Why no Earth connection to LED streetlight?

Hello,
1:52 of this video shows no earth connection to a 12W LED streetlight.
Why is the teardowner so particularly worried about this?

https://www.youtube.com/watch?v=8dxqRiXFGC8

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Thsi is also about LED streetlights.....

From 15:05 of this video onwards, you can see that the LED streetlight circuitry is covered in silicone...

https://www.youtube.com/watch?v=KKd2L9Exw0M
..this is to stop it getting afflicted by damp and moisture, Do you also believe as I do that it would actually also keep the componentry very slightly cooler than if silicone covering had not been used?
The streetlight enclosure has a rubber gasket and so why are they so worried about moisture?

Why is the teardowner so particularly worried about this?

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He needn't be. If all the internal electrics are sufficiently out of reach from outside no Earth is required. In any case, the post itself would be set in the ground with a lowish resistance to Earth, at least good enough to pop a trip somewhere if a short occurred.

..this is to stop it getting afflicted by damp and moisture, Do you also believe as I do that it would actually also keep the componentry very slightly cooler than if silicone covering had not been used?

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About 99% of the heat is conducted back to the substrate, some of the remaining 1% will still be conducted through the sealant and carried away by convection or be radiated. It isn't a big problem.

The streetlight enclosure has a rubber gasket and so why are they so worried about moisture?

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Don't think of moisture as being direct wetting per se. You get a pumping action as the enclosure heats up and cools down. As it heats, the air inside the enclosure expands and the pressure builds up, inevitably some air leaks out around the gasket and wiring entry glands. When it cools down again the reverse happens, humid air gets drawn in by the vacuum and condensation forms. Over many cycles, and there will be at least one per day, they can get pooled water in them. There is some merit in leaving a vent hole in position where direct ingress isn't possible.

Brian.

Thanks, also it is often said that having a large earthed enclosure round the electronics reduces noise, would you agree? {(i refer to "being earthed" by an earth wire, and not the fact that its a metal post sunk deeply into the ground (earth)}

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Don't think of moisture as being direct wetting per se. You get a pumping action as the enclosure heats up and cools down. As it heats, the air inside the enclosure expands and the pressure builds up, inevitably some air leaks out around the gasket and wiring entry glands. When it cools down again the reverse happens, humid air gets drawn in by the vacuum and condensation forms. Over many cycles, and there will be at least one per day, they can get pooled water in them. There is some merit in leaving a vent hole in position where direct ingress isn't possible.

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Thanks, that rings the bell of good sense to me, that’s about the best explanation I’ve ever heard of the moisture problem in outdoor lighting.
When I worked at a LED beacon company, nobody could agree whether to conformally coat the PCBs or not…..we were getting failures of unknown cause, and people thought it might be moisture…as you know, the problem with conformal coating is that it does slightly result in heating the components up more…specially those that don’t have a mode direct connection to the heatsink.
I wonder if it’s a golden rule that all outdoor led lighting should be conformal coated, or coated in silicone?

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Don't think of moisture as being direct wetting per se. You get a pumping action as the enclosure heats up and cools down. As it heats, the air inside the enclosure expands and the pressure builds up, inevitably some air leaks out around the gasket and wiring entry glands. When it cools down again the reverse happens, humid air gets drawn in by the vacuum and condensation forms. Over many cycles, and there will be at least one per day, they can get pooled water in them. There is some merit in leaving a vent hole in position where direct ingress isn't possible.

Click to expand...

Thanks, that rings the bell of good sense to me, that’s about the best explanation I’ve ever heard of the moisture problem in outdoor lighting.
When I worked at a LED beacon company, nobody could agree whether to conformally coat the PCBs or not…..we were getting failures of unknown cause, and people thought it might be moisture…as you know, the problem with conformal coating is that it does slightly result in heating the components up more…specially those that don’t have a more direct connection to the heatsink.
I wonder if it’s a golden rule that all outdoor led lighting PCBs should be conformal coated, or coated in silicone?

I agree with betwixt for the most part. I suspect with that much grease and low pressure on the LED against the MCPCB the conduction is less than 99% and more like 95% worst case with convection being 5% while conduction loss being less than ideal for a MCPCB with not being substrate soldered but rather edge lead.

REMEMBER the measure of how good a thermal conductor is , how not it gets not how cool the case is at 20'C ambient but how hot the junction rise is above outside ambient, which is easily measured as all Diodes are thermometers when used in pulse mode while being LEDs the rest of the time. The case rise of 20'C is not affected by the contact pressure and excess grease and only affects the inside junction temperature.

Also REMEMBER that all plastic and silicones are insulators unless contain metal conductors like zinc, silver etc.

Zinc Oxide has a thermal conductance about 1000x air so any air bubble or void means lost surface area and any excess paste means less conductance as zinc oxide paste is also an insulator when excess thickness. Normally the ideal thickness is only to displace air caused by gas bubbles , voids or lack of coplanarity, twist,warp gougues, contaminants, scratches which for a CPU &LED heatsink should be less than 2 um but often much much larger. Thus each multiple of this thickness is proportional to the insulation property of the thermal grease.

