Fuse Technology Bussmann - Cooper Industries

1 Basic fuse Technology Fuses serve two main purposes:a. To protect components and equipment from costly damage causedby To isolate sub-systems from the main system once a fault exist when the normal load for a circuit is exceeded. It canbe either an overload or short circuit. An overload is any current flowingwithin the normal circuit path that is higher than the circuit normal full short circuit is an overcurrent which greatly exceeds the normal full loadcurrent of the circuit. Also, as the name infers, a short circuit leaves thenormal current carrying path of the circuit and takes a short-cut aroundthe load and back to the power source. Components and equipment canbe damaged by both types of Overcurrent ProtectionDuring normal conditions, the fuse must carry the load current of the cir-cuit without nuisance openings.

2 However, when an overcurrent occursthe fuse must interrupt the overcurrent, and withstand the voltage acrossthe fuse after arcing. To properly select a fuse the following items must beconsidered: Voltage rating (AC or DC voltage) Characteristics of equipment or Full load currents (RMS Amperes)components to be protected Available short circuit current Ambient conditions In-rush characteristics Standards requirementsIn addition consideration must be given to: Available board space Reliability Type of mounting Ease of field service Automatic or manual insertionElectronic circuits frequently exhibit surges, caused by capacitors charg-ing, motors being momentarily stalled, or high voltage components spark-ing over. It is important that designers take account of these temporary conditions during fuse selection.

3 The ability to resist surges is a functionof the fuse design relative to the surge pulse, duration, frequency of a manufacturer s I2t value alone is not sufficient, andBussmann would be pleased to advise on specific RatingsThe voltage rating of the fuse must be greater than or equal to the circuitvoltage. Because the fuse has such low resistance, the voltage ratingbecomes critical only when the fuse is trying to open. The fuse must beable to open quickly, extinguish the arc after the fuse element has melt-ed and prevent the system open-circuit voltage from restriking across theopen fuse RatingsEach fuse is marked with a nominal current factors can actually affect the ability of the fuse to carry this ratedcurrent. First the base material of the clip in which the fuse is mounted maygreatly affect the performance of the important factor is the conductor size used to connect the fuseto other circuit components.

4 If the conductor is too small, it will generatea heat rise. That extra heat will be seen by the fuse , causing the fuse toopen before it should. It is also important that the fuse be installed withclean and tight connections. If the connections are dirty or loose, they willcause increased resistance, generating extra heat. That heat will lead toa shortened fuse Rating (Breaking Capacity)A fuse must be able to open the circuit under a short circuit without los-ing case integrity. The breaking capacity of a protective device is the max-imum available current, at the rated voltage, that the device can safelyopen without ResistanceIn most applications, the voltage drop across the fuse due to its internaland contact resistances is negligible.

5 There are, however, certain criticalapplications where the fuse resistance must be considered, and it is impor-tant that the circuit designer understands the fuse characteristics in orderto select the proper SizesThere are numerous physical sizes of electronic small dimension fuses,including sub-miniature fuses. The most common are 5mm @20mmand 9@1 9 ( fuses are designed for applications where board footprintusage is of critical Standards are very different from North AmericanStandards. International Electrotechnical Commission (IEC) writes the stan-dards followed by many European and Asian countries. Among the com-mission members are:AustraliaJapanSouth the electrical characteristics of these fuses are so different, NorthAmerican and IEC rated fuses are not interchangeable.)

6 When designingproducts to go international , it is important to consider that world stan-dards may require different of these countries conduct their own testing, such as VDE, theGerman testing agency. However, most accept the testing of SvenskaElektriska Material Kontrollanstalten or Semko, the Swedish testing products tested by Semko that pass the IEC requirements aremarked with testing by European agencies revolves around voltage drop, time/current characteristics, breaking capacity and endurance tests. However,the biggest differences between North American and European standardsare the time current characteristics. As shown on the next page, fuses built to North American Standards are not compatible with European Sizes Of Fuses5mm 9@fi 92AG (5mm 9@1 94AG* 9@1 95AG 9@1 97AG* 9@ 98AG* 9@19*Not popular for new designsFuse TechnologyBussmann Form No.)

