This is why small transformers that don't need a fuse were developed. They have a special type of coated wire for the primary winding, which goes quietly open-circuit if it gets too hot. However, I suppose your product standard might not allow them. The limits based on load current are technically indefensible, but that may not help. The right way to determine what fuse you want is to take account of the worst-case inrush current, which occurs when the core is left fully magnetized at switch-off and the supply is switched on at the point where it is trying to magnetize it further. The current is then limited only by the DC resistance of the primary winding, and you need to look at the fuse I^2T curves to select one (usually it needs a T-type) that doesn't fail on inrush but does fail on about 1.5 times the full-load current of the transformer. That is most unlikely to be the 1.1 mA you mention (32 mA being 3000% of it). I doubt you can get a transformer rated at less than about 1.5 VA, which with 120 V input means a full-load primary current of 12.5 mA. I measured one like that and the primary resistance is 600 ohms, so the inrush current could be 200 mA. That means that you need a fuse that will pass 200 mA for at least half a cycle of 60 Hz, while breaking within an acceptable time at, say, 2 x 12.5 mA. It may indeed be difficult to find one, but at least you know what the component is actually required to do. I suspect that you can find a fuse that works for a 3 VA transformer; 1.5 VA is very extreme. With best wishes DESIGN IT IN! OOO – Own Opinions Only <http://www.jmwa.demon.co.uk/> www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England Sylvae in aeternum manent. From: Kunde, Brian [mailto:brian_ku...@lecotc.com] Sent: Monday, October 31, 2016 3:59 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Low current Transformer OC Protection Greeting Experts. I often run into issues with safety inspectors during Field Evaluations regarding overcurrent protection of small transformers that are not thermally protected. These small transformers can draw such small amounts of current on both the primary and secondary, that finding a fuse within 300% (250%) of the primary or 167% of the secondary max load current is impossible. Even if we use the smallest fuse we can find (Littelfuse 218 series is 0.032A), this value can be over 3000% of the load current. Even if we provide thermal test data to an inspector, they will reply that our data is worthless to them as they only accept data from their own lab or other NRTLs. So am I missing something here? The NEC says I can use the next highest common fuse value. Is this acceptable even if the value is exceeds 1000% of the load current? Has anyone ran across this issue with inspectors? How best do we protect small transformers and meet the electric codes? Thanks in advance. The Other Brian _____
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