Open frame SMPS safety - Page 1
Forum section: electronics / projects

author: Lomax
author_rank: Frequent Contributor
timestamp: November 03, 2015, 09:55:04 pm
content: Hi all,In a project I'm working on I need to include an SMPS inside the equipment, which is housed in a top-ventilated aluminium desktop enclosure. I would like to use an "open frame" supply - for size, ventilation and cost reasons - but I'm unsure what the safety considerations are. It would sit on its own shelf inside the enclosure, with a 1cm clearance on all sides, but located directly below the ventilation holes (a row of 0.5x2cm holes) in the lid of the enclosure. This means any liquids spilled, or (small) objects dropped, could end up on the SMPS. Considering the high voltages present I thought it wise to ask here before proceeding - is this placement ok, or do I need to better protect the SMPS from ingress?Edit: This is the SMPS I'm looking at,Cincon CFM60S15

author: calzap
author_rank: Frequent Contributor
timestamp: November 03, 2015, 10:03:27 pm
content: You don't mention what voltages are involved. I'm assuming Vmax is 240 VAC; if it's higher, other considerations are needed. To stop metal objects (screws, pens,etc.), plastic screening on the underside of the vents will do. If there is a credible threat of liquids, you should reconsider the design of the case.Mike in California

author: Lomax
author_rank: Frequent Contributor
timestamp: November 03, 2015, 10:14:08 pm
content: Thanks Mike!Quote from: calzap on November 03, 2015, 10:03:27 pmSorry, I forgot to add that. Yes it is, 240VAC -> 15VDC @ 60WQuote from: calzap on November 03, 2015, 10:03:27 pmGreat idea, I might include a small strip of screening under the ventilation holes in the design.Quote from: calzap on November 03, 2015, 10:03:27 pmWell it's not intended for out-door use, that's for sure! I'm thinking more of accidental spills; tea, coffee, beer, over-watered house plants, that sort of thing. Obviously the thing would go bang if that were to happen, but then wouldn't most (ventilated) electronics? I've certainly seen a CRT television die a (somewhat spectacular) death from house-plant overflow, and that had every safety certification known to man. I guess what I'm really asking is if anyone knows what regulations apply, or where I might find that kind of information - hours of googling has not turned up any relevant info...

author: nctnico
author_rank: Super Contributor
timestamp: November 03, 2015, 10:19:51 pm
content: How about mounting the SMPS upside down? I guess it has an aluminium base plate.

author: Lomax
author_rank: Frequent Contributor
timestamp: November 03, 2015, 10:46:40 pm
content: Quote from: nctnico on November 03, 2015, 10:19:51 pmNice lateral thinking there! This might be an option, but tricky to do in this case. Do I take your reply as indication that my current configuration is "unsafe"?

author: Ian.M
author_rank: Super Contributor
timestamp: November 03, 2015, 11:10:11 pm
content: The enclosure isn't paperclip-proof, so at the minimum, for desktop usage you need a fairly fine metal mesh cover on the PSU if you don't want a lovely fireworks display. Bend up a top hat section and bolt it down. If liquids are a concern, what about a job lot of 2" standoffs and a solid 'shelf' top right over the whole case top? It makes it a legitimate place to put stuff too.

author: Lomax
author_rank: Frequent Contributor
timestamp: November 03, 2015, 11:45:07 pm
content: Quote from: Ian.M on November 03, 2015, 11:10:11 pmYes, I can put a mesh on the inside of the lid, to prevent solid objects from entering, this is a good idea.Quote from: Ian.M on November 03, 2015, 11:10:11 pmThe case isn't tall enough for this arrangement; any protective sheet above the SMPS would effectively cover the ventilation holes, which are there for a reasonI might be able to re-arrange things to increase the available height though. I so need to get a sheet metal brake, but I don't really have room for one!Quote from: AcHmed99 on November 03, 2015, 11:36:03 pmThanks. I do indeed need to shield the SMPS from the rest of the electronics - this is why it's on a shelf of its own; the shelf fits the width of the case and the ends are bent up, so that when the lid is in place the SMPS is in its own "sub-enclosure". There is another PCB 2.5cm below the shelf.

author: Ian.M
author_rank: Super Contributor
timestamp: November 04, 2015, 12:00:46 am
content: See how the vents are protected on the top of this old Hartley Electromotive Type 13A Oscilloscope 'boatanchor'. I used to own one and it made a nice room heater!Now imagine that with a bit more vertical spacing and no gap in the middle for the handle. That's what I mean by a shelf top. You can see by how beat up the vent covers are, that its had a lot of kit stood on top of it over the years.

author: Lomax
author_rank: Frequent Contributor
timestamp: November 04, 2015, 12:28:43 am
content: Ah. Now I see how you mean. Not a bad idea, I've made a note to explore that option. Wonderful old scope, looks like a military model?

