2022-09-18

Weird science

LED panel (when switched on)
I have LED lighting panels in my bedroom, two of them. Nothing complicated.

They each have a power supply behind them in the ceiling, and connect to the power with just live and neutral (no earth).

The live goes via a switch, obviously. It's a Shelly, so a clean relay contact.

So, when they are off, they should be off. Simple. They only have the neutral connected at that point.

The weird bit

However, during the night, I can see that they are not quite off. They have a faint glow some of the time. It can only be seen in the pitch black of night, and when your eyes have adapted to the dark. So if I wake up in the night I can see it.

At first I did wonder if I was imagining it, it was so faint, but no, I have seen them glow turn on and off, not fading in/out but sudden on/off. So very real. They are not glowing all the time, and not at the same time necessarily - sometimes one or the other. But most of the time as far a I can see, i.e. if I wake up in the night they are glowing. And frankly they are annoying me.

But with only neutral connected I don't see how this can happen at all!

The weirder bit

This only started happening the day my battery was installed. I have had these over 18 months and never seen then glow before, but they do now, every night I see them.

How can my battery install have triggered this?

The even weirder bit

The point the battery stopped powering the house
As I say, I now see this every night, well, except for last night. Last night I turned off the hot tub and allowed the house to run on battery all night for the first time. To be fair, I kept checking the battery charge in the night. Sad, I know. But yes, the battery lasted not only until sunrise, but until enough solar to run things and start charging it again. That means I have not used any power from the grid for over 24 hours, and in fact exported 9½kWh, even using the hot tub in the day yesterday. Amazing.

But, guess what, the lights did not glow last night, not once did I catch them doing so.

So they only glow, once the battery was installed, but only when it is in standby, not powering the house.

I repeat, there is no earth on these lights. And the extra earth rod for the battery install was done 6 months ago anyway. So all that changed when the battery was installed was, err, the battery and gateway box.

I may play with switching the neutral as well, but wow, why would I need to do that.

Explanations welcome!

Update

Usually the switch for a light like this would be L/N to the ceiling, the L going all the way down to a light switch, and all the way back, in the same wire/sleeve, to the ceiling, and then in to the light with N. This long wire acts as a capacitor allowing small amounts of AC current to flow.

But sorry. My lights are L/N to a shelly, L via the relay in shelly, and the L(switched)/N on to the light. No cable to act as a capacitor. Nice try twitter.

Further: We are planning to take up floor boards tomorrow (Friday) to confirm wiring is as I remember it, and try some things - I'll post more details here when I know. Thank you all for the ideas on this.

Final answer

Firstly, there is no switch lead, so capacitive pick up L to Switched-L was not the cause.

One person sort of got it right, a lively neutral. But not quite what you may think. In fact it was L/N tails to CU reverse, so all neutrals were very live and unprotected. The cable to the light therefor had a live (on neutral wire), and an earth (not used at light, but obviously connected to earth), and next to the earth was the neutral (on the switched live wire), which capacitively picked up from earth and power the lights.

Yes, there was a big flash and a bang in discovering this. No, I am not going to point fingers (was not me, obviously).

25 comments:

  1. The live wire, though an open circuit at the control end, acts as an (inefficient) antenna and can thus pick up energy that happens to excite it near its resonant frequency. It might only take a few microamps rectified through the LED driver circuit to produce such a faint glow. Adding the battery has changed the load profile of the DC-AC inverters associated with your solar installation and might therefore have caused some switching harmonics to shift into the effective range, some of the time. Adding a ferrite choke near the lamp end of the cable might help.

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    Replies
    1. Maybe, but this at night with battery on standby - neither inverter is running, just power from grid. But I'll have to try that (choke).

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  2. Probably due to leakage current on the Shelly. It is just enought to make the LEDs glow but wouldnt be noticeable for incandescent. Not sure why battery affects it (maybe power factor issues). A capacitor across live and neutral seems to fix it Many YouTube electricians mention it at times. See following from Thomas Nagy https://youtu.be/BpeBXrL6vDs?t=1512

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  3. Replies
    1. Except there is no G. Also, I cannot see how the shelly can have "leakage". It's a dry relay contact. Some capacitive effect is the best explanation I have seen so far, but again not clear why battery install could cause it.

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  4. A noisy neutral will do this.... it's effectively what ElectroBoom is doing in this video: https://youtu.be/KPNhNEqohVY

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    1. Agreed, I came here to suggest a lively neutral. It's not always as "neutral" as you might like.

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    2. Except there is no earth - a noisy neutral to earth would make sense if there was an earth. Also the neutral is well earthed.

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    3. Lack of an earth does not matter. My parents have one light fitting where there is no earth (to the bulb), and Neutral is switched rather than Live because of cable length issues. I can't put an LED bulb in it because it glows dimly when off, even though only connected to Live at the time.

      How lively "Neutral" is in any house depends on a lot of things. This is why mains sockets generally have double pole switches these days.

