Descriptive Bulletin 242-32 - BimexC Power Fuses - Type SMD-20 With SMU-20 Fuse Units Outdoor Distribution (14.4 kv through 34.5 kv) current without risk of unwanted operation due

S&C Power Fuses - Type SMD-20 With SMU-20 Fuse UnitsOutdoor Distribution (14.4 kv through 34.5 kv)

The SMD-20 Power Fuse. . . a dropout power fusewith unique low-arc-energy fault interruption and amild exhaust; tailored for pole-top or station mounting-Loadbuster’” operable for convenient load switching

Although it looks and handles like a cutout, the “SMD-20” is a power fuse, offering the full-fault-spectrum protection of all S&C Power Fuses. The SMD-20 Power Fuse is ideally suitedfor protection of transformers, lines, cables, and capacitors located on outdoor distribution feedersor in distribution substations where the higher fault currents, higher voltages, and higher loadsencountered on today’s systems may push cutouts beyond their capabilities. These solid-materialpower fuses provide unique low-arc-energy fault interruption with a mild exhaust-so they’remuch quieter operating than conventional distribution cutouts or horn-fiber power fuses.SMD-20 Power Fuses incorporate precision-engineered nondamageable silver or pretensionednickel-chrome fusible elements. Consequently, the time-current characteristics of these fuses
are permanently accurate and precise-assuring not onlyreliable and predictable performance, but also the continuedintegrity of carefully engineered system coordination plans.The precise time-current characteristics and nondamage-ability of these power fuses permit upstream protectivedevices to be set for faster operation than may be practicalwith current-limiting fuses or with other makes of powerfuses . . . providing better system protection withoutcompromising coordination.
S&C Type SMD-20 Power Fuses-with their universallyused S&C SMU-20 Fuse Units?--are available for loadsthrough 200 amperes at distribution system voltages through34 .5 kv , and p rov ide f au l t - i n t e r rup t ing r a t i ngs o f22,400 amperes rms asymmetrical on systems through16.5 kv; 20,000 amperes rms asymmetrical on systemsthrough 24.9 kv; and 16,000 amperes rms asymmetrical onsystems through 34.5 kv.

SMU-20 Fuse Units are available in a wide variety ofampere ratings, and in four different speeds, i.e., S&C “K,”Standard, Slow, and Very Slow. The broad selection ofavailable ampere ratings and speeds permits close fusing formaximum protection and optimum coordination.

Every SMD-20 Power Fuse is equipped with attachmenthooks for universal load switching using Loadbuster, S&C’sportable loadbreak tool. No disconnects (isolators) orinterrupter switches need to be installed in series with thefuse-providing greatly improved appearance and immediatecost savings. And a selection of four different mountingstyles provides the flexibility required for application in avariety of installations.

† SMU-20 Fuse Units are also utilized in S&C SM-20 and SML-20 Mountings for indoor distribution applications and for application in metal-enclosed gear.

242-32 DESCRIPTIVE BULLETINPage 2 of 20February 10, 1986

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Suitable for a Multitude ofProtection DutiesThe S&C Type SMD-20 Power Fuse has the superiorperformance characteristics and reliability required toprovide two-fold protection-protection for the systemupstream, and protection for downstream lines andcables, as well as for downstream equipment such astransformers and capacitors. With respect to systemprotection, SMD-20 Power Fuses operate promptly onshort circuits, thus minimizing the stresses on theremainder of the system, and they isolate only the faultedsegment, limiting the extent of service interruption. Withrespect to equipment protection, these power fusesprovide full-fault-spectrum protection for single trans-formers and for feeder circuits serving multiple loads.In addition, SMD-20 Power Fuses provide fast isolationof faulted capacitors to guard against capacitor-unit caserupture. In all applications, the positive dropout actionof these power fuses following a fault interruption assurespermanent isolation of faulted circuits and equipment,and provides clearly visible indication of fuse operation.