This is why there is a temperature difference between the body of the LED and the body of the heatsink.

Hopefully tHe LED does not have silicone underneath it for thermal conductance but perhaps something thermally conductive.

(snip) Example: IP65 (NEMA 4)
The two digits represent different forms of environmental influence:
• The first digit represents protection against ingress of solid objects.
• The second digit represents protection against ingress of liquids.

In this case silicone is an OK but imperfect protector of liquids because interlayer creapage can occur and still oxide the interface and rust. But at least it slows it down.
Just look at any bathtub and see if it lasts forever. (Not)

In order to be IP65 for dust and moisture it is 6 for dust and 5 for moisture
5 Limited protection against dust ingress. (no harmful deposit) Protected against low pressure water jets from any direction. Limited ingress permitted.
6 Totally protected against dust ingress. Protected against high pressure water jets from any direction. Limited ingress permitted.

If the metal housing is connected to the lamp-post;
a) the access hole needs to be grounded from the lamp
b) the access hole needs to seal from dust
c) the access hole needs to seal from moisture

I suspect the problem with a seal is pressurization limits with silicone but it should have some moisture protection prevent moisture creapage into the LEDs for longest life.

I'm not sure but the lenses sounded like glass, but were probably plastic floating so they would not melt from the LED heat.

it should read|" it is not how cool the case temp rise is at 20'C ambient but how hot the junction rise is above outside ambient," either way poor heat transfer LED to board , the outside case will be the same

and if I did not make the point clear The LED board does not need an earth ground wire, the lamp pole does that automatically. by housing connections It does however need at least 10kV insulation for lightning protection and probably better CM rejection in case of a lamp stand hit by lightning to raise the CM impedance and thus attenuate induced current by at least 100dB from 10kA, otherwise it must be replaced once struck..

I suspect with that much grease and low pressure on the LED against the MCPCB the conduction is less than 99% and more like 95% worst case with convection being 5% while conduction loss being less than ideal for a MCPCB with not being substrate soldered but rather edge lead.

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Thanks, so you think it is much worse in terms of led Tj rise because the thermal pad of the led is not soldered to the mcpcb, but instead is just "thermal greased" to the pcb?...how many degrees worse do you think?.....because the "edge lead" only method looks really handy for replaing the leds in case of failure....those leds look easy to hand replace.

Try it with any power Diode and put 1 Watt thru it and measure Tj rise with pressure by measuring the Vf when you pulse off the current from say 1.2A *0.8V on a 1 Watt Silicon Diode then measure Vf at near <1mA such that ESR has a fixed very low current thus reading temperature.

So convert Vf (1mA) to Tj and only solder the leads but with some silicon 1 diode with edge leads with comparable surface area to any heatsink and measure force ( some weight vs Tj rise above ambient from the diode voltage using say 100Hz 99% duty cycle from 1.2A or 1W . Then plot Force vs Tj rise and see for yourself. THen experiment with soldering edge lead Silicon diode or LED while pressing with some hard silicon coated finger and thermal grease in between to see what Rth you can achieve with the resulting Tj rise and see how that shifts from CTE material differences in an oven or on a hot plate after applying heat to see if the LED contact force reduces and thus the Tj rise increases then report back your results.

Got it?

You should understand now how to measure Tj in any LED from above test method or similar and how to measure Contact force vs Rth thermal resistance. Remember Air has a Rth 1000x higher than zinc oxide -silicon grease so any gap squeezed by pressure to reduce the air content will be like biasing a logic FET for RdsOn.

this experiment should take about an hour with skill and a day for the 1st time.

Either buy some LEDs or use AWG 30 magnet wire on a 1W LED instead to simulate the similar 1W LED not solder bonded to the heat sink. Then use a kitchen milligram scale to measure weights. YOu may have some Schottky Diodes with a DO-219AB (SMF) package in stock that will work too.

Hello,
This is related to teardown of LED streetlights , so please may i post it here?

Why has this LED streetlight not got a seal where the electric cable enters the product?
...please see 12:57 onwards of this video...
https://www.youtube.com/watch?v=1ZiB9rLTSRo

....Surely flys, insects and moisture from fog can get in and short out the electronics.?
Do you recomend a rubber grommet coated in silicone paste to seal it?

I suspect the pole mount may have a grommet or at least a tight fit or could be added. The pole could be grounded by the screws but electronics grounded by wire for unshared earthing, which may be what you want for a lightning strike,

With 24 Watts it will fry any bugs from the internal heat. Gnd is optional and isolated for a reason, so that pole becomes a more likely conduit for a strike. An open pipe may allow additional convection air cooling from the inside. With excessive grease and insufficient screws for a thin alum clad board, there will be warp and suboptimal heat transfer. xx um conductor gap variation can make a difference in CPU heatsinks. The case temp should be higher than 20'C for this small area, indicating the LED junction temp is likely scalding hot after 1hr

I like the lens design. Everything else looks ok to me.