Lots of failing outdoor LED lights...long mains cable

[Flyback]

Hello,

We have installed one hundred 25W outdoor LED lamps in a town in Bosnia. Sixty failed in just a few weeks.

They all are fed from a single 240VAC generator which delivers power to the 2500 metres of mains cable. A contactor near the generator is used to switch them all on/off. The lamps are fitted every 25 metres along this cable. The cable is overhead (not buried). It’s aluminium cable. Its twisted, though the installers tell us that in places the twisting is “not so tight”. We suspect there is an awful lot of mains stray wiring inductance.

The strange thing is that they have another run of one hundred of the exact same lamps, but in this case the 2500 metre mains cable is buried in the ground. –None of these lamps with the buried cable have failed despite being installed months ago.

The lamps are controlled by microcontrollers in them which slowly dim the lamps as the night goes past the small hours, so as to save energy when few people are really outside.

We suspect that maybe there is some kind of transmission line effect going on, with the long mains cable. We have advised the installers to fit an RC snubber across the contactor, and another across Live/neutral, just downstream of the contactor. We also wonder that with the “not so tightly twisted” mains cable, maybe noise is getting into the long overhead mains cable, and disrupting the microcontroller.

The lamps are not SMPS based, but instead are sucessively switched linear current regulators. There is no mains input filter in each lamp, and the input stage is as in the attached. (ltspice sim also attached, including ZR431 model)

May I request what are your thoughts on why they have failed?

 

[Colin]

How many LEDs in each lamp? How many volts across the LEDs

Replace each damaged lamp with a different model.

 

[Flyback]

Thanks,
We are going to ask the installer to put small boxes across live/neutral. In these will be a series combo of TVS and 32mA quick blow fuse.
The TVS will be SMCJ400CA.
In another box we will have two series SMCJ250CA TVS's in series with a fuse. (this gives a bit higher voltage detection than a single SMCJ400CA)

....If the fuse blows, then we at least know there was an overvoltage transient
Do you think a lighting strike to one bit of the overhead cable would take out say 30 lamps either side of the strike point?.....i am guessing it would?

 

[Colin]

You obviously don't have a clue because the circuit works on pulses and turns off when the current reaches 700mA, so a 32mA is not going to tell you anything.
I have been working on projects that take huge currents from the mains for only a fraction of a cycle per second to about 10 cycles of the 50 cycles in the form of fridge doors in a run of 20 doors and this puts enormous strain on the supply for fractions of the cycle.
The same thing is happening with your 100 LEDs and it could be 50 amps for part of a cycle. This will upset a primitive generator and may produce spikes that get into the LEDs. That's why I have asked you the first and most important question.
Going by your figures, you have 25watts and 700mA so there is 35v across the LEDs and this makes it 10 x 2.5watt LEDs.
These LEDs are directly across the mains and any hiccup will put extra current through the LEDs and blow them. It would be much better to have a capacitor-fed power supply and some protection resistors.

 

[kubeek]

Is the generator loaded by anything else other than the LEDs? Generators are notorious for high voltage spikes and often have poor regulation. So I expect that your OVP is not working as it should.

 

[alec_t]

Do you think a lighting strike to one bit of the overhead cable would take out say 30 lamps either side of the strike point?

Yes.
Since the only difference between the two cables seems to be that one is buried and the other is not, then presumably they have similar inductance values. If the two generators are similar and causing lamp failure then logically you would expect both lamp sets to fail. That hasn't happened, so I think that rules out the generator and leaves exposure of the non-buried cable to lightning strikes or strong interference as the cause of the failures.

 

[ChrisP58]

It sounds like the two groups of lamps were installed at different times.

Were they manufactured at the same time? With identical PCBs, components and software revisions?

Have you looked at any of the failed units yet to see what parts of the circuit is failing?

 

[Flyback]

It sounds like the two groups of lamps were installed at different times.
Were they manufactured at the same time? With identical PCBs, components and software revisions?
Have you looked at any of the failed units yet to see what parts of the circuit is failing?

Thanks,
Yes they are identical in all ways.
It will cost a lot to ship then back.
Nobody wants to go there because we know from some of our ex-servicemen that Bosnia is lawless….memories of civilian workers wandering out of the camp and being brutally butchered and killed etc.

Since the only difference between the two cables seems to be that one is buried and the other is not, then presumably they have similar inductance values. If the two generators are similar and causing lamp failure then logically you would expect both lamp sets to fail. That hasn't happened, so I think that rules out the generator and leaves exposure of the non-buried cable to lightning strikes or strong interference as the cause of the failures.

Thanks, they even swapped the generators and the lamps fed by buried cable still did not fail. So yes it does seem to rule out the generators…unless its generator voltage spikes inter-acting with the greater mains stray wiring inductance of the overhead wire. I believe underground cables are more capacitive (?)

You obviously don't have a clue because the circuit works on pulses and turns off when the current reaches 700mA, so a 32mA is not going to tell you anything.

We are going to ask then to put the TVS/fuse ccts across live/neutral so that we can proove that overvoltages are occurring...at the moment they are saying our lamps are bad...but we think there are overvoltages....and they dont yet seem to agree.....so the TVS/fuse devices will proove the prescence of overvoltages.

 

[alec_t]

I believe underground cables are more capacitive (?)

Have you found out what cables are actually being used overhead and underground?

 

[Flyback]

Sorry no, they dont have the part number for us...they just tell us that he overhead one is aluminium and is twisted. It was some kind of general cable used for lighting cables, something like "ABC" ..."something Bus Cable".

Is it possible for lighting to strike the ground near to outdoor streetlights, but not kill them..just "weaken them", so that they fail some weeks later?

 

[alec_t]

A near-hit from a lightning strike might well involve circuit overloads, which could cause component degradation. I've no idea how long it might take after that for complete failure to occur.