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  5. I have observed the same phenomenon in an off grid situation where the power source that was disconnected from the bulb was an inverter. I suspect it is due to inverter 's HF harmonics, capacitively coupled across the switch. A suitable filter should fix, but it never bothered me enough to do anything about.

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    Replies
    1. Except this is happening at night (solar inverter not on, and did not happen before, when we had solar not battery), and when the battery inverter is off. In fact when running off battery, so using its inverter, it does not happen.

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  6. You have a wiring fault somewhere in your lighting circuits - almost invariably it'll be on a circuit with two or more switches controlling the same lamp where the sparky/previous owner has wired the switches wrong.

    We had exactly the same in the previous house and it took bloody ages to find it. The fault wasn't apparent until a halogen bulb was replaced with LED - NB that wasn't the LED which glowed, it was in a completely different area.

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    Replies
    1. Nope, it is L/N to a Shelly and then on to light. New
      Light. No lead to light switch. Really simple.

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    2. In our case it was an earth loop elsewhere on the lighting circuit causing current on neutral. Bear in mind your lighting circuit is a loop circuit - ie its all chained together, not spurs.

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    3. Yes, but the current on neutral has to be relative to something (such as earth). When off, this light has only neutral connected, and has no earth. The best explanation is capacitive pick up on the switched live, but there is not the usual wiring with a live/switch pair to a light switch, where that would normally happen, so still a bit of a mystery.

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  7. I've seen the pictures you posted of lighting switches that you said the previous owners did. I'd be willing to bet you that you have a fault.

    Get a sparky in - a decent one, not some idiot with a £10 multimeter....

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    Replies
    1. Fortunately this is a separate circuit, which has already been re-done. There is not the usual loop of live down to a switch and switched live back, instead this is a Shelly in the loft, dry relay contact, switching live going with neutral to the light. There is not somewhere for the switched live to capacitively pick up from live. There is not something for a loaded neutral to bounce off earth, as there is no earth. I mean, it is tempting to put a scope on it. But the lighting circuit is fine (and was sorted by a proper sparky when we changed it all to use Shelly).

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  8. I suspect that this is just very long time constant on the phosphorescent coating. The LEDs driving the light will be blue and firing into a phosphor (typically yellow-ish) to get the wider spectrum of light you want to seem like white light. The emission from the phosphor follows a very long curve.

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    1. Nope, I have literally seen it come on and off in the night, not just “stay sim after you turn it off and fade”. I said in the post…

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  9. No mystery here, it is just the very long time-constant of the phosphor decay. The LEDs in the lamp will be blue, and will be exciting a phosphor (usually yellow-ish) to get the colour spectrum you want for white. The more you use the lamp the more the phosphors will be charging up.

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  10. My money's on capacitive coupling - it doesn't matter where the wires go or what they're connected to, they can still pick up just enough power to dimly light LEDs.
    To prove it one way or the other, put a high value resistor (say 510K 1/4W) across the mains supply to the LEDs at the LED end of the cable run and if it is capacitive coupling, the LEDs won't glow any more.

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  11. Just yesterday I fitted an OSRAM LED luminaire with a Shelly 1 in a 'bog standard' UK lighting circuit (albeit with no fewer than 5 FTE cables at the ceiling fitting) and I observe exactly the same effect. With the Shelly relay off, the LEDs light (very) dimly, and more annoyingly, I can hear a slight mains 'buzz', which may of course be coming from the Shelly itself.

    I have a second lamp + Shelly combination wired up ready to install, so I tried measuring the stray impedance across the relay, and from each relay contact to the live & neutral supply terminals. In all cases it was about 5pF + 1MΩ, except for live to the relay contact at the end of the terminal block, which is 13pF + 100kΩ. Both of which are surely too low to cause significant illumination?

    It's all very mysterious!

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    Replies
    1. And it turns out my problem was also L/N reversed at the Shelly. In my case, the wiring to the ceiling was OK, but I had fitted an extra terminal block as the block in the luminaire wasn't big enough to handle all 5 cables. The L/N in the jumper wiring from this block to the luminaire were crossed.

      My excuse is that I was in a hurry to finish off the wiring before I lost the daylight, as the whole thing had taken longer than planned (clearly a 5-minute job!) Naturally, the very last connection I wired up was the problem.

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  12. Are we perhaps looking at something from the weird and wonderful world of switched mode power supplies operating (spuriously oscillating????) outside their expected comfort zones?

    Oscilloscopes and meters might help (as mentioned), but a simpler first step might be a good old analogue radio tuned off-station being walked around the potentially relevant parts of the building a few times. Do it when the spurious illumination is present and (for comparison) when it is not present.

    (No I've never worked for or with BT/Openreach SFI and I've no knowledge of REIN dear).

    Have a lot of fun!

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  13. So is this also explained? "This only started happening the day my battery was installed. I have had these over 18 months and never seen then glow before, but they do now, every night I see them. How can my battery install have triggered this?"

    ReplyDelete

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