Transformer ProtectionInstalled on the primaries of power transformers locatedin substations-or on the primaries of pole-mountedtransformers on distribution feeders – SMD-20 PowerFuses provide full-fault-spectrum protection; that is,these fuses will detect and interrupt all faults-large,medium, and small (even down to minimum meltingcurrent); whether the fault is on the primary or secondaryside; with line-to-line or line-to-ground voltage acrossthe fuse; whether the transformer is adjacent to the fuseor connected to it from a remote location; and regardlessof transformer winding connections. Moreover, theSMD-20 Power Fuse has proven capable of handlingthe full range of transient-recovery-voltage severityassociated with all of these conditions.

This superior protection, particularly with regard totransformer secondary-side faults, is possible withSMD-20 Power Fuses because they utilize silver orpretensioned nickel-chrome fusible elements that are notdamaged by transient surges that may heat the elementnearly to the severing point; they are available in a varietyof speeds that provide time-current characteristicsespecially suited to protecting transformers for very-low-magnitude fault currents; and because they possesssubstantial overload capabilities and surge capacity morethan adequate to withstand transformer magnetizing-inrush currents as well as severe hot- and cold-loadpickup currents. As a result of these unique design andperformance characteristics, SMD-20 Power Fuses makeit possible to fuse close to the transformer full-load

ESCRIPTIVE BULLETIN 242-32Page 3 of 20

February 10, 1986


current without risk of unwanted operation due toroutine overloads or harmless transient surges. Closefusing with SMD-20 Power Fuses, coupled with theexceptional low-current fault-interrupting performance,assures maximum protection for the transformer-withrespect to the transformer through-fault protection curvedefined in the latest ANSI Standards-for a broad rangeof secondary-side fault currents, thus minimizing life-shortening thermal and mechanical stress associated withprolonged transformer through-faults.

SMD-20 Power Fuses are also ideal for protection ofauxiliary (station service) transformers and voltagetransformers. Regardless of the application, the main-tenance-free SMD-20 Power Fuse-with the S&CSMU-20 Fuse Unit-provides full-fault-spectrumprotection plus reliable, permanent, and precise coordi-nation with line or secondary breakers and with otherpower fuses.

SMD-50 Power Fuses protecting a three-phase pole-top capacinstallation.


Line and Cable Protection (Sectionalizing)When applied at pole-top locations on distribution feed-ers, or on the secondaries of distribution substation trans-formers, the SMD-20 Power Fuse can interrupt allclasses of permanent faults on overhead lines or under-ground cables, and yet not operate unnecessarily or bedamaged by transient faults. The permanently accuratetime-current characteristics of these power fuses, plusthe wide selection of available ampere ratings and speeds,make SMD-20 Power Fuses ideal for coordination withsubstation reclosers or circuit breakers in “fuse saving”schemes, and also for series coordination with other fuseswhenever greater system segmentation is desired to limitthe scope of outages following permanent faults. And,since every SMD-20 Power Fuse is Loadbuster operable,the convenience and versatility of full-load switching canbe provided anywhere on the distribution system.

Capacitor Bank ProtectionSMD-20 Power Fuses are particularly well suited forprotection of pole-top or station capacitor banks sincethe substantial continuous peak-load capability of thesepower fuses frequently permits the use of smaller ampereratings than is possible with distribution fuse links, othermakes of power fuses, or current-limiting fuses-andwithout nuisance fuse operations (“sneakouts”) due tocapacitor-bank inrush or outrush currents. Close fusingwith SMD-20 Power Fuses results in superior protectionfor the capacitor bank to the extent that evolving faultswithin individual capacitor units, the most commonmode of capacitor-unit failure, can be detected andcleared before case rupture occurs.

Superior to Cutouts orCurrent-Limiting FusesSMD-20 Power Fuses are the ideal protective device foroverhead distribution feeder applications and outdoor dis-tribution substation installations through 34.5 kv, andare a made-to-order alternative to cutouts where:l system available fault currents exceed the capabilities

of distribution cutouts, and wherel the noise and exhaust associated with cutout operation

are unacceptable because of the application or becauseof environmental considerations.Likewise, the SMD-20 is a superior alternative to

current-limiting fuses in applications where:l current-limiting fuses are unsuitable because of their

less-than-adequate time-current characteristics andtheir susceptibility to damage from surge currents

itor

commonly experienced in outdoor distribution appli-cations, and also where

high continuous-current requirements and fuse-handling considerations make current-limiting fuses animpractical selection.

