LED tubes failure mode

[timhoward]

I just wondered if anyone else has seen LED tubes do this (2 different fittings)



I’m assuming this is a bad batch as this is apparently the 3rd to do this.

Unfortunately testing the wiring was a little difficult due to a right rats nest installed behind two fire-trip wires that shut off the gas.

 

[marconi]

If you look at the fitting is there a component located along the location of the tube damage which runs hot?

 

[timhoward]

If you look at the fitting is there a component located along the location of the tube damage which runs hot?

There is actually nearly nothing in the fitting at all. Just the lamp holders and a capacitor in parallel with the supply which is a good distance away.

These were originally fluorescent fittings and have been converted by someone else to run LED tubes.

(I've done a similar conversion at other sites and I'm having second thoughts about the wisdom of this now; if a faulty bulb starts a fire and the fitting was 'made' by me this carries more responsibility than I want!)

But anyway the ballast is removed, and the incoming supply is wired directly to the bulb holders, leaving the capactitor in place to absorb the in-rush current.

Not a great photo, but this demonstrates how minimalist the fittings are, and the effect this fault has had on the supply cable. I thought it was new wire colours for a second!
The protective device for the circuit is a Type 2 10 amp MCB by the way.
I can only assume it has been sitting just under the trip threshold for a long time?

 

[James]

The capacitor was originally part of the fitting to bring the power factor back to near unity, it has nothing to do with inrush currents.
they should be removed if the ballast is removed as one was to compliment the other.

 

[timhoward]

The capacitor was originally part of the fitting to bring the power factor back to near unity, it has nothing to do with inrush currents.
they should be removed if the ballast is removed as one was to compliment the other.

I realise their original function, but I did once find that turning on about 8 panels at once with 4 LED tubes each was tripping a B6 breaker, and reinstating the capacitors seemed to prevent this occurring. Was that unwise or messed up logic?

This further backs up my thought that I don't want to be messing around with converting fittings any more!
But in any case with respect to the tubes pictured above, it wasn't me what did it honest guvnor.....

 

[James]

I realise their original function, but I did once find that turning on about 8 panels at once with 4 LED tubes each was tripping a B6 breaker, and reinstating the capacitors seemed to prevent this occurring. Was that unwise or messed up logic?

This further backs up my thought that I don't want to be messing around with converting fittings any more!
But in any case with respect to the tubes pictured above, it wasn't me what did it honest guvnor.....

It was not unwise, to be honest I don’t know why it might improve (reduce) the inrush. when it gets as deep as this into theory, I think tho only thing to do is say HELP! @Lucien Nunes

 

[timhoward]

It was not unwise, to be honest I don’t know why it might improve (reduce) the inrush. when it gets as deep as this into theory, I think tho only thing to do is say HELP! @Lucien Nunes

An excellent idea, we rub the lamp and see if the genie is able to appear. I would completely understand if not, this may not be very high on the list for obvious reasons.

Primitive research suggests the capacitors should have increased inrush current not reduced it, so this is actually totally illogical!

While this is all interesting stuff, my main concern in this thread is that this customer has now had 3 LED tubes fail in a potentially dangerous way. I can't see how the fixed wiring or the fittings could be causing this.
Is it reasonably to conclude these are defective tubes?

I did ascertain that the entire reception area was all converted at the same time, and all of the tubes were purchased together. As a precaution we agreed to change them all. These are the tubes in question by the way:

 

[Lucien Nunes]

It looks from the pictures like the burn marks correspond to a line of LEDs (and possibly resistors along the same strip) within the tube. I could imagine that a progressive S/C failure of LED chips in one series string could have diverted excess current into that string and the ballasting resistors have overheated the plastic. The only odd thing is that where it has burnt right through, it looks hollow behind, but perhaps that's an illusion. It would certainly be interesting to see the tube cut open.

The damage to the cables at the terminal block does not strike me as related. If that red insulation has discoloured through overheating caused by overcurrent, then the current is much too high to be the results of the tube fault(s) and also for a B10. Even if it's 1.0sqmm, that would take 20-30A to cook it like that. Is it possible that it was caused in the past by exposure to the heat of a wirewound ballast that is now removed? Or is it discoloured without being overheated, due to exposure to smoke from the smouldering plastic of the tube? It looks like the CPC sleeving is discoloured too which would more or less rule out overcurrent.

The capacitor business is interesting but without knowing the exact circuit of the driver within the LED tubes, it's difficult to say what is going on. The original function of the capacitor was power-factor correction, as a fluorescent fitting with wirewound ballasts has heavy lagging pf, and that doesn't apply to the replacement LED tubes. However, low pf can result from two different effects: displacement (where a linear but reactive load draws a sinusoidal current that is out of phase with the voltage, e.g. wirewound ballasts) and distortion (where a non-linear load draws a non-sinusoidal current.) It is possible that the LED tubes have distorted current waveforms containing significant harmonics, and although the capacitor is not correcting for a lagging pf it is nonetheless compensating for that and reducing the effective circuit current for a given load wattage. All would be revealed with a scope.

 

[timhoward]

@Lucien Nunes as usual I wish I could press "like", "thank you" and "informative" all at once....

The damage to the cables at the terminal block does not strike me as related.

I popped in today and found another fitting with similar condition cable so it's indeed the right conclusion.

I've found product recalls for a few LED tubes citing similar problems but not this brand. Interestingly it seems these ones aren't on the market anymore.
This job might be the one that finally pushes me to purchase a thermal camera.

I met the facilities manager today and they are now checking every fitting in the building and replacing any of this make. It seemed the only sensible solution.