Regarding fuse breaking capacity, 10kA fuses are readily available. As is the case with all critical component assessments, compliance to 60950-1 is partly established by verifying correct choice. Appropriately approved components are not tested as part of the 60905-1 test.
It is probably a short coming of 60950-1 that it requires only fuses of "adequate breaking capacity" rather than specifying minimum values. Although there is a (informative) note that suggests that 1500A breaking capacity fuses should be used. There is no lab in the UK that uses a special 1000A peak current source for testing IT equipment - that I know of - 60950-1 does not specify test current specs of the supply. Domestic products are tested in the environment that they will be used, and test labs simulate this. If 1000A can be drawn for a few ms in a fault test then it will be - the supply current is not monitored. The test lab current source will be nominally the same as the domestic one. If the EUT blows up it’s a fail! – and clearly a correctly rated fuse is required, in order to make it pass. Clause 2.7.1 b) ' For components in series with the mains input to the equipment such as the supply cord, appliance coupler, r.f.i. filter and switch, short-circuit and earth fault protection may be provided by protective devices in the building installation.' This means that faults in these parts do not need to be tested, since the installation protection will surely operate; and its not a product specific issue. For other parts an internal fuse is required if faults can produce hazardous currents. But the installation protection is not allowed to operate - the internal fuse must operate first. Also, on another point raised; Clause 5.3.1 of 60950, does allow tests to be applied on incomplete parts of the equipment, if it’s the only practical way to do it. In fact this approach is usually adopted in order to be sure to determine the worst case. A 'real' test may then be able to be performed on a worst case scenario. The 'Note' of 2.7.1 has been removed from 60950-1, and anyway 'notes' in IEC/EN standards are informative and do not have to be read in order to establish compliance (IEC rules). Andy Clifford ________________________________ From: Mark Gandler [mailto:[email protected]] Sent: 21 May 2009 05:52 To: [email protected] Subject: Re: [PSES] IEC 60950-1 short circuit test criteria Pete, this is incredible info for me personally. But what's more intriguing are very few responses this question produced. It seems like then a question raised on a voltage matter, it will get much more researched and UL, IEC will get every spec, test and procedure how to deal with overvoltage, surge, hi-pot, clearances, creepages and so on. when it comes to current, almost nothing. section 5.3 says do the short circuit test and you can relay on circuit breaker for secondary backup protection. Really? by the time it gets to the circuit breaker it is all over with 900A going through UL/TUV/CB/CE/GS certified adapter which explodes like there is no tomorrow. How many people, even HW engineers, realize or understand breaking capacity of the fuse, not fuse rating/tripping/opening point, but how much current fuse can clear before it gets destroyed? You do not want metal fuse to gets blown to pieces inside your product. Isq2/t anyone? Adapter vendor picks 3.15A fuse for 0.2A device, claiming surge, inrush current and over life derating to avoid nuisance tripping and it has 35A breaking capacity. Little solder ball on one of the diodes and boom. Although 1.2 parts per million, but does anyone wants to take a chance? Again, went through all of the testing/certs and all short tests being performed and documented. Put the short on the cap behind the inductor, and it will raise voltage/arcing even higher driven by huge inductance, prolonging the event. I know I do not have extensive expertise in power supplies design and fuse ratings/regulations, but you would expect people on a different committees would get together and discuss it. Would you? Or is it one of those stones better kept unturned? Mark > From: [email protected] > To: [email protected]; [email protected] > Subject: RE: IEC 60950-1 short circuit test criteria > Date: Wed, 20 May 2009 15:52:21 -0700 > > Mark, Brian et al, > > Without trying to deal with all of the issues that this question > raises, I would like to comment on just one aspect from my experience. > > In the North American distribution system the short circuit current > that can be expected from the usual 115V domestic & commercial outlet is on > the order of 7000A. Because of this fuses and circuit breakers providing > protection for these circuits must comply with a 10,000A breaking capacity. > > > In my other life I remember testing a current measurement product > subjected to this 10kA current which produced a surprising failure that > provided ionized/arcing fire from the front panel which ate up the 3AG > instrumentation fuse proposed along with the panel connector and several > inches of the lead wire coming into the product. Needless to say the design > engineer then recognized the inadequacy of the small fuse and designed the > product to accommodate the proper breaking capacity fuse (which was quite a > bit larger). Experience is a great teacher; unfortunately we sometimes > forget too fast. > > br, Pete > > Peter E Perkins, PE > Principal Product Safety & Regulatory Consultant > Tigard, ORe 97281-3427 > > 503/452-1201 fone/fax > [email protected] > > - > ---------------------------------------------------------------- > 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]> ________________________________ Windows Live™: Keep your life in sync. Check it out. <http://windowslive.com/explore?ocid=TXT_TAGLM_BR_life_in_synch_052009> - 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]> - 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]>
