Separate Earth connections to group of 150W offline led lamps? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: March 30, 2018, 06:47:08 pm
content: Hello,We are selling a number of our 150W offline 230VAC LED lights to customers.Our LED lights are all linear regulator based (yes, really), but do have a 1W Buck bias supply in them.All the components are SMD and the PCB sits on an earthed heatsink (on a thermal pad).There is no earth connection to the actual PCB, but we do connect the earth wire to the heatsink on which the PCB sits. This earth wire comes in the cable which also carries live and neutral.However, this particular customer will be supplying our Lamps from twin core Live/Neutral cable….there is no earth conductor in the customer’s cable. The customer has asked if they can just separately connect the earth to the heatsinks?…ie running “lone” earth wires up to the heatsinks……that is, not an earth wire that runs with the live/neutral conductors inside the sheath of the cabling itself.…What do you think of this?……we have never before supplied our products with anything other than 3 core cable containing live/neutral/earth.Will we be running the risk of noise/susceptibility issues with these separate earth wire connections?

author: capt bullshot
author_rank: Super Contributor
timestamp: March 30, 2018, 07:08:36 pm
content: Oh, I really like this kind of customer questionsIMO, there's only one valid answer: Use the lights as specified, and they'll work as specified. Anything else may work or not and is on your own risk. Please don't ask or tell us about it.Yes, I can overhear conversations like this every so and so weeks or month, each time the customer wants to do something that isn't specified and asks if he can. And then there are people that believe answering these questions would help selling stuff and they go and bugger a developer. If he says no (see above), they bugger the next developer until they find one who says maybe, or "I'll have a look on it". Usually this kind of stuff turns into some kind of boomerang, hitting you somewhen later when something unexpected happened, maybe involving costs larger than the sale. But people refuse to learn from that, so they'll happily come back next week with next customers question.

author: dmills
author_rank: Super Contributor
timestamp: March 30, 2018, 07:41:26 pm
content: Hey we have that guy as well....Never worth playing that game (But you try to convince a salesman of this!), they are the systems integrator, it is their problem not yours.Have you managed to kick that thing thru EMC and immunity testing yet?Regards, Dan.

author: ocset
author_rank: Super Contributor
timestamp: March 31, 2018, 03:41:48 pm
content: thanks, we're getting there, not completely through yet though.

author: ocset
author_rank: Super Contributor
timestamp: March 31, 2018, 03:43:40 pm
content: I actually suspect that if our product was totally smps based, and we had hundreds of them being supplied in a small space, with all having separate earth wires, then i suspect a major RF disturbance "storm" type event would take place....and nearby equipment like phones and stuff would all go wrong. Do you agree?

author: dmills
author_rank: Super Contributor
timestamp: March 31, 2018, 04:18:29 pm
content: Depends on if you do it right or do it crap!Anyone who has worked in the entertainment lighting industry will have used hundreds of switched mode supplies in close proximity, it is not a problem provided you actually do them right and don't cheap out.The common problem we used to see with LED based kit was cumulative earth leakage currents getting large enough to take out RCDs, you can hang a lot of 100W switchers on a 32A supply, and when they each leak 500uA (Led screen vendors, looking at you, also pixelline)....The cure was a isolation transformer to produce a separately derived supply (Earthed neutral in the obvious way) and then many individual RCDs on the output.The other gotcha (But more common with early discharge fixtures, Martin....) was triplen harmonics causing neutral overloads and excessive heating in delta/star transformers (And alternators).Nothing that should be a surprise to anyone working in the game.Regards, Dan.

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2018, 07:51:55 pm
content: Politely tell the customer that using a three wire cable is strongly recommended and that doing as they suggest can create EMC and potentially safety issues.If you have an alternative power supply, which doesn't require an earth connection or can get hold of a suitable one, then recommend that.

author: ocset
author_rank: Super Contributor
timestamp: March 31, 2018, 10:26:03 pm
content: Our LED drivers do work without an earth connection. Since the earth doesnt go to our PCB....earth is only connected to the heatsink. And this is only for safety so that should (goodness knows how) the live somehow connect onto the heatsink, then the fuse would blow.But realistically, i dont see how the live could connect to the heatsink, so i reckon we could get away without earthing the heatsink at all.I mean, our PCB sits on top of the heatsink, but it is separated from it by one of those rubbery thermal pads which is electrically insulating.....so i dont see how live can get connected to the heatsink..........so i reckon realistically we can do away with the earth connection to the heatsinks.What do you think?I mean, in the lab, we always run the product without bothering to earth the heatsink.

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2018, 11:05:17 pm
content: Quote from: treez on March 31, 2018, 10:26:03 pmAll mains appliances will work without an earth connection. It's only required for safety and on some occasions EMC.QuoteIf the heatink can come into contact with the user or another metallic part which the user might touch, it needs to be earthed, unless it is isolated from the live parts by reinforced or double insulation. The insulation needs to be good enough to withstand a high voltage (8kV for 10 seconds, if I rfemember rightly but look it up), be mechanically robust and meet certain minimum creepage and clearances, otherwise it needs to be earthed.

author: dmills
author_rank: Super Contributor
timestamp: March 31, 2018, 11:29:40 pm
content: Treez, how is this even a question on an internet forumAsk your test house to certify the thing as suitable for use in a class II product without the customer having to supply external secondary insulation around the heatsink, if they say no then the heatsink needs earthing, simple as.Note that you will also probably need to re run your EMC tests in a class II configuration.Regards, Dan.

author: Nusa
author_rank: Super Contributor
timestamp: March 31, 2018, 11:45:06 pm
content: As I read the question, it was: Can I connect directly to local earth instead of running a wire from the source earth?Whether it meets testing and regulations is a regional subject I won't touch. However, I'd think it would satisfyactual safety concerns.That's a very different question than: Do I need to connect earth at all?Which is what much of the above seems to have jumped to. Legally you probably aren't in a position to endorse such a thing even if there's really nothing seriously wrong with it.

author: Eka
author_rank: Regular Contributor
timestamp: April 01, 2018, 04:26:06 am
content: Quote from: capt bullshot on March 30, 2018, 07:08:36 pmIt is the only answer the the question. To give any answer that says it is OK is to accept liability for messed up customer installs.