video investigates how LEDs can be powered from the radio signals from a cellphone, no battery required. The little circuit collects RF power and converts it into current to light an LED. They are actually sold as fingernail stickers, so when you use your phone your fingers twinkle. (Yes it's a "thing")

NFC powered flashing fingernail sticker (with schematic) - YouTube 

The weird staying on / then not restarting after removing the lamp? Weird, but if the drive circuit needs a bootstrap of voltage to start operating it may not start with that residual power. However if it's running, it may continue to run. Also some circuits will be more sensitive that others, so one will hog most of the power, and drop the voltage down to a level where the other lamps shut down.  So yeah, weird behaviour that can be very puzzling.  

The LED driver circuits do often contain capacitors, but they are only small. They would be discharged in less than a second, although that would be a reason some LEDs aren't instant off. Also the explanation for the LED's locking up and not coming up until the driver circuit is reset. It can get "stuck" in a situation where the bootstrap circuit doesn't function, and has to be left off for several seconds to discharge some capacitor so it's able to kick into life when the power is returned  We see this occasionally with computer power supplies, especially after a power glitch or brownout. PC won't restart. Remove power from the wall, count to 10 and try again. 

Big Clive also takes apart various LED lamps, just to see what the circuity actually is. 

[21incher]

Be careful.  You never know if it's a person down the road with an improperly wired generator.  

[moodnacreek]

Be careful.  You never know if it's a person down the road with an improperly wired generator.  

Anybody who knows how to back feed can [and must] put in a double throw switch.

[aigheadish]

Twinkle fingers sounds cool.

[Don P]

The backfeeding generator is a real thing, I live next door to that kind of genius.
 Reading Ian's post made me think though. When I walk around in the "dark" house, there are lights twinkling everywhere. When the line dies, all of that is discharging back to the panel at whatever rate for however long.

[Corley5]

LEDs don't like lightning.

[Ianab]

LEDs don't like lightning.

Very few electrical devices do   

[kelLOGg]

Very educational replies. Ianab, that was an interesting video too. 
Now I know that I was not crazy when I made the observation. 

The main breaker was closed when I observed the lights; I didn't think to open it in the event something downstream was causing the effect. 

I'm always super careful to open the main breaker when I connect my generator to prevent electrifying the grid. If I was a lineman and heard a generator running I think I would go check it out but no one ever has. 

[Ianab]

The lines crews usually attach grounding cables to the "dead" lines before working on them. That would ground out any stray or generator created voltages. See in the picture here how he has connected all the lines to each other and to an earth peg.  

The Importance of Cluster Bars While Performing De-Energized Line Maintenance.

Years ago some friends owned a farm with one of the main power pylon lines running through it. They wondered why the electric fences stayed "live" with the energiser turned off?  It was picking up enough current from the 220kv lines overhead to give you a small shock.  

It would also be safer to wire in an actual change-over switch, so you can't accidentally forget to flip the breaker open. A "Safety System" that relies on remembering to do something first isn't as safe it as could be, especially if someone else has to connect the generator one day. Doesn't have to be a fancy automatic switch, a manual one prevents you from back-feeding the dead mains. If you forget to throw the switch before you start the gen, then simply nothing at all happens, until you remember the switch.