No Need to Push CutoutsBeyond Their LimitDistribution cutouts typically have fault-interruptingratings of 16,000 amperes or less, and may be subjectto application restrictions when applied at systemvoltages of 25 kv or higher. With its higher voltage andinterrupting ratings, the SMD-20 Power Fuse neatlybridges the protection gap between distribution cutoutsand other, more expensive, high-capacity power fusesor current-limiting fuses . . . so there’s no need tooverduty distribution cutouts and settle for incompleteprotection of system conductors or equipment.

SMD-20 Power Fuses provide full-fault-spectrum pro-tection by means of a solid-material low-arc-energy tech-nique of fault interruption having a mild exhaust. Theoscillograms shown at right compare the arc energy ofS&C’s SMD-20 Power Fuse with that of both a typicaldouble-vented and single-vented fuse cutout. In the testrecorded, the relative arc energy for the SMD-20 PowerFuse was only 18% of the energy for the double-ventedcutout, and merely 20% of that for the single-vented fusecutout. Peak arc power-one measure of exhaustenergy-was 9.6 Mw for the SMD-20 Power Fuse,compared to 96.8 Mw and 72.8 Mw for the double-vented and single-vented cutouts, respectively. Likewise,arc voltage-an indirect measure of arc power-was alsosubstantially lower for the SMD-20 Power Fuse thanfor either of the distribution cutouts. Consequently, theSMD-20 Power Fuse is quieter and gentler than acutout.

Graphical comparison of arc energies of the SMD-20 PowerFuse versus double-vented and single-vented distributionfuse cutouts.

S&C Test No. 6605Test Circuit Data-Test voltage (across fuse): 9.0 kv rmsAvailable fault current: 11.4 ka rms asymmetricalTransient-recovery-voltage natural frequency: 2.2 khzTransient-recovery-voltage peak factor: 1.24

DESCRIPTIVE BULLETIN 242-32Page 5 of 20

February 10, 1986


And, No Need to ResortTo Current-Limiting FusesNot only are Type SMD-20 Power Fuses a superior alter-native to distribution cutouts for many applications, butalso-because of their special construction-they aresuperior to current-limiting fuses as well.

Type SMD-20 Power Fuses have helically coiled silverfusible elements that are of solderless construction andare surrounded by air. Because of this construction, thefusible element is free from mechanical and thermalstress and confining support, and therefore is not subjectto damage-even by inrush currents that approach butdo not exceed the fuse’s minimum melting time-currentcharacteristic curve. Current-limiting fuses, in contrast,have fusible elements which consist of a number of veryfine diameter wires, or one or more perforated or notchedribbons, surrounded by, and in contact with, a fillermaterial such as silica sand. Because of this construction,current-limiting fuses are susceptible to element damagecaused by current surges that approach the fuse’sminimum melting time-current characteristic curve.

Such damage may be compounded on overhead distri-bution systems by repetitive current surges occasionedby open/close operations of upstream circuit reclosers.This damage may occur in one or more of the followingways:l The fusible element may melt, but not completely sepa-

rate because the molten metal is constrained by thefiller material-resulting, possibly, in resolidificationof the element with a different cross-sectional area.

l One or more, but not all, of the parallel wires orribbons of the fusible element may melt and separate.

l The fusible element may break as a result of fatiguebrought about by current-cycling that can cause local-ized buckling from thermal expansion and contraction.

Damage to fusible elements of current-limiting fuses,as described above, may shift or alter their time-currentcharacteristics, resulting in a loss of complete coordi-nation between the fuse and other downstream overcur-rent protective devices. Moreover, a damaged current-limiting fuse element may melt due to an otherwiseharmless inrush current, but the fuse may fail to clear

Station Style SMD-50 Power Fuses applied for voltage trans-former protection.


the circuit due to insufficient power flow-with the fusecontinuing to arc and burn internally due to load-currentflow.

Because of the potential for damage to the fusible ele-ment from inrush currents, and because of the effectsof loading and manufacturing tolerances, current-limiting fuse manufacturers typically require that, whenapplying such fuses, adjustments be made to theminimum melting time-current characteristic curves.These adjustments are referred to as “safety zones” or“setback allowances,” and range from 25% in terms oftime to 25% in terms of current. The latter can resultin an adjustment of 250% or more in terms of time,

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depending on the slope of the time-current characteristiccurve at the point where the safety zone or setbackallowance is measured.