7 SFB fuse 1 of 3 BIF Document 800512-18-98SB98107 Rev. ABussmann fuse TechnologyNotice that at 135% of its rated current a North American fuse must openwithin one hour while an IEC fuse must be able to carry 150% of its ratedcurrent for at least one IEC uses symbols to denote fuse time/current characteristics, andbreaking Symbol:LETTER COLOR BREAKING CAPACITYCODECODESYMBOLFF Super Quick ActingBlackL: Low Breaking CapacityF Quick ActingRed35A @ 250V AC or 10 @Rated CurrentM Medium Time LagYellowH: High Breaking CapacityT Time LagBlue1500A @ 250V ACTT Super Time Lag`GreyTypical interrupting ratings of North American Listed electronic fuses is10,000 amperes for all voltage levels except 250 volt AC fuses.

8 250 voltAC listed fuses are first tested as 125 volt AC fuses, in which case theymust have a 10,000 ampere interrupting rating. However, at 250 volts, thesefuses, as shown below, may have lower interrupting Ratings (Breaking Capacity)of North American Listed FusesVoltageAmp RatingRating (AC)of FuseShort Circuit Current UL Minimums125V*All10, 301,500A300 VAll10,000A500 VAll10,000A600 VAll10,000A, 50,000A, or 100,000A*Does not include micro Electrotechnical Commission (IEC)Fuses designed to IEC standards must meet a breaking capacity test. Thevarious breaking capacities for the four main IEC fuse types are Ratings (Breaking Capacity)Per IEC StandardsTypeCase SizeBreaking CapacityQuick ActingHigh-Breaking 5mm @20mm1,500 ACapacityQuick-Acting35A or 10 times rated currentLow-Breaking5mm @20mmCapacitywhichever is or 10 times rated currentLow-Breaking5mm @20mmCapacitywhichever is or 10 times rated @32mmCapacitywhichever is , both North American and IEC fuses interrupting/breaking capac-ity are tested using AC.

9 Their DC ratings could be different because ofcircuit time constant considerations. It is generally easier for a fuse tooperate under AC than Versus North American StandardsNorth American Listed or Certified FusesInternational Electrotechnical Commission FusesMiscellaneous and Miniature Types*(Publication 127)PercentFast-ActingTime-DelayFast-Act ing FusesFast-Acting FusesTime-DelayOf FuseAmpereFusesFusesSheet 1 Sheet 2 Fuses Sheet Cont. 135%0-30A 1 hr. 1 1 hr. 1 hr. 1 hr. 200% 2 2 30 min. 30 min. 2 sec. sec. 32-100mA .01 .05 .02 sec..02 .02 sec..02 sec.*Does not include micro fusesForm No. SFB fuse 2 of 3 BIF Document 800512-18-98SB98107 Rev.

10 ABussmann Glossary of TermsForm No. SFB fuse 3 of 3 BIF Document 8005 AmpereThe measurement of intensity of rate of flowof electrons in an electric circuit. An ampereis the amount of current that will flowthrough a resistance of one ohm under apressure of one RatingThe current carrying capacity of a a fuse is subjected to a current aboveits ampere rating, it will open the circuit aftera predetermined period of Squared Seconds, I2tThe measure of heat energy developed with-in a circuit during the fuse s clearing. It canbe expressed as melting I2t . I stands foreffective let-through current (RMS), which issquared, and t stands for time of opening,in TimeThe amount of time from the instant the fuselink has melted until the overcurrent is inter-rupted, or Capacity(See Interrupting Rating)Cartridge FuseA fuse consisting of a current responsive ele-ment inside a fuse tube with terminals onboth CC Fuses600V, 200,000 ampere interrupting rating,branch circuit fuses with overall dimensionsof 13/32 x 11/2.