1. actually, i only said that the O/L test is done without the protection device in circuit. no comment was made on the S/C test - left as an exercise for the reader.
2. my comments were in the context of NFPA 70, typically refered to as national electric code - which can, for some installation categories, can be very different from BS7671. E.G., there are some installations in various north american electrical codes where the xfmr must be tested to many days during both o/L and s/c, with out any external protection device in the circuit. 3. xfmrs must, in fact, have some form of 'inherent' limits to their output impendance. > -----Original Message----- > From: Andy Clifford [mailto:[email protected]] > Sent: Wednesday, May 27, 2009 8:50 AM > To: 'Brian O'Connell'; [email protected] > Subject: RE: [PSES] Protecting Transformers and the NEC > > Actually EN61010-1 does leave transformer protecting devices > in circuit for > the short circuit test. For the overload test the protecting > device is > shorted and the current for the test limited to the > theoretical maximum that > the protecting device can take without blowing. The > transformer does not > have to be inherently safe or impedance limiting, just > designed to cope with > the fault current. > > Best regards > > Andy Clifford > > -----Original Message----- > From: Brian O'Connell [mailto:[email protected]] > Sent: 27 May 2009 16:15 > To: [email protected] > Subject: Re: [PSES] Protecting Transformers and the NEC > > did not see any replies, so here we go... > > 1. inspector viewpoint -> UL61010-1 states conformity to the > standard = > NEC conformity - the device has NOT been tested to UL61010-1, so no > presumption of NFPA conformity can be assumed. > 2. in general, if xfmr outputs are accessible or not > insulated, then the > transformer should be inherently limited per Class 2/3 > requirements (see > UL5085-3), or all outputs shall be protected. > 3. EN/UL61010-1 has specific overload and short tests for > all secondary > windings of xfmrs connected to mains. The fuse is NOT in the circuit > during the O/L test, so the xfmer must have some limiting > impedance built > in to pass this test. > 4. the 9A CB in the input can only be depended for > protection (that is, > used during Type Tests) of equipment if it is permanently connected. > otherwise, assume 20A input. > 5. if all xfmrs are inherently limited, i.e., certified as a > fail-safe > according to IEC61558-2-6 and/or Class 2/3 inherently > limited according to > UL5085-3, and if proper materials for input wiring are used, > input ckt of > unit should conform to NEC. > 6. NFPA70 Table 450.3 is scoped for building equipment, i.e., a > distribution xfmr that forms the input to a branch ckt. your box is > connected to a branch ckt - WHY IS THE INSPECTOR NOT USING > ARTICLE 725 ?? > > do not feel alone - i get similar stupidity from > 'inspectors' that audit > the end-use boxes that my customers build. is there a fire > marshal lurking > that can comment on this ?? > > -----Original Message----- > From: [email protected] [mailto:[email protected]]On Behalf Of Kunde, > Brian > Sent: Friday, May 22, 2009 7:01 AM > To: [email protected] > Subject: Protecting Transformers and the NEC > > 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 - 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. 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