汽车电器保险.5.08

Circuit Protection Products and Application TrainingLittelfuse Introduction Product trainingApplication trainingLittelfuse IntroductionProduct trainingApplication training–Basic FuseologyTerms and definitionsFuse rating and wire sizeGeneral fuse selectionFuse selection -steady state currentFuse selection -transient pulseFuse selection -transient currentWhat is a Fuse?1.A fuse is a thermally responding device.2.A fuse is designed and manufactured to protectelectrical components by being the intentional weak link in an electrical circuit.3.Each rating is characterized by a range of resistancethat defines its overload characteristics“Blow Time”The amount of time required for a current to melt the fusing element.It is determined by the amount of current passing through a fuse and the fuses rating.Blow time is widely interchanged with Opening -, Melting -or Prearcing time.Arcing TimeThe amount of time elapsing from the end of the melting time to the final interruption of the circuit.Arcing time is significantly affected by the voltage in the circuit and the fuse design.Clearing TimeClearing Time = Melting Time + Arcing TimeCold ResistanceResistance measured @ room temperatureResistance measured without electrical loadCold resistance has a direct correlation to the Blow Time (next slide)Cold Resistance is used to check 100 % of the fuses for it’s performance3.503.102.702.301.901.501.100.700.30-0.106.807.007.207.407.607.808.008.208.408.608.80L o g (135% B l o w t i m e i n s e c )Cold resistance (mOhm)Cold Resistance vs. Blow timeOverloads / Short circuitsAn Overload is an over current which is confined to normal current paths.An Overload is usually 100 and 600 % of rated current (In).Overloads can occur when:–Too many devices are connected to a circuit.– a device on the circuit draws higher than normal current because of amalfunction. A Short Circuit is an over currentwhich is out of its normal path.A Short Circuit is usually above600 % of rated current (In).Short Circuits can occur when:–Collision -Wiring harness damage creates a direct path fromalternator or battery to ground.–Malfunction -Pinched, worn ordegraded wire insulation causesdirect path to ground.T/C Gates (Time / Current Gates)40 Ampere MAXI fuse T/C GatesPercent of I n Opening Time100 135 200 350 600100 h. min.60 -1800 sec.2 -60 sec.0.2 -7 sec.1.0 sec. max.T/C Curves (Time / Current Curves)MAXI T/C Curves0.010.11101001000110100100010000Current [A]T i m e [s ]20 A30 A 40 A 50 A 60 ATime in seconds (logarithmic)Current in Ampere (logarithmic)T/Ccharacteristics for each ratingTypical 40 Amp T/C Curve0.010.1110100100010100100010000Current [A]T i m e [s ]Reading T/C Curves1.Choose point of interest2.Draw a horizontal line3.Draw a vertical line4.Read Time and current In this example Opening time would be1 s @ ~200 Afuses earlier (Shift to the left) Lower ambientfuses later(Shift to the right)Maxifuse Temperature Rerating Curve7580859095100105110115-40-30-20-100102030405060708090100110120130140Temperature (°C)R e r a t i n g F a c t o r (%)How Re-rating is DeterminedFast Acting FusesA fuse which opens on overloadand short circuits very quickly.This type of fuse is notdesigned to withstandtemporary overload currents.Time Delay FuseA fuse with a built in time delaythat allows temporary andharmless inrush currents topass without opening, but is so designed to open on sustained overloads and short circuits.M i n iM a x iJ C A S EContentLittelfuse IntroductionProduct trainingApplication training–Basic FuseologyTerms and definitionsFuse rating and wire sizeGeneral fuse selectionFuse selection -steady state currentFuse Selection -transient pulseFuse selection -transient currentFuse rating and wire sizeMAXI T/C Curves0.010.1110100100010100100010000Current [A]T i m e [s ]20 A30 A 40 A 50 A 60 ACable T/C Curves0.010.1110100100010100100010000Current [A]T i m e [s ]1 mm^22 mm^23 mm^25 mm^2MAXI & Cable T/C Curves0.010.11101001000101001000Current [A]T i m e [s ]20 A 30 A 40 A50 A 60 A1 mm^22 mm^23 mm^25 mm^2Fuse rating and wire sizeContentLittelfuse IntroductionProduct trainingApplication training–Basic FuseologyTerms and definitionsFuse rating and wire sizeGeneral fuse selectionFuse selection -steady state currentFuse selection -transient pulseFuse selection -transient currentCase differentiations02468100246810Time [ms]C u r r e n t [A ]02468100246810Time [ms]C u r r e n t [A ]2468100246810Time [ms]C u r r e n t [A ]1. Steady state2. Transient pulse3. Transient currentRealistically, most applications are acombination of steady state and transient over currents.General Fuse SelectionApplicable for all three cases–Voltage rating–Interrupting rating (Breaking capacity)–Time current characteristics–Re-rating for ambient temperatureUnsafe Fuse OpeningContentLittelfuse IntroductionProduct trainingApplication training–Basic FuseologyTerms and definitionsFuse ratings and wire sizesGeneral fuse selectionFuse selection -steady state currentFuse selection -transient pulseFuse selection -transient current02468100246810Time [ms]C u r r e n t [A ]Recommended loading is 75% of the fuse rating Recommended loading is 75% of the fuse rating Calculation of the ideal fuse rating (I f ) depending of the nominal operating current (I n )Ideal fuse rating (If)Nominal operating current (In)Temperature re-rating factor (RR)Application formula:In(RR*0.75) I f=Conditions:Nominal current = 15 A Temperature = 105 °C Find: Ideal fuse ratingRe-rating for MINI fuse @ 105 °C = 88 %I f = = Choose next highest rating = 25 AmpI n(RR*0.75)Fuse selection -steady state current Example15 A(0.88*0.75)= 22.7 AContentLittelfuse IntroductionProduct trainingApplication training–Basic FuseologyTerms and definitionsFuse rating and wire sizeGeneral fuse selectionFuse selection -steady state currentFuse selection -transient pulseFuse selection -transient current0246810246810Time [ms]C u r r e n t [A ]Fuse selection -transient pulseTransient pulse has no steady state current Calculation of how many pulses the fuse can withstand This is done with the comparison of I2t of the pulse and the fuse (relative I 2t) I 2t is the heat energy of the pulse and the fuseMelting I 2t (Fuse)What is melting time?–The amount of time required for current to melt the fusing element.C u r r e n t (A )Timemelting time arcing timeElement melted,Arcing beginsIntegrated to calculate Melting I 2ti(t)Time (t)C u r r e n t (A )t 1t 2i 2(t)Time (t)C u r r e n t (A )t 1t 2i 2(t)I 2t of a PulseI 2t = area under I 2curveExampleWill it survive 10’000 pulses?5 amp ATO fuses subjected to repetitive current pulses of the following form:TimeI 2t of pulse = I 2(0.10 sec.)C u r r e n tI = 10 A100 ms max.10 sec.min.Relative I 2t =I2t of Pulse Melting I 2t of FuseExampleI 2t of the pulse = 0.1 s * 10 A * 10 A = 10 A 2s Melting I 2t of the fuse = 26 A 2s (from catalogue) Relative I 2t = 10 A 2s / 26 A 2s = 38 %Relative I 2t =I 2t of Pulse Melting I 2t of FuseExamplePulse Cycle Withstand Curve for typical blade FuseActual # of pulsesconfidence limit The fuse could withstand a minimum of 10,000 pulsesContentLittelfuse IntroductionProduct trainingApplication training–Basic FuseologyTerms and definitionsFuse rating and wire sizeGeneral fuse selectionFuse selection -steady state currentFuse selection -transient pulseFuse selection -transient current2468100246810Time [ms]C u r r e n t [A ]Fuse selection -transient currentTransient current (inrush current) is a transient pulse followed bya steady state current Calculation of how many pulses the fuse can withstand and ifthe fuse is strong enough for the steady state This is done with the comparison of the cumulative I 2t of the pulse and the fuse (relative I 2t) Cumulative I 2t is cumulated energy over time01234567891000.20.40.60.811.21.4Time (seconds)C u r r e n t (A m p e r e s )i(t)ExampleAssume a non-adiabatic pulsing event.Example: Consider the transient pulse i(t) shown. Will a 5 amp ATO Fuse survive 1000 cycles of this pulse?ExampleCumulated I 2t of the pulse Determine the cumulative I 2t of the pulse versus time by:1.) Plotting I 2(t) vs. time2.) Numerically integrating under the I 2(t) vs. time graph.010203040506070809000.20.40.60.811.2 1.4Time (seconds)I 2(t) (in A 2)cumulative I 2t of pulse (in A 2sec)Example0.010.11101001000110100Current (Amperes)O p e n i n g T i m e (S e c o n d s )17.48Melting I 2t of the fuseSelect a fuse rating based on an initial best guess (5 amp for our example). Calculated the fuse’s melting I 2t at different times using a time-current curve (as shown at right).at 0.1 sec, I 2t = (17.4 A)2(0.1 s) = 30.28 A 2s at 1.0 sec, I 2t = 64 A 2sExamplePlot the fuse’s Melting I 2t vs. Opening Time0102030405060708000.20.40.60.811.21.4Opening Time (seconds)M e l t i n g I 2t (A 2s )30.4 A 2s @ 0.1 s64 A 2s @ 1.0 sExamplePlot Cumulative I2t of the pulse and Melting I2t of the fuse on the same graph. Compute the relative I2t ratio at each time interval:Relative I2t =I2t of PulseMelting I2t of Fusemelting I2t of fusecumulative I2t of pulse102030405060708000.51 1.5Time (seconds)I2t(amperes)2(seconds)1956Time (seconds)Relative I2t Ratio0.20.410.40.410.60.370.80.341.00.311.20.291.40.2711010010001000010000010000000.11relative I 2tn u m b e r o f p u l s e s252050200500200050002000050000200000500000 Use the Pulse Cycle Withstand Curve to predict the number of pulses the fuse will survive.For example, a fuse with a 0.41 relative I 2t pulse would survive at least 4,500 cycles of the given pulse.Relative I 2t RatioPredicted # ofPulses0.60 10 0.50 200 0.41 4,500 <0.37 >10,0000.414500ExampleMutual Heating EffectsA fuse which is energized radiates heat (I2R) which effects the fuses surrounding it.This mutual heating causes the fuses to be hotter than they would be if they were placed separately.Mutual Heating EffectsspacingSpacing between fuses willinfluence performanceThe highest temperature in the fuse box can be reduced to an acceptable levelby allowing more space between fuses.Widen fuse spacingMutual Heating EffectsMutual Heating EffectsThe strategic placement of fusescan help alleviate the localizedheat buildup without changing the spacing.。