Greetings Experts. I need some understanding regarding properly protecting a transformer (according to the 610101-1) vs the requirements of the US National Electric Code (NEC).
Background: A high tech piece of laboratory equipment about the size of a dishwasher, designed and tested to the safety requirements of the UL/IEC/EN 61010-1 (currently not NRTL listed). Within this instrument we have several linear power supplies each fed by a step down transformer about the size of you fist (or smaller). These transformers step down 230VAC 50/60hz to 24V, 12V, 5V, 3.3V (you get the idea). These transformers are panel mounted separate from the rest of the power supply circuit for ease of assembly and to improve cooling and air flow. The instrument has a 9 amp circuit breaker on the Mains. Because the primary current on these transformers are so low (in some cases 0.01 amp), we use secondary fuses to protect the transformers from overheating during the secondary Overload and Short-Circuit fault tests according to the 61010-1 standard. The 61010-1 standard does not dictate how to protect the transformer, only that is must be done. (generally we use a primary fuse, or a secondary fuse, or both, or a thermally protected transformer). However…. On occasion our instruments get inspected at the Customer Site to the requirements of the NEC. This is either done by the local electrical inspector or the company will hire an inspector to satisfy their insurance company and/or OSHA inspector. Whatever the case, these inspectors will sometimes say that Article 450 of the NEC requires a primary over-current protection device on ALL transformers. In our case, they say that the requirements of Table 450-3(b) apply where we have to have a primary fuse no larger than 250% of the primary current. Finding a 0.05 amp fuse is not easy and we fear that the inrush current on these transformers will fault trip such a small fuse value. When I argue that such a primary fuse would not protect the transformer they say, “I’m only going by the letter of the law”, or something like that. When I ask if Exception No. 2: applies (Dry-type transformers that constitute a component part of other apparatus and comply with the requirements for such apparatus.) they say NO because the transformers are not mounted as part of the linear power supply circuit. So (finally getting to the question), do I really need to have a primary over-current protection device on every transformer regardless if it serves any purpose or not? OR should the NEC requirements not apply to such an application? A little more info: The transformer primaries are current limited by a 9 amp circuit breaker in a branch circuit. The wiring can handle this current so a primary fuse is not needed to protect the conductors. Thanks to all. The Other Brian _________________________ LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - 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.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html 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]>
