How do you find out that 8mm is min creepage distance in offline isolated SMPS? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: August 12, 2017, 04:31:14 pm
content: In an offline, totally enclosed SMPS of 60W for domestic product use in UK and EU and Australia and USA, the creepage distance along the PCB from primary to isolated secondary needs to be >8mm. The clearance distance needs to be >5mm.How do you find out which section of which UK/European standards document formally states this?..This brings up a point of interest….i know these creepage and clearance values because of being around for over a decade in the industry….however, if you asked me which section of which regulation standard document I need to look at in order to prove this, -then I would be lost.In fact, I defy anybody to be able to quote here what is the exact sub-section, section and regulation standard document number which tells what is the safety creepage distance along the surface of an offline SMPS PCB for UK/EU/USA/Australia domestic product of power level <60W. I guarantee that nobody here will be able to do this.Of course, the huge Corporations can afford to pay an internal department of “standards engineers” to keep abreast of all the standards so that their products conform to all of the multitudinous regulations. Also, they are able to, shall we say “garner the attentions” of the standards institutions so that they can be sure of acquiring the knowledge needed for conforming to the standards.Small start-up companies simply cannot afford to do this. These small startups not only cannot afford to buy the standards…but also, they wouldn’t actually know which particular multitude of the standards that they must conform to.And heaven forbid, should a small company accidentally introduce a product into the market that doesn’t conform fully to the standards, then the huge corporations would get one of their highly payed lawyers to ensure that small startup was sued for this. –Putting the startup well out of business. Destroying the competition.Some say that the standards are simply “common sense”. However, I beg to differ. For example, exactly why is 8mm of creepage needed on a totally enclosed offline 60W SMPS PCB for domestic use? Why wouldn’t 4mm be adequate? 4mm is well above the flashover distance of any overvoltage transient that might occur. For such a PCB placed inside a totally enclosed plastic enclosure with no vent holes, 4mm of creepage would be absolutely safe and adequate…….yet as we speak, small startups are being sued into liquidation because they simply cannot afford the activity needed to ensure total standards conformance. They are being sued because they only have for example, 4mm of creepage where the standards, quite non-sensically, say 8mm is needed. 4mm would have been perfectly safe.The big corporations win here…the small startups die off, and the jobs go with them.Has anybody tried to read the standards?...they are written in such a convoluted way that the vast majority of engineers would struggle to fully & accurately interpret them...let alone be cognisant of exactly which of the multitudinous number of standards they actually need to conform to for any particular product...and if small companies should make a mistake and fall foul of a standard...then the lawyers of the huge corporations are circling overhead, ready to sue them into non-existence.Of course, the lawyers of all the big European and UK institutions are too scared to sue the Chinese…..China is big and powerful so they daren’t sue the Chinese……hence we in UK have loads of literally lethal Chinese products flooding us……such as this lethal LED lightbulb…Here's another one...Here's what happens when your country imports fridges that catch fire.......all the UK’s own companies got closed down because they made some much lesser standards infringement, and were sued out of existence.

author: Benta
author_rank: Super Contributor
timestamp: August 12, 2017, 08:43:52 pm
content: If your blood pressure has returned to normal by now, what's your question?

author: coppice
author_rank: Super Contributor
timestamp: August 12, 2017, 10:21:17 pm
content: The requirement used to be 4mm clearance across an insulating surface, and it proved to be inadequate far too often. That's why it was increased to 8mm.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 13, 2017, 03:25:24 am
content: I don't find the standard documents especially hard to read. They are long and detailed and finding the section you care about takes a while, but safety and electronics are both complicated. Yes, it is technical, and it is work, but it isn't dramatically different from reading data sheets. I think people who say they are so hard to understand are thinking of it as "administrative overhead." But it is not -- if you are designing electronics that will be plugged into mains or subject to high voltage, electrical safety is part of your job, just like all of the other performance requirements. I imagine it would be hard for a person without high voltage electronics training or experience. But reading an opamp datasheet is hard or impossible for someone who doesn't know anything about analog electronics.We could have simple requirements, but that would be no fun and no good-- they would be overkill / worst possible rating. Medical, office, outdoor, sealed, avionic, applications all have different requirements. Live-neutral is different than live-ground. Different insulation materials have different properties. Manufacturers and consumers would *hate* it if they had to design every device to the worst case.No knowledgeable person says the regulation are common sense. It is common sense that you need some creepage spec, but there only way to figure out what it should be is measurement and experience. People who think safety is just common sense are a hazard to themselves and those around them.I do agree that we should crack down on non-compliant imports and fraudulent certifications. That part I agree with you 100%. It is dangerous and unfair to people who do play by the rules.