Furthermore, most current-limiting fuses inherentlyhave steep, relatively straight time-current characteristiccurves which, together with the required large safety-zone or setback-allowance adjustments, force theselection of larger current-limiting fuse ampere ratingsin order to withstand transformer magnetizing-inrushcurrents and hot- and cold-load pickup currents, andalso to coordinate with downstream protective devices.The selection of such large fuse ampere ratings resultsin reduced protection for the distribution system andequipment. In addition, because the current-limitingfuse ampere rating may substantially exceed transformerfull-load current, coordination with upstream devices canbe severely impaired.

Finally, current-limiting fuses are heavy and diffi-cult to maneuver, particularly with a hookstick; linemenmust carefully steer these fuses when performingopening and closing operations. S&C SMD-20 PowerFuses, in contrast, are easy to handle. The SMU-20Fuse Unit is light and readily maneuvered at the endof a hookstick, and the positive, self-guiding action ofthe fuse hinge and trunnion permits nearly effortlessopening and closing operations.

Select the Best in Power Fuse ProtectionWhen a cutout cannot fully satisfy application require-ments, and when the need for close fusing, precisecoordination, and dependable field-proven performancepreclude current-limiting fuses or other makes of powerfuses, specify S&C SMD-20 Power Fuses for econom-ical, reliable, full-fault-spectrum protection and conve-nient full-load switching (with Loadbuster) of distribu-tion circuits and equipment. Complete applicationinformation and technical data are readily available fromthe fuse specialist at your nearest S&C Sales Office. Callfor complete details.

SMD-20 Power Fuses applied at cable dip for undergroundfeeder-circuit protection.

ESCRIPTIVE BULLETIN 242-32Page 7 of 20

February 10, 1986


The Fusible ElementS&C Type SMD-20 Power Fuses possess the perform-ance characteristics and quality that make them espe-cially suitable for fault protection on distribution systemsrated through 34.5 kv. The fuses are available in a widevariety of ampere ratings and time-current characteris-tics, permitting close fusing to achieve maximumprotection and optimum coordination. The initial andsustained accuracy of their melting time-current char-acteristics assures that these fuses can be depended uponto operate exactly when they should and-equallyimportant-not to operate when they shouldn’t. Thispermanent accuracy is achieved principally in the designand construction of the fusible element.

Nondamageable ConstructionS&C Power Fuses have silver or pretensioned nickel-chrome elements with these characteristics: (1) they aredrawn through precision dies to very accurate diame-ters; and (2) they are of solderless construction, brazedinto their terminals. Their melting time-currentcharacteristics are precise, with only 10% total tolerancein melting current, compared to the 20% tolerance of

242-32242-32DESCRIPTIVE BULLETINPage 8 of 20February 10, 1986

many fuses (20% and 40% respectively, in terms oftime). And their design and construction features assurethat they will conform to their time-current character-istics not only initially, but on a sustained basis . . .they are corrosion resistant and nondamageable . . .neither age, vibration, nor surges that heat the elementnearly to the severing point will affect the character-istics of these fuses.

The construction features illustrated below make S&Cfusible elements nondamageable with these advantages:1. Superior transformer protection. SMD-20 Power

Fuses make it possible to fuse close to the transformerfull-load current, thus providing protection against abroad range of secondary-side faults.

2. Higher levels of service continuity. “Sneakouts”(unnecessary fuse operations) are eliminated.

3. Close coordination with other overcurrent protectivedevices . . . attainable because of the initial and sus-tained precision of the fusible elements, and becauseno “safety zones” or “setback allowances” need beapplied to the published time-current characteristicsto protect the element against damage.

4. Operating economies. There is no need to replaceunblown companion fuses on suspicion of damage fol-lowing a fuse operation.

The SMU-20 Fuse UnitThe SMU-20 Fuse Unit consists of a fusible element,an arcing rod, and a solid-material arc-extinguishingmedium contained within a filament-wound glass-epoxytube.

