Hi Amund:
There is no requirement in 60950-1 or in 62368-1 that Class II insulation must consist of two layers. Rather, the use of two layers (or three layers) is simply a permitted option if the product designer would prefer to use an insulation thickness that is less than the prescribed amount. As I have noted previously, I have more experience applying 60950-1 than with applying 62368-1, but I believe the basic concepts for insulation are the same. Some of the specific values may be different in 62368-1, but otherwise the concepts are similar. My impression is that compared to 60950-1, 62368-1 may allow slightly smaller creepage and clearance distances, but may require larger thickness for solid insulation (depending on the material being used). In general, under 60950-1 for "double insulation" or "reinforced insulation,", the following requirements must be met for the insulation barrier that separates a 230 Vrms AC mains from an ordinary user: 1) Minimum hipot breakdown of at least 3000 Vrms 2) Minimum 5 mm creepage distance (distance across a surface) 3) Minimum 4 mm clearance distance (distance through air) 4) Minimum 0.4 mm distance through solid insulation Keep in mind that the 0.4 mm distance through solid insulation applies to the barrier between the 230 Vrms AC mains and anything that an ordinary user can touch. It does not apply to the AC mains transformer unless an ordinary user can directly touch the transformer's secondary circuits. This is why an alternate solution for providing the isolation barrier inside the transformer is to use a plastic enclosure with no accessible conductive parts. Also note that the 0.4 mm thickness requirement for solid insulation applies to insulation with just a single layer. The various allowances for using either two layers or three layers are options that are available for designers who wish to use an insulation thickness of less than 0.4 mm. In general, manufacturers of transformers prefer to use primary-to-secondary insulation that is thinner than 0.4 mm. The thinner insulation allows for a more compact transformer construction and also reduces an undesired electrical parameter called "leakage inductance". In view of this, transformers designed to provide "double insulation" or "reinforced insulation" typically use two layers of extremely thin Kapton tape rather than a single layer of something thicker. There is no thickness requirement if two or three layers are used. My main point here is that the use of two layers is not a requirement. It is simply an option. And, my other point is that the AC mains transformer is not required to have any isolation barrier whatsoever, if the secondary circuits have acceptable isolation from ordinary users (such as a plastic enclosure with no accessible ports). Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) <mailto:[email protected]> [email protected] <http://www.randolph-telecom.com/> http://www.randolph-telecom.com From: Amund Westin [mailto:[email protected]] Sent: Thursday, February 24, 2022 1:27 PM To: [email protected] Subject: [PSES] SV: [PSES] SV: [PSES] Numbers of insulation layers in transformer (IEC62368) A Class II or double insulated electrical appliance (bear the double insulation symbol), is it required to have a transformer with 2 insulating layers? Or is one layer in transformer ok, if the plastic enclosure is giving enough insulation, thus in total you will have 2 layers of insulation? Ref. 62368-1 product (AC powered WiFi HUB). No cabling/wiring on secondary side. Cheers! BR Amund Fra: Joe Randolph Sendt: 18. februar 2022 05:09 Til: [email protected] <mailto:[email protected]> Emne: Re: [PSES] SV: [PSES] Numbers of insulation layers in transformer (IEC62368) Hi Amund: In general, a safety lab does not spend time thinking about hypothetical scenarios. They usually focus on simply complying with the requirements of the applicable safety standard, which in this case is 62368-1. I'm still relatively new at using 62368-1, but I have lots of experience using 60950-1. While there are differences in how the two standards approach the issue of safety for 230 Vac mains, I think the underlying philosophy is the same: 1) 230 Vac mains should not be "accessible" to ordinary users 2) 230 Vac mains should not become "accessible" without a double-fault condition There are lots of ways to meet these criteria. Key to both of the above criteria is the definition of "accessible." The method for making sure that a 230 Vac circuit is not accessible is to carefully apply the definition of "accessible" in the standard. Safety standards typically define "accessible" with various criteria including various specialized probes that simulate a human finger, along with various criteria regarding anticipated use. I'm not sure how 62368-1 defines accessibility, but in 60950-1, a 230 Vac circuit is considered to be accessible if it can be touched by the specified probes without the use of a "tool" (such as a screwdriver). Further, even if a "tool" is required to be able to touch a 230 Vac circuit, that circuit is considered to be accessible if the user is instructed to use a tool to access the area, such as a battery compartment. I don't know the details of the removable part on the product in question, but perhaps the above considerations will help you evaluate the available options. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) [email protected] <mailto:[email protected]> http://www.randolph-telecom.com <http://www.randolph-telecom.com/> From: Amund Westin [mailto:[email protected]] Sent: Thursday, February 17, 2022 2:57 AM To: [email protected] <mailto:[email protected]> Subject: [PSES] SV: [PSES] Numbers of insulation layers in transformer (IEC62368) Thanks, guys! I have not received a fully explanation from the safety lab, but I assume they are concerned about this: - With only one insulation layer between Prim/Sec winding in the transformer, a fault in the insulation tape will result in 230VAC in the secondary circuits. OK, but there are no cable interfaces on the secondary side and no conductive parts. The entire enclosure is flame category V-1 or V-0. But one part of the enclosure is a little piece that is pressed onto the main part of the enclosure. This little part can be demounted, and since the product is plugged into a 230VAC socket that is located 20 cm above the floor, a child could manage to remove this little part and access the secondary circuit (now 230VAC) by its fingers. I guess this is the reason for requiring two layers in the transformer. Or make it impossible to remove this part which can give access to secondary circuits. Best regards Amund Fra: Richard Nute Sendt: 16. februar 2022 23:09 Til: 'Amund Westin' <[email protected] <mailto:[email protected]> >; [email protected] <mailto:[email protected]> Emne: RE: [PSES] Numbers of insulation layers in transformer (IEC62368) Hi Amund: Since the unit does not have grounding/earthing, it is therefore double-insulated with respect to the input 230 volts (and must bear the double-insulated symbol). This requires all accessible conductive parts and accessible surfaces to be double-insulated from the 230 volts. If the secondary circuit has any accessible conductive parts (e.g., metal antenna), then the secondary must be double-insulated from the 230 volts: transformer must have two insulation layers, basic and supplementary. (This is the most common construction.) If the secondary has no accessible conductive parts, then the insulation between the secondary and accessible surfaces (whether or not conductive) is "supplementary insulation": transformer can have one insulation layer, basic. (This is uncommon construction and will likely be not acceptable to many certification houses.) Good luck, and best regards, Rich From: Amund Westin <[email protected] <mailto:[email protected]> > Sent: Wednesday, February 16, 2022 1:02 PM To: [email protected] <mailto:[email protected]> Subject: [PSES] Numbers of insulation layers in transformer (IEC62368) Do I need one or two insulation layers (tape) between the primary and secondary windings? Can find an absolute answer in the standard IEC62368. Product: Input 230VAC Secondary 24VDC for other Low volt circuits No grounding /earthing No cable outputs Plastic enclosure Radio HUB product Thanks. Best regards Amund - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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