author: floobydust
author_rank: Super Contributor
timestamp: August 13, 2017, 03:54:37 am
content: Where is this 8mm spec, in what standard?I find that large for a 240VAC mains SMPS; I only encounter 8mm+ in HazLoc.The standards are much simpler if you understand the intent of the clause, and are not filled with hate towards making safe products.I learned that after working with many certification agencies, their senior certifiers use that approach.A literal interpretation of a standard (clause) is only of so much use.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 13, 2017, 04:38:36 am
content: To complicate matters more, creepage tables ususally go hand in hand with pollution degrees. So its not just a number but a choice of them depending on application, security level...Google images for 'creepage pollution degree'

author: Tomorokoshi
author_rank: Super Contributor
timestamp: August 13, 2017, 04:43:50 am
content: Let's consider a standard such as NFPA 70 (National Electric Code, or NEC) in USA. It's almost 1000 pages long and doesn't cover your case. However, and while this is notliterallytrue, it's useful to think about each clause being in a standard like that because someone got hurt or something burned up, with the standard developing over time to address those cases until some amount of reasonable safety is met.The wide range of additional industrial standards cover the specialized cases of construction or application. In my case we have a conversation of the expected approval agency before getting too deep into a product cycle with a new direction to determine the most likely applicable standards. This is done for both safety and EMC. It doesn't eliminate, but it does reduce approval risks.These upfront costs are difficult to justify for unenlightened management, for there is no return. It's only an unrecoverable cost, even though the potential for costs are very high if design proceeds without properly understanding the standards and the approval process.So where might your 8 mm creepage distance come from? For what it's worth, in my 2012 copy of 61010-1 Table K.4 lists 8 mm most of the way across for > 300 < 600. Now, whether or not this is where your number comes from, or if it's even the right categorization, or if it's the right standard, is where it may be useful to consult with a local agency.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 13, 2017, 05:05:52 am
content: Quote from: Tomorokoshi on August 13, 2017, 04:43:50 amThat number you mention is the CTI, comparative tracking index or sometimes found as the materail group. It takes into account that not all dielectric materials are the same and some will form a conductive path sooner than others. Here its all about the surface, and the test involves closely spaced electrodes in the sureface of the material where drops of conductive liquid are applied in between them

author: Tomorokoshi
author_rank: Super Contributor
timestamp: August 13, 2017, 05:15:27 am
content: It looks like several standards reference IEC 60664-4 when it comes to creepage and clearance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2017, 02:45:49 pm
content: Quote from: treez on August 12, 2017, 04:31:14 pmWell, if you'd stop projectingyour own ignoranceupon everyone else, sit down, and brew a cup of tea, and think, instead, you'd realize that those standards are easily found.Proper purchased copies of them are not burdensome even to small companies. If your margins are so razor-thin that you can't spare $1000 for a standard*, your business approach needs to be re-designed, not your products.*And I think that's more about UL standards. IEC may be cheaper.As for quoting section, I don't know domestic rules offhand, but I am familiar with IEC/UL (harmonized) 60950-1. The corresponding section you are referring to there is Section 2.10, and Table 2H. You may find this especially handy:http://www.creepage.com/but again, purchase the standard and read it head to toe before you end up with a safety issue in that market (60950 is for commercial ITE and is often used for office, server, and medical equipment).Tim

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2017, 04:36:22 pm
content: Where the heck did you get 8mm from? The distance depends on the environment, moisture, pollution, how likely there is to be an over voltage. I don't have the relevant standard handy but I'm pretty sure it's less than 8mm of creepage between the mains and user, for reinforced insulation, on a PCB in an indoor, domestic environment. If I remember rightly,8mm is the creepage distance specified for a board used in an industrial environment.No doubt someone else here will have access to the relevant standards and will be able to clear this up.

