Dear Jeff, Yes I have a suggestion. Talk to your test house. Convince them that transformer meets all insulation requirements in the standards (ie applicable clearances, creepages, distances through insulation, electric strength tests, and insulation system (A, B, C, E, F, H) as applicable) and that the likelihood of a transformer shorting or overloading at the windings is very unlikely. Thus, you could provide an Approved fuse or protective device for each of the secondaries but as a first component after the windings. This type of construction is acceptable for Medical Electrical Equipment investigated to UL2601-1.
Of course, the transformer circuit would need to be additionally evaluated. Unreliable components after the fuse or protective device would be opened or shorted to determine for any hazards. Also, with the fuse or protective device bypassed, an overload test would be conducted at a percentage of the fuse rating; check the standard and/or your test house for additional details regarding these tests. Remember, safety engineers (like many of us!) at the various test houses are used to seeing thermal cutoffs and/or automatic or manual reset overtemperature protective devices within the windings and/or fuses/circuit breakers in the primary. You must lead the way and prove that you meet all construction and performance criteria in the applicable standards. Also, you mention you are sure that with a 500 V/100 mA fuse in primary applying the 2.1 x 100 mA test in EN60742 would burn the transformer. What kind of UL1446 Recognized/Certified insulation system (not insulating materials) are you using? One solution is to try a higher level of insulation system than the one you are using today. Best Regards and Happy and Safe Holidays and please don't let that transformer burn. > Hi folks, > > I have a question about transformer fusing. I have an application that uses > a 10W transformer connected across 400Vac mains. It is my understanding > that this requires a fuse rated to at least 400V, which would mean 500V > because that's the next larger size available. However, the smallest > amperage fuse available at that voltage rating is 100mA. This by far > exceeds the transformer rating. I thought about performing the EN 60742 > test of loading the transformer until the primary current is 2.1 x 100 mA = > 210 mA and taking thermal measurements, but I am quite sure the transformer > will burn up under this large a load. > > The standards being used to evaluate this product are: > EN 50178 > UL 1012 > CSA C22.2 No. 107.1 > > Any suggestions? > > Regards, > > Jeff Jenkins > Senior Regulatory Compliance Engineer > Advanced Energy Industries, Inc. > Fort Collins, CO USA 80525 > > Opinions are my own and not necessarily shared by Advanced Energy > Industries, Inc. or its affiliates. > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to [email protected] > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to [email protected], > [email protected], [email protected], or > [email protected] (the list administrators). > PETER S. MERGUERIAN MANAGING DIRECTOR PRODUCT TESTING DIVISION I.T.L. (PRODUCT TESTING) LTD. HACHAROSHET 26, P.O.B. 211 OR YEHUDA 60251, ISRAEL TEL: 972-3-5339022 FAX: 972-3-5339019 E-MAIL: [email protected] Visit our Website: http://www.itl.co.il --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