The fusible element is connected at one end-througha current-transfer bridge-to the fuse-unit exhaustferrule. At the other end, the fusible element is connectedto the arcing rod which extends upward through thestepped bore in the fuse unit. A drive spring locatedinside the fuse unit provides the stored energy to drivethe arcing rod upward through the arc-extinguishingmedium during fault-current interruption. The actuatingpin located at the upper end of the spring-driven arcingrod initiates dropout action of the fuse unit by pene-trating the fuse-unit upper seal and tripping the upper-live-part latch (see pages 12 and 13).

DESCRIPTIVE BULLETIN 242-32242-32Page 9 of 20

February 10, 1986


The Power FuseSMD-20 Power Fuses consist of two basic compo-nents-a complete mounting and a replaceable SMU-20Fuse Unit. The mounting includes the base (or mountingbracket in the case of overhead-pole-top style mount-ings), insulator(s), latch-and-upper-contact assembly,


hinge-and-lower-contact assembly, and reusable upperand lower fuse-unit end fittings.

Fuse-unit end fittings are available separately to enableusers to equip spare SMU-20 Fuse Units for quickreplacement when re-fusing.

Reliable LatchingAs shown in the illustration below, latching is accom-plished as the latch rides over and drops in behind theroller on the fuse-unit upper end fitting.

Due to the impact-absorbing action of the spring-backed contact fingers, the fuse unit will not recoil fromthe latched position when being closed, and cannot dropout due to vibration or shock. And the spray shield helpsprevent accidental tripping of the latch during any high-pressure washing of the insulators.

Superb Current TransferSuperb current transfer between the SMU-20 Fuse Unitand the upper and lower contacts is assured-even afterexposure to the elements for an extended period of time.

Latch-and-upper-contact assembly (fully closed).

Lower contacts-silver-to-silver, p

The silver-clad lower contacts feature embossed sur-faces for built-in wiping action, and are also backed upby pre-stressed loading springs for efficient currenttransfer between the lower contact assembly and thefuse-unit lower end fitting.

The wiping-in, rolling-out contact design featured atthe upper contacts results in minimum electricalresistance between the upper contact assembly and thefuse unit. As the fuse unit is closed into the upper contactassembly, silver-clad contact fingers first engage and“wipe” across the silver-clad surface of the fuse-unitupper end fitting. Then, during latching, as the contactfingers enter the contact detent of the upper end fitting,a high-pressure, low-resistance contact is created byflexing of the contact fingers, augmented by backup fromthe pre-stressed loading spring.

rovide dual current path independent or hinge pivot.


February 10, 1986


Fault Interruption inSMU-20 Fuse UnitsFast, positive fault interruption is achieved in theSMU-20 Fuse Unit-after the fusible elementmelts-by the following means:l High-speed elongation of the arc in the solid-material-

lined bore (as produced by rapid movement of thespring-driven arcing rod), and

l The efficient deionizing action of the gases generatedthrough thermal reaction of the solid material dueto the heat of the confined arc.

Positive Dropout ActionWhen the fuse unit is blown, the force of the drive springcauses the latch-actuating pin at the upper end of thearcing rod to penetrate the fuse-unit upper seal and liftthe latch above the roller on the fuse-unit upper endfitting. After the roller is free of the latch, the spring-backed contact fingers thrust the fuse unit outward,permitting it to swing (by force of gravity) to the fullyopen position. Tripping of the latch and the start of thedropout action during fault interruption are illustratedat right.

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February 10, 1986


Easy to HandleWhen the SMD-20 Power Fuse operates, the fuse unitswings to the open position where it can easily beremoved by inserting a hookstick into the lifting ringof the fuse-unit lower end fitting. Replacement isequally easy. The fuse-unit end fittings are reusable,and are readily removed from the blown fuse and re-installed on a new SMU-20 Fuse Unit. The replacementfuse unit with end fittings is simply dropped into thehinge and closed, as described below.

Easy to CloseSMD-20 Power Fuses may be closed-using a hook-stick-from practically any angle. At the start of theclosing operation, the fuse unit is laterally restrainedby engagement of guiding surfaces on the hinge withsubstantial trunnions and cams on the fuse-unit lowerend fitting. Loadbuster attachment hooks guide thefuse-unit upper end fitting into proper alignment forlatching at the end of the closing operation.

