Output Earthing of SMPS and Appliance Class (I or II) - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: January 22, 2020, 10:58:10 pm
content: Hi all,We've been having a bit of a philosophical(?) debate in my office about proper classification of switch-mode power adapters. I'm hesitant to call it a philosophical debate because I feel there has to be a right and wrong answer here.The company I work for manufacturers switch-mode power adapters...predominantly wall-plug in type.Many people are probably familiar with the standard classification of appliances which plug into mains:Class I:The transformer (and primary-secondary isolation barrier, in general) incorporates a single mean of protection against electric shock. A protective earth wire connection to output negative (and the metal enclosure, if there is one) forms two means of protection.Class II:Two means of protection directly built into the insulation between primary and secondary, thereby making the Earth wire unnecessary to meet global safety standards.These days, the vast majority of switching power supplies, are designed with "double insulated" transformers, as triple-insulated wire (TIW) has made it possible to design a double-insulated transformer which is hardly any bigger than a hypothetical equivalent single-insulated version.So the question is:If you attach an earth wire to the output negative in a power supply which has a Class II insulation rating (based on transformer construction and other creepages/clearances), what class does the power supply become? Class I? Still Class II?The company I work for calls it Class I, but my problem with that is: Class I implies an inherentneedfor the earth conductor to be safe. By my logic, the addition of the earth wire should only make the power supplymore safe. If I dare say, I might even call it a "triple insulated" power supply in this configuration. Am I wrong? Is it somehow possible that defeating the earth isolation somehow degrades the power supply to a true Class I designation?It's interesting that, for the most stringent medical applications, earthed power supplies are not allowed, and in fact, must be floating. (These classifications are called Type-BF, for body floating and Type-CF, for cardiac floating.) The medical standard IEC60601 notes that earthing a patient (via power supply earth connection) opens up the possibility that an unintended external voltage could be applied to the patient, causing a current to flow through the body to earth.Anyone have any experience with this?Thanks,Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 22, 2020, 11:33:02 pm
content: Earthing the secondary of a Class II power supply does not change its classification back to Class I. After all, the device being powered by a Class II supply could itself be earthed (intentionally or incidentally), and if this could change its classification then Class II wouldn't be a thing in the first place (or, conversely, every device would require the power supply be built into it).

author: TimNJ
author_rank: Super Contributor
timestamp: January 23, 2020, 05:06:21 am
content: Thanks a lot. That is a solid example to prove the point. Another guy is trying to say that connecting output neg to mains earth voids some protection because it is now "connected to the mains supply"...but the logic there doesn't really hold, because in a true Class I system, the earth connection is added as an extra safety provision, so how could it have some opposite effect here?I think it's important to remember that the earth terminal on a mains output is less part of the mains "supply" and more a convenient way to connect to the physical earth.

author: Someone
author_rank: Super Contributor
timestamp: January 23, 2020, 07:01:14 am
content: Quote from: MagicSmoker on January 22, 2020, 11:33:02 pmThis depends on where and how the earthing is done.Quote from: IEChttp://std.iec.ch/terms/terms.nsf/3385f156e728849bc1256e8c00278ad2/767c9cffaa8610d8c12577ac00499ce8?OpenDocumentGenerally if the mains inlet is an IEC C14 (and its connected to something???) thats immediately Class I, even if there are no accessible parts that have continuity back through to it.

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 23, 2020, 11:14:47 am
content: Quote from: Someone on January 23, 2020, 07:01:14 amHuh... Just when you think regulations can't get any more stupid you see something like this. I'd definitely check with your safety agency testing lab on this one - I do design power supplies and motor drives but nothing that could even remotely qualify as Class II (or SELV, etc.) - and my understanding is that Class II prescribes a set of construction techniques that allow you to avoid earth grounding, not prohibit you from earth grounding. Footnote 2 in the above link does seem to suggest the very opposite, however.

author: TimNJ
author_rank: Super Contributor
timestamp: January 23, 2020, 04:05:18 pm
content: From IEC 62257-7-1 (linked to above):QuoteFrom IEC 60601-1 (Medical safety):QuoteThey don't even want you to have aprovisionfor protective earthing? I really want to know the logic here.Also from IEC 60601 (Medical safety):QuoteThis standard assumes equipment will fall under Class I, where second mean of protection is derived from earth connection, Class II with no earth connection and double insulated barrier, and Class II with Functional Earth, where an earth connection serves a purpose which is not for protection against shock (most probably something with EMI or immunity). The above seems to say, "Once the earth conductor becomes user accessible (tied to chassis or output negative), then it's no longer Class II equipment"...why?? I don't get it.Thanks all.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 23, 2020, 05:00:38 pm
content: Probably a leakage current restriction. If two devices with ground referenced output are connected to each other or to a structural ground you can get ground current flowing between them regardless of the insulation on the line voltage. It is low voltage, but for medical standards that might not matter.