author: Lomax
author_rank: Frequent Contributor
timestamp: November 04, 2015, 12:35:45 am
content: The question remains though: regardless of whatcouldbe done to protect the SMPS, do I actuallyneedto do anything? While I'm grateful for the suggestions provided, and will take them into consideration, I wasn't really asking how to improve ingress protection, but whether regulations/best practice demands it.

author: nctnico
author_rank: Super Contributor
timestamp: November 04, 2015, 01:01:55 am
content: I'd ask a company involved with product safety compliance testing. I vaguely recall there are requirements for objects entering a case.

author: DanielS
author_rank: Frequent Contributor
timestamp: November 04, 2015, 04:37:00 am
content: If you want to pass certain UL certifications, there are maximum hole sizes and depth that you can poke stuff in before making contact with a live circuit that apply to ventilated enclosures but I don't remember the standard's number. IIRC, one of the tests mimics baby fingers.

author: KJDS
author_rank: Super Contributor
timestamp: November 04, 2015, 08:57:03 am
content: http://www.ergonomicsusa.com/prod-category/product-safety-test-equipment/accessibility-and-safety-probes/finger-probes/

author: Neilm
author_rank: Super Contributor
timestamp: November 04, 2015, 07:13:41 pm
content: I guess you are aiming at Europe, so you will need CE marking. I don't know too much about other regulations, but if I remember IEC61010 one test involves putting rod (3mm diameter 150mm long it think) through any holes in the case. Check what ever safety standard you have to meet.Quote from: nctnico on November 03, 2015, 10:19:51 pmNice idea, but check that this does not degrade the performance.

author: Lomax
author_rank: Frequent Contributor
timestamp: November 04, 2015, 07:41:39 pm
content: Quote from: nctnico on November 04, 2015, 01:01:55 amI could do, if I had £10k plus to burn. Sadly I don't, so if I ever wanted to build something that had a reasonable chance of passing certification I'd need to find a cheaper optionQuote from: KJDS on November 04, 2015, 08:57:03 amNice find! Is it just me or are some of those probes kinda creepy looking?Quote from: Neilm on November 04, 2015, 07:13:41 pmThanks, the UL standard says 4x100mm, but I'll assume 3x150mm to be sure. More on this below.Quote from: DanielS on November 04, 2015, 04:37:00 amAh yes, some of the probes in KJDS' post appear to be intended for this. It seems for a device of the type I'm working on, the applicable UL standard is "UL 6500 - Standard for Audio/Video and Musical Instrument Apparatus for Household, Commercial, and Similar General Use", and specifically for the question at hand:QuoteAnd once I figured out which document and section I was looking for:QuoteSo there we have it, to be compliant I would need to include a shield that prevents the entry of a 4mm diameter probe, which the metal mesh suggested by calzap and Ian.M will easily take care of. There is a qualification WRT to the voltages presentQuoteBut this would of course be moot if the openings do not allow the probe to even enter, as long as the distance PSU live -> case is sufficient. Surpsisingly, nothing much is mentioned about liquid ingress, apart from the following:QuoteAnd "Annex A" only applies to equipment specifically intended for out-door usage:QuoteSo it would appear that no special considerations are required with regards to liquids; only physical object entry as described in section 9.1.3 is required. Thanks everyone for your input - I will add a steel mesh fine enough to prevent entry of anything > 2.5mm dia. Case closed

author: nctnico
author_rank: Super Contributor
timestamp: November 04, 2015, 07:42:54 pm
content: Quote from: Lomax on November 04, 2015, 07:41:39 pmAdvice isn't that expensive. Actually the test house which provides the info for free is probably a good place to have your products tested if the need arises.

author: Lomax
author_rank: Frequent Contributor
timestamp: November 04, 2015, 07:46:38 pm
content: I seriously doubt this project would ever turn into a product which I wil be selling, I just wanted to ensure that a) I'm not doing something dangerously stupid and b) that a possible future certification won't require a complete re-design.

author: DanielS
author_rank: Frequent Contributor
timestamp: November 04, 2015, 09:00:47 pm
content: Quote from: Lomax on November 04, 2015, 07:41:39 pmAs long as the chassis is properly grounded, liquid ingress should pose little to no shock hazard. The enclosure still needs to be able to contain the possible resulting explosions (ex.: capacitors blowing up) and fire though.

author: Neilm
author_rank: Super Contributor
timestamp: November 04, 2015, 09:25:53 pm
content: Quote from: Lomax on November 04, 2015, 07:46:38 pmI know from (bad) experience it is easier to pass if EMC and safety are designed in from the start. Finding which standards you have too meet is not too hard. Start by looking at the guide for the Low Voltage Directive - there is a list of standards that if you meet you can presume compliance with the LVD. Read the free part that is online. This will include the scope of equipment that is covered to double check that the standard applies. If you are working for a company there is no excuse for not spending £150 on a standard. Doing some of your own pre testing will easily pay for it if you don't have to retest.