Class I instead of class II PSU for "medical" equipment? - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: January 29, 2025, 10:36:24 am
content: Hi,I have no experience in medical devices and the required safety, but someone gave me a "foot detox" unit to check (they are getting very popular apparently - see attached picture). Regardless of what people believe about them, it is nothing more than a 10 to 20 volts electrolysis of salt water occurringwhile the person keeps their feet in the water. And despite the operating voltage being low, the main PSU is ultimately connected to the mains posing a potential hazard.They explicitly mention: "External power supply: Class I Equipment (Grounded Type):Electrical grounding is required for safety."Should it not be Class II with 2 x MOPP for maximum safety given the user places their feet right in the water where the electrodes are and while the unit is working?Thank you

author: coppercone2
author_rank: Super Contributor
timestamp: January 29, 2025, 11:21:11 am
content: its not that far fetched, if you look at foot cream it has some caustic ingredients. (sodium or potassium hydroxide is one, and its made through simple electrolysis)I think it should be a medical supplyThough I noticed if you stop working, your damaged feet will simply molt after a while, unless its hard-prison labor damaged. I guess people don't like that in bed.

author: ricko_uk
author_rank: Super Contributor
timestamp: January 30, 2025, 01:56:57 pm
content: Thank you @copperconeCould anybody who has designed medical devices also provide feedback? Should it not be a Class II PSU? Why do they say it "Electrical grounding is required for safety." in such application instead of making the PSU Class II?Thank you as always.

author: TimNJ
author_rank: Super Contributor
timestamp: January 30, 2025, 04:47:26 pm
content: 2 x MOPP can be achieved by Class II (double/reinforced) insulation (no earth) or it can be achieved by Class I construction (basic insulation + protective earthing).Equipment designed for direct and prolonged patient contact is usually a floating design though; you are right. There are three designations for patient applied parts: Type-B "Body", Type-BF "Body Floating", and Type-CF "Cardiac Floating".8.3 Classification of APPLIED PARTSb) * An APPLIED PART that includes a PATIENT CONNECTION that is intended to deliver electrical energy or an electrophysiological signal to or from the PATIENT shall be a TYPE BF APPLIED PART or TYPE CF APPLIED PART.Earthing a patient is not really dangerous in and of itself, since the power supply can still provide 2 x MOPP using Class I construction. Leakage current is also minimal because the patient is at earth potential, and leakage currents arise when the output of a power supply isnotat earth potential (e.g. when it's floating). The issue with earthing a patient is more so that, if an uncontrolled external voltage comes in contact with the patient, there is a particularly low impedance path for current to flow, much lower than the typical bulk impedance of building materials, earth, etc.

author: ricko_uk
author_rank: Super Contributor
timestamp: February 03, 2025, 02:44:35 am
content: Thank you @TimNJ,that was very useful.Following your feedback I did a bit more searching and could not find anywhere about the 2 x MOPP being achievable by class I through basic insulation + protective earthing. All the resources I came across say that 2 x MOPP can only be achieved by Class II.Could you please point me to some resources where I can learn about 2 x MOPP being achieved with Class I?Thank you

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 03, 2025, 04:54:29 am
content: I recall Big Clive looked at a few of those and they're basically scams.

