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Does Functional Insulation always need to be evaluated per Annex B.4.4, including for Class III products?

More specifically you asked: “I have a couple of questions from 60950-1 but may equally applicable to 62368-1: (1) If you have a product which is classified as functional safe, do we need to validate the insulation as per 5.3.4?  (2) If you take the path of 5.3.4 b) to prove the clearance and creepage of your product, the insulation tested is between the input conductors and the conducting chassis? i.e. we bundle al the input conductors together and apply the test voltage across the bundled conductors and the chassis?”


In response, it is noted that Annex B.4.4 of IEC 62368-1 includes similar though not identical requirements for evaluation of Functional Insulation to those in sub-cl. 5.3.4 of IEC 60950-1. In this forum we focus on application of IEC 62368-1 so your questions will be addressed in this context.  Also, although Class III equipment often is referred to as ‘inherently safe’ or ‘functional safe’, these terms are not used in IEC 62368-1 and can be misleading. 


IEC 62368-1 uses Hazard Based Safety Engineering principles, which include identification and classification of Energy Sources and then identification and qualification of Safeguards. While Class III equipment includes circuits with voltage and touch currents that are considered reduced risk of electric shock, they still are required to have their energy sources classified. 


Why? Class III equipment may include both class ES1 and ES2 circuits, combined with PS1 through PS3 energy sources, which need classification, including under Single Fault Conditions. 


As a consequence of classification, Class III equipment, even if not requiring safeguards for electric shock, still may require other safeguards against fire, including Basic and possibly a Supplementary Safeguard. When applicable, effectiveness of safeguards are required to be verified, including during single fault conditions, which are described in Annex B.4. 


Therefore, in response to Question 1, yes, evaluation of Functional Insulation per Annex B.4.4 is always required per one of the options allowed for, either minimum spacings (Clearance and Creepage Distance), minimum Electric Strength, or Short-circuit. 


The answer to Question 2 would require a more in-depth understanding of the specific construction.  For an in-depth consultation you are encouraged to contact UL, either via your local account executive or via

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