Hi Brian:
I can't help you with the CEC. Both the NEC and CEC are quite wrong to specify overcurrent protection as a function of rated input current. Overcurrent protection is protection for fault condition current, which is not a function of rated input current. The fuse value is always greater than the input current rating, but how much greater? Overcurrent of a mains-frequency transformer is caused by three major faults: 1) Shorted turns of the primary winding; 2) Shorted turns of the secondary winding; 3) Overload of the secondary winding. Each of these faults results in heating of the transformer insulation. Shorted turns are usually mechanically-caused or thermally-caused. Thermally-caused insulation failure can be controlled by a fuse in the primary. Load the secondary and measure the insulation temperature. When the temperature reaches the transformer insulation temperature rating, note the input current. Select a fuse value that is about the same as the overload input current. This will protect the transformer. I've attached a paper that first appeared in the "Product Safety Newsletter," Volume 3, Number 3, May-June 1990 entitled "Selecting the fuse value." The PSN is available to IEEE members at https://ewh.ieee.org/soc/pses/Downloads/newsletter s/90v03n3.pdf Good luck, Rich From: Kunde, Brian [mailto:[email protected]] Sent: Friday, March 27, 2015 1:27 PM To: [email protected] <mailto:[email protected]> Subject: [PSES] NEC vs CEC for Transformer Protection Please help. We have a product, laboratory equipment, that has a 330 watt 1:1 230Vac isolation transformer. 330w / 230V = 1.34 amps. To protect this transformer we applied the US-NEC table 450.3(B) to where the primary protector can be up to 300% of the of 1.34 amps. The transformer manufacturer recommended a 2.25A time lag circuit breaker to handle the inrush current from this transformer. Life is good. Then, we had this product inspected in Canada to which they apply the Canadian Electric Code section 26-256, "Overcurrent protection for dry=type transformer circuits rated 750V or less", which states the primary overcurrent protection device cannot exceed 125% of the transformer current rating. That's 134 amps * 1.25 (125%) = 1.78 amps. Rounded up, the inspector said we had to use a fuse or breaker no larger than 1.8 amps. We notified the transformer manufacturer who said (and we confirmed) that 1.8 amp protection device will nuisance trip due to Inrush Currents. The transformer CSA inspector and a representative from Littelfuse both are telling us that the inspector applied the wrong section of the Electric Code and that section 26-254, "Overcurrent protection for power and distribution transformer circuits rated 750 V or less, other than dry-type transformers" should be applied. This section of the CEC does alien better with the US-NEC. However, what is a "dry-type transformer"?? From my understanding, our transformer is not an oil or dielectric cooled transformer and it should be considered a "Dry-Type" transformer which would make the inspector correct in applying section 26-256. Am I missing something here? It makes sense that the NEC and CEC should track closely in this regard but it appears as Canada's requirement for dry-type transformers trump these low current transformers that would be impossible to properly protect according to their code. Thanks to all for any information or advice. The Other Brian - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