author: Nominal Animal
author_rank: Super Contributor
timestamp: February 03, 2025, 04:54:51 am
content: I'm only a hobbyist who uses various DC PSUs in a country where mains sockets are unpolarized (CEE 7/3 "Schuko" sockets), with some devices using grounded supplies and others ungrounded supplies, and dealing with the ensuing potential differences. Members here have laughed at me for using isolators too often, for example.You'll find that Mean Well has both class I (grounded) and class II (no ground) medical-rated (2×MOPP)AC-DC adapters: NGEnn, GSMnnU, GSMnnE, and GSMnnB are class II (no ground), and GSTnnA are class I (requires a grounded socket).The medical rating means that even if the output is connected to ground, the current via that connection will be less than 500µA AC or 50µA DC in normal operation, and less than 1000µA = 1mA AC or 100µA DC in a single failure condition. Essentially, if you use your finger or any part of your body to connect the output to ground, the current is so low you won't have any negative effects. 1mA AC current at 50/60 Hz is considered "barely perceptible", i.e. youmightfeel it (if you use sharp probes and stick them through the outer layers of your skin into the corium/dermis with much lower resistance, or e.g. use your tongue), but it won't be dangerous. There are other safety features, like 2×MOPP requiring double isolation; see e.g. Wikipedia article onMOPPfor details.Thus, whether the supply is grounded or referenced to ground, does not matter for the medical rating, Means of Patient Protection (MOPP), and both class I and class II medical-rated supplies exist and are readily available.The brown gunk in those foot baths comes from one of the electrodes. The iron in it is eroded, gets into solution, and oxidizes. You can see the pitting in the electrode (or both electrodes, if the supply periodically changes the polarity of the electrodes). My elderly mom was hoodwinked to get one of those things; they're bunk.Warm foot bath with Epsom salt (magnesium sulfate heptahydrate, MgSO₄·7H₂O) or just magnesium sulfate (MgSO₄) will give much better results. While its efficacy is not precisely scientifically proven, there is lots of anecdotal reports wrt. reducing inflammation and soreness/pain (with magnesium absorption through the skin well explaining those effects). In any case, it is definitely classified as safe, even if you use enough of it for your feet to float –– as that's exactly what they use in sensory deprivation tanks to increase the specific gravity so the body is neutrally buoyant in it ––, unlike those electric footbath things.

author: TimNJ
author_rank: Super Contributor
timestamp: February 03, 2025, 05:34:10 am
content: The choice of wording here is a little weird, but I guess it's suppose to cover a variety of scenarios.Subclause 8.5.1 – MEANS OF PROTECTIONTwo MEANS OF PROTECTION can be provided in several ways. The following are examples:1) PATIENT CONNECTIONS and other ACCESSIBLE PARTS are separated from parts different from earth potential by BASIC INSULATION only, but PROTECTIVELY EARTHED and have such a low internal impedance to earth that LEAKAGE CURRENTS do not exceed the allowable values inNORMAL CONDITION and SINGLE FAULT CONDITION.[...]3) PATIENT CONNECTIONS and other ACCESSIBLE PARTS are separated from parts different from earth potential by DOUBLE or REINFORCED INSULATION.Basically, the standard wants the insulation system to have tolerance to a "single fault". So, this is the rationale for needing *two means* of protection. If the first means fails, you still don't get fried.Just supposing we are talking about a mains transformer...The two means are most readily obvious with the Class I case, I think. There is the enamel insulation on the wires in the transformer (1 x MOPP) and the protective earthing of the output (1 x MOPP). Add them up and you have 2 x MOPP.For the Class II case, the two means typically come in the form of specialized wire insulation which counts as 2 x MOPP. The special wire *does* have multiple layers of insulation, so there are indeed physically more means of separation, but it's not immediately obvious to the naked eye because they are extruded layers. When the layers of insulation cannot be readily separated, this is called reinforced insulation. Three layers are required and all layers are tested together. The special wire is called "triple insulated wire" or TIW. If the layers can be separated, this is called double insulation. The first layer is called "Basic insulation" and the second layer is called "supplementary insulation".There is actually no minimum material requirements for "basic insulation", besides perhaps a temperature category rating(?). Basic insulation really just needs to pass dielectric strength testing. In the Class I case, the enamel coating on the transformer wire is considered basic insulation. For supplementary insulation (i.e. insulation to be added to basic insulation to form double insulation) *does* have strict material requirements. I guess the rationale is that this supplementary insulation needs to be "as reliable" as an protective earth connection.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 03, 2025, 05:41:47 am
content: Quote from: TimNJ on January 30, 2025, 04:47:26 pmPossibly the subtlety here could be with the purpose of the device. One can argue that these devices are *not* intended to deliver electrical energy to the feet, but merely circulate ions in the solution.Actually one could argue that these devices are not medical devices by any stretch of the imagination and thus cannot be classified as ones. In which case, the regulation to comply with would be similar to that applying to any electrical household device that can be operated in a wet environment, in which case, either class I with mandatory earthing, or class II (double insulation) without earthing should be acceptable.If that were considered a medical device with an *applied part*, it would be difficult to justify a class I power supply though, given the potential, but very probable risks of misuse. But given the context (putting your feet in a highly conductive solution), the risks are still not negligible.These are generally considered a scam:https://en.wikipedia.org/wiki/Detoxification_foot_bathsAnd here are some points about foot detox in general:https://health.clevelandclinic.org/foot-detox

author: ricko_uk
author_rank: Super Contributor
timestamp: February 03, 2025, 11:37:15 am
content: Thank you all for the detailed replies.Am I correct in saying that, across the board, regardless of the application, Class II with 2 x MOPP can safely replaceanyClass I PSU?Or are there particular cases where that is not correct or safe and you have to stick to Class I?Thank you

author: daisizhou
author_rank: Super Contributor
timestamp: February 03, 2025, 12:00:07 pm
content: There is an analyzer called an electrical safety analyzer, for example: ESA620 Electrical Safety Analyzer.The Class II and Class I you care about are just a mark in my opinion. Only the data after the actual test is the most important.Because inferior goods always have the mark of looking good