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2017, 09:47:41 pm
content: I've just done some Googling.IEC60950-1 says 5mm creepage, worst case for 250V, pollution degree 2, material group IIIa or IIIb.http://www.pcbtechguide.com/2009/02/pollution-degree.html#.WZDCyquQwWMSo what's pollution degree 2?Quotehttp://www.pcbtechguide.com/2009/02/pollution-degree.html#.WZDCyquQwWMWhat does material group mean?https://en.wikipedia.org/wiki/Comparative_Tracking_IndexNote, I've found different figures on other sites. I've only posted figures I've found which are consistent across multiple sites. If in doubt, refer to the proper standards.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2017, 12:02:48 am
content: FR-4 is usually the worst (IIIb). In other words, it carbonizes quickly!Tim

author: floobydust
author_rank: Super Contributor
timestamp: August 14, 2017, 12:09:39 am
content: Even if 8mm is the creepage distance OP requires, there are ways to use less:Lower the pollution degree or lower the over-voltage category or use PCB slots or conformal coat the PCB etc. etc.I can't find this big 8mm number in IEC 60664, 60730, 60950, 61010 even under double-insulation for 240VAC

author: CopperCone
author_rank: Super Contributor
timestamp: August 14, 2017, 08:25:33 am
content: the best way to understand this is to get a high voltage high energy source and plug PCBs into it.May I recommend playing with a boiler transformer? If done safely it will give you a very good visual indicator of requirements.I recommend trying it both ways. playing with high voltage will illuminate many things for you.btw its not nearly enough, if people cared you would have HV construction on these things, teflon cans, potting etc. its a reactive not a proactive standard.if you really wanted to do protection right, stuff you buy would have little protection modules that you need to replace every so often, in the end tho you would need to monitor your power grid all the time to count transients,energy absorption by electronic devices.... otherwise its just guess work.a little like baking soda for the refrigerator. or galvanizing steel. over time the sacrificial elements degrade. Makes me wonder if you can put windows on some protective elements so you can visually inspect them for degradation (i.e. MOV)... some things seem difficult without extensive equipment though. You can probably analyze fuses in a x-ray crystallography machine to detect probable failure due to thermal cycling.. it seems that active devices would require an electron microscope however.you can probably look for pitting on the surface of a discharge device (gdt) to determine its ability to perform.diodes/special elements seem the most difficult. most failure analysis I see is done with a electron microscope.but hey, its the bottom dollar, fuck yo house and yo kids come bonus time

author: VK3DRB
author_rank: Super Contributor
timestamp: August 14, 2017, 11:15:01 am
content: Quote from: blueskull on August 12, 2017, 08:59:25 pmClearance in China is how much hoarded money the communist leader bourgeoisie can clear out of the country illegally to buy trophy homes in the west or hide in Swiss bank accounts. The biggest creepage in China are the creeps who run the country.

author: FlyingGecko
author_rank: Newbie
timestamp: April 16, 2018, 07:46:21 pm
content: To anyway wondering where the 8mm creepage number comes from:The separation between primary and secondary circuits is consideredreinforced insulation. According to section 5.2.4 of IEC 60664-1: "Creepage distances for reinforced insulation shall be twice the creepage distance for basic insulation from Table F.4"According to Table F.4 in IEC 60664-1 the creepage distance for pollution degree 3 and material group III at 250V gives 4mm.Double that number because of the reinforced insulation and you get8mmfor the creepage distance between primary and secondary in a power supply. This is a worst case scenario that many people designing mains powered devices use even if they could get away with lower pollution degree or material group.

author: coppice
author_rank: Super Contributor
timestamp: April 16, 2018, 07:58:50 pm
content: Quote from: FlyingGecko on April 16, 2018, 07:46:21 pmPeople use the 8mm reinforced spec because 25 years ago, when everyone used 4mm, there were problems in high humidity and dirty conditions. 8mm was a response to the real world.

author: james_s
author_rank: Super Contributor
timestamp: April 16, 2018, 09:12:44 pm
content: I don't think people are afraid to sue the Chinese, the problem is who do you sue? You can't sue the whole nation, at least it wouldn't make any sense to do that. A lot of this junk is made by little fly by night operations and chasing those down is an epic game of Whack A Mole. It's really more the fault of people in other places who buy the cheapest of cheap everything. To a point you get what you pay for.

author: Giaime
author_rank: Regular Contributor
timestamp: April 17, 2018, 10:16:10 am
content: I think that the right standard in this case is IEC61347-1 and -2-13 for LED drivers. Creepage/clearance distances are given in section 16 of -1.

