Peter Tarver wrote in reply to my comment "The rating of a UL rated fuse is more or less the current at which it blows. The rating of a fuse to IEC 127 (used throughout Europe) is more or less the working current of the fuse and the circuit it protects"
> This may or may not be true. Last I looked, UL Listed > miniature fuses (typically 1 X 1-1/4in cartridge size) and > branch circuit protection fuses are required to carry 110% > of their rated current for a minimum specified time and 100% > continuously; Listed microfuses are required to carry 100% > of current continuously. For a UL Recognized fuse > (including 5 X 20mm cartridge sizes, of which you most > likely refer), this is not necessarily the case, though it > may be. > > Thus, "the rating of a fuse ... is more or less the working > current of the fuse" is as true for a UL Listed fuse as it > is for an IEC 127 fuse. > > For Recognized Component fuses, any deviation from the base > requirements for Listing is rationale to allow only > Recognition. These base requirements include, but are not > limited to: physical dimensions, current carrying capacity, > "calibration" or time-to-open characteristics, time delay > characteristics for time delay rated fuses, etc. > There is a substantial difference in the rating of UL and IEC127 fuses. UL listed fuses such as 1 x 1ΒΌ in fuses are required to blow at 135% of rating in one hour. IEC127 fuses such as 5 x 20mm are required not to blow in one hour at 150% of rating. The difference in "must blow" and "must not blow" means that the actual current the fuses blow at is very much wider than the 135% to 150% ratio. The first "must blow" current mentioned in IEC127 is at 210% of rating. With fuses only sure to hold at 110% percent of rating you would not use a load with a nominal load of 100% of the circuit rating as small deviations upward of nominal may take out the fuse. That is why it is common to derate branch circuits in the US. This is not needed with IEC127 fuses. It is quite reasonable to have the nominal load equal to the rating of the protection. My characterisation of the two rating systems being "blowing current" and "working current" may be fairly crude but it is not that far from true I do not have the figures for circuit breakers to hand but I believe there is a similar disparity in European and US ratings. The current carrying rating of European wiring regulations obviously reflect these differences. If fact these differences might have something to do with the differences in the rating of IEC320 C13 & C14 connectors in the US and Europe discussed in this forum recently, 15A in the US 10A in Europe. Peter further wrote "One is left with the question: are fuses used throughout Europe as an integral part of mains circuit protection? By this I include the power supply cord as an extension of the mains, whether or not it is included by definition or is absolutely correct in everyone's perspective." There is certainly a move towards circuit breakers and away from fuses but fuse protection is still being used in new domestic premises and some industrial ones also and there is a huge installed base of fuse protected wiring. The latest issue of the UK wiring regulations have tried to push the move to circuit breakers by allowing smaller wire to be used in circuit breaker protected circuits of the same rating as fuse protected circuits to reflect the closer ratio of must break to must hold currents of circuit breakers. The UK ring main system is unusual if not unique. Because the ring main will be protected by a 30A or 50A fuse or breaker and the plugs and sockets on the ring rated at 13A (despite their vast size) and the power cord possibly rated considerably less than this, the fuses in BS1363 plugs used in the UK are an essential part of electrical safety in the UK. > > One is left with the question: are fuses used throughout > Europe as an integral part of mains circuit protection? By > this I include the power supply cord as an extension of the > mains, whether or not it is included by definition or is > absolutely correct in everyone's perspective. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall,"