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 03, 2025, 01:44:55 pm
content: I design medical devices and just wanted to confirm overall what TimNJ said above.Also Class I/II does not describe power supplies, but rather the medical equipment (ME) as a whole. AFAIK IEC 60601-1 leaves it up to the device maker which class to go with**. Both should be safe when implemented correctly. Often the choice is made based on whether the ME will have a metal or plastic enclosure/chassis. Metal usually goes class I while plastic goes class II (but not always).**oh almost forgot one big exception which is "home healthcare" ME (which falls under IEC 60601-1-11), which only allows Class I if the PE connection is "fixed" (i.e. not a third prong on a plug).Quote from: ricko_uk on February 03, 2025, 11:37:15 amIn general no, such assumptions are not justified. As a designer of ME, I would never suggest that an end user can swap the power supply. The power supply is considered part of the ME itself, and changing the power supply invalidates any safety/EMC certifications.Quote from: daisizhou on February 03, 2025, 12:00:07 pmWhether or not you choose to believe the marks are genuine, the purpose of the marks are mainly to inform users how to use the device safely. For example if it has the Class II equipment mark it does not need a protective earth connection, but if it has a PE mark then it does.

author: daisizhou
author_rank: Super Contributor
timestamp: February 03, 2025, 01:57:04 pm
content: What kind of medical device are you designing?

author: TimNJ
author_rank: Super Contributor
timestamp: February 03, 2025, 04:44:14 pm
content: Quote from: SiliconWizard on February 03, 2025, 05:41:47 amYeah, I suppose, but I feel it is reasonable to apply medical device thinking if that's what the makers and consumers fundamentally see it as, whether or not it is certified or accepted as a such.If you have access to IEC60601-1, read subclause 8.3 on what constitutes an "Applied Part". Seems pretty clear that this would be an applied part, and more specifically, a "patient connection", even if incidental.I guess the logic still sounds a little funny because we interact with earthed equipment *all the time*, and earthed equipment is not thought to pose a hazard in general. I guess the thinking is that "patients" are possibly more vulnerable(?), whatever that may mean, and that our interactions with other earthed equipment are typically intermittent...Like, I don't attach a ground strap between my body and my computer every day when I show up to work.

author: coppercone2
author_rank: Super Contributor
timestamp: February 03, 2025, 04:55:34 pm
content: the thing that stands out to me is that its wet. when do you use appliances wet? people regard that as not safepeople buy gas pressure washers because their scared of the cord.DOn't they use the same tech to sterilize pools?its salt water. very unusual to use appliance if you are in salt water. Maybe a commercial aquarium does it, with giant salt water fish tanks, if they need to climb in with a vacuum cleaner to vacuum the sturgeon tank....hardly the standard consumer experience

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 04, 2025, 01:06:15 pm
content: First of all, looking at the webpage for that footbath thing, it says:QuoteSo yeah, the manufacturer doesn't treat this as a medical device.Their website does does provide downloads of their EMC and safety test reports. Safety is certified per EN 60335-1: Household and Similar Electrical Appliances - Safety-Part 1: General Requirements. So less stringent than medical devices per IEC 60601-1. But it also mentions that the device is provided with an external medical-grade power supply.Quote from: TimNJ on February 03, 2025, 04:44:14 pmUnder IEC 60601-1 it certainly would be considered an applied part (at least the bath part of it; often ME is broken down into multiple parts, only some of which are considered applied parts). Whether it would be type B or BF is much less clear, the logic there is really wonky IME.

author: ricko_uk
author_rank: Super Contributor
timestamp: February 07, 2025, 02:48:47 pm
content: Thank you all for the extensive feedback@daisizhou I have been asked to design the same product.So given that I am designing a new one from scratch, it would be best and safest for me to specify a class II with 2 x MOPP correct?I am think that also because of what has been mentioned including "almost forgot one big exception which is 'home healthcare' ME (which falls under IEC 60601-1-11), which only allows Class I if the PE connection is "fixed" (i.e. not a third prong on a plug)".Thank you