author: dmills
author_rank: Super Contributor
timestamp: April 17, 2018, 11:05:52 am
content: Which standard are you declaring conformity to?Then you just follow whatever that standard says.ITE (Usually 60950, but there is a new one on the horizon that is not yet widely ratified, makes deciding what to do a pain), is different to Lighting, is different to AV (Which is interesting as most AV is basically IT these days).Of course if going for CB scheme you then get into the various national variations, Japan (PSE) being the one making me swear at the moment.Regards, Dan.

author: Phoenix
author_rank: Frequent Contributor
timestamp: April 18, 2018, 01:03:04 pm
content: At my old company we made smps of all sorts. A key component was an isolation diagram and table of required creepage and clearance for any 2 adjacent nodes or functional blocks before any pcb layout is done. I've been involved with both 62109 and 60950. They aren't too bad when you find the right tables.Sent from my G8441 using Tapatalk

author: CopperCone
author_rank: Super Contributor
timestamp: April 18, 2018, 01:54:26 pm
content: The transformers in dcdc converters are mega shady anyway. To have any kind of semblence of saftey you would want large donut cores to seperate the primary and secondary with potting in the middle. And the core would have a thick plastic shell. And probobly there should be a special enclosure that goes down the middle of the core and only lets the magnetic energy path through.These standards are basically telling you not to stick your dick in the outlet and your complaining??They are designed to favor mass production and cheap manufacturing 100%.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 18, 2018, 03:31:20 pm
content: Quote from: CopperCone on April 18, 2018, 01:54:26 pm

author: CopperCone
author_rank: Super Contributor
timestamp: April 18, 2018, 03:57:21 pm
content: Well you are relying on protection due to a layer of tape and some enamel... Look at some of those switching cores.I mean ei cores are not that much beter but still.R core looks safer so long it has thick plastic coat, like some torroidal inductors that have a plastic snap cover

author: dmills
author_rank: Super Contributor
timestamp: April 18, 2018, 05:29:52 pm
content: That is why triple insulated wire exists, it is a standard thing in transformer winding.Also, those mylar tapes used inter winding are tough and as long as you design the windings to ensure sufficient creapage around the edge of the tape, a few layers is just fine and I would have no concerns.The issue with PCBs is surface contamination and that FR4 fairly readily carbonises leaving conductive tracking, been there, seen it, not much of an issue in a transformer, which has at least two thicknesses of insulation to fail even if you took no precautions what so ever (And as I say, triple insulated wire exists for a reason).Regards, Dan.

author: CopperCone
author_rank: Super Contributor
timestamp: April 19, 2018, 12:32:55 am
content: im reconsidering what I said, proper would be an R-core or ring core, with a ceramic shell, with the wire on top.I took apart a small transformer, I am not impressed by this so called triple insulation. Maybe something special on medical products? But really...If its gonna be handled by a person, do you really wanna trust some flexible plastic shit?Of course, being around humans, they do plenty of dumb ass shit all the time to save like, fractions of a penny.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2018, 12:48:09 am
content: Flexible plastic shit is a long sight better than inflexible plastic shit. Polyimide insulation caught several airliners on fire back in the day.Tim

author: CopperCone
author_rank: Super Contributor
timestamp: April 19, 2018, 12:52:02 am
content: i wonder if those materials made someone a 50 dollar or 100 dollar bonus loli wanna see a plot of the management bonus money vs airliner destruction costs

author: dmills
author_rank: Super Contributor
timestamp: April 19, 2018, 08:24:09 pm
content: How many millions of designed to the appropriate standard switchmode transformers are out there?How often do you hear about a primary/secondary insulation failure in such a transformer?The things are pretty bloody reliable at least if designed to an appropriate specification.As to the airliner thing, that is what airworthyness directives are for, we LEARN from the mistakes, which is why an airliner going down today is a much rarer event per mile flown then it was in say 1955.Companies behaving really badly WRT safety to save a buck are actually much rarer in my experience then you would think, fraud is just more profitable if you are that way inclined (Enron, Worldcom) then say Union Carbide in India cheaping out on cooling plant or BP in the gulf of Mexico not doing the maintenance.CEOs may sometimes be sociopaths (Not even sure I buy that common meme) but the folk they put in charge of multi million dollar facilities usually know their trade and understand the risks.Regards, Dan.