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 07, 2025, 03:48:32 pm
content: Quote from: ricko_uk on February 07, 2025, 02:48:47 pmThe real question is what safety standard this product is to be tested to, and whether it's going to be treated as an actual medical device or not.

author: grumpydoc
author_rank: Super Contributor
timestamp: February 07, 2025, 05:00:14 pm
content: Quote from: NiHaoMike on February 03, 2025, 04:54:29 amYep

author: daisizhou
author_rank: Super Contributor
timestamp: February 08, 2025, 02:29:04 am
content: Quote from: ricko_uk on February 07, 2025, 02:48:47 pm@ricko_ukUsually real medical devices need to obtain a CFDA registration number,In the United States, it should be the FDA.Just like a car needs a license to drive on the road.However, many times, physical therapy equipment is not considered medical equipment.Or they are just auxiliary.I'm curious why you need to redesign it，I think a lot of cheap Chinese products have the same functionality.Unless your design has a very specific purpose, I don't understand

author: ricko_uk
author_rank: Super Contributor
timestamp: February 08, 2025, 10:44:58 pm
content: Thank you all@mtwieg I asked what standard they need to make it comply with and they are looking into it but they said they are not looking to have any medical certification. What I was wondering is, regardless of any standard, if you had to design one yourself and did not care about the cost, which PSU would you choose to be on the safe side considering the information I shared so far which is all I have at the moment. After all the feedback I am inclined to go for a Class II with 2 x MOPP but I don't have any experience in this area which is the reason why I am asking here for feedback.@daisizhou I have been asked to design one for another client, nothing to do with the company that makes the one I mentioned and linked to. The reason why they asked me that is that they want their own enclosure (same look and feel to their other products) and also additional software functionality@grumpydoc yes, I saw that video by Big Clive a while agoThank you

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 09, 2025, 02:20:13 pm
content: Quote from: ricko_uk on February 08, 2025, 10:44:58 pmAgain, describing power supplies as being class I/II is nonsense, so you'll have to rephrase the question. Yes I'm aware that some PSU vendors will describe their power supplies as being class I/II, but this has no grounding in any standard AFAIK. For example, class II medical equipment may have a three-prong power plug, if the earth connection is for device functionality and not for patient/operator safety (functional earth vs protective earth). This can be the case if the earth connection is necessary to pass EMC testing, or to make a capacitive touch interface work properly. In this ME, would the power supply be class I or II? Trick question, as the classification is not meant for power supplies.Anyways what you should care about regarding power supplies are MOOPs and MOPPS, and leakage currents.

author: Krotow
author_rank: Regular Contributor
timestamp: February 09, 2025, 04:37:11 pm
content: Foot bath with bath salts are Ok for recreation purpose. However mentioned electrolysis "baths" are 100% true snake oil. Or in simpler term - pure BS. Pity that people still fall on them.

author: Nominal Animal
author_rank: Super Contributor
timestamp: February 09, 2025, 05:58:45 pm
content: As a hobbyist myself, I foundthe explanation of the difference between class I and class II supplies by David Buck, TDK-Lambda product manager(PDF atwww.emea.lambda.tdk.com), quite clear. He even shows how some class I supplies can be used in equipment to achieve class II 2×MOPP requirements for the whole.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 10, 2025, 01:19:50 am
content: Quote from: Nominal Animal on February 09, 2025, 05:58:45 pmYes, this document sums it up nicely and in a very digestible way.One important point to have in mind: "However, estimates indicate that less than half the houses in North America and Europe have reliable earth grounding. (...) Therefore, the medical home healthcare standard IEC-60601-1-11 stipulates that Class II equipment must be used."

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: February 10, 2025, 04:46:47 am
content: Quote from: TimNJ on January 30, 2025, 04:47:26 pm100%. The safer option is not to earth, IMO.I've done a silly thing standing on wooden floorboards and, having gotten away with it, did the silly thing again while standing on damp soil. The second time was quite unpleasant.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 10, 2025, 07:49:59 pm
content: I don't think anyone was actually suggesting earthing the patient by any means in this case. That has nothing to do with earthing the power supply and/or enclosure.