SMD-20 Power Fuses may be closed with a hookstick fromnearly any angle. During closing, the fuse unit is broughtto within several inches of the upper live parts and then,while looking away, fully closed with a purposeful thrust.The Loadbuster attachment hooks guide the fuse unitduring the final approach into the upper-contact assembly.


Easy to Operate with Loadbuster,S&C’s Portable Loadbreak ToolAll SMD-20 Power Fuses are equipped with Load-buster hooks and may be operated with Loadbuster,S&C’s portable loadbreak tool, to provide full-loadswitching at maximum system voltage, as well asswitching of associated magnetizing and line-chargingcurrents. No disconnects (isolators) or interrupterswitches need to be installed in series with thefuse-providing greatly improved appearance andimmediate cost savings. Moreover, because the inter-

1. ATTACH: Reach across thefront of the SMD-20 Power Fuseand attach Loadbuster’s anchorto the attachment hook on the farside of the fuse mounting. Then,engage the power fuse pull ringwith Loadbuster’s pull-ring hook.Loadbuster’s pull-ring latch pre-vents inadvertent disengagement ofthe power fuse pull ring and Load-buster’s pull-ring hook.

S&C ELECTRIC COMPANY- ChicagoS&C ELECTRIC CANADA LTD - Rexdale

a smooth operating stroke, without hesitation, until the

rupting unit is in the Loadbuster tool-and becauseonly one Loadbuster is needed for each truck-theadvantages of low-cost, universal load switching areavailable anywhere on the distribution system.

Switching with Loadbuster is a quick and simpleoperation, as illustrated below. Circuit interruptionoccurs internally, and without any external arc or flame.The only sound is that of Loadbuster tripping. Becausecircuit interruption is independent of the speed at whichthe Loadbuster tool is operated, all that is required is

tool is extended to its maximum length. The resettinglatch retains the tool in the open position for removalfrom the SMD-20-until released to reset Loadbusterfor its next operation.

Resetting Loadbuster is easy, too. Merely release theresetting latch and firmly close the extended tool toits fully telescoped position. It’s that simple.

For detailed information on Loadbuster, S&C’s port-able loadbreak tool , see S&C Descr ip t iveBulletin 811-30.


February 10, 1986


Overhead-Pole-Top Style

242-32 DESCRIPTIVE BULLETIN

Page 16 of 20February 10, 1986

SMU-20 Fuse Units used with these power fuses are available in Refer to table on page 20 for additional, detailed information onratings through 200K amperes as well as 200E amperes. interrupting ratings.


February 10, 1986

S&C Power Fuses - Type SMD-20 With SMU-20 Fuse Units

Outdoor Distribution (14.4 kv through 34.5 kv)

Station Styles


SMU-20 Fuse Units used with these power fuses are available inratings through 200K amperes as well as 200E amperes.

Refer to table on page 20 for additional, detailed information oninterrupting ratings.


February 10, 1986


Short-Circuit Interrupting RatingsThe ratings shown below are the maximum interruptingratings of the fuses listed based upon full line-to-linevoltage across a single fuse. Obviously, this is only onecriterion of fuse performance. These fuses have alsobeen rigorously tested through the full spectrum of faultcurrents, from the lowest to the highest fault-not onlyprimary faults but also secondary-side faults as seenfrom the primary side of the transformer-and underall realistic conditions of circuitry. In all S&C testing,especial attention is given to establishing and control-ling circuit parameters to duplicate conditions as severeas those which will be encountered in the field. Thisinvolves testing at all degrees of asymmetry andmatching of the rate of rise of transient recovery voltageof the test circuit to that found in actual field appli-cations. This rate of rise depends, in turn, on carefully

60-HERTZ SHORT-CIRCUIT INTERRUPTING RATING

SApplies to 25-kv Overhead-Pole-Top Style only, for protection ofsingle-phase-to-neutral circuits (lines or transformers) only.

Nominal rating.


established laboratory test conditions to obtain realisticnatural frequencies and typical amplitudes of transientrecovery voltage.

The short-circuit interrupting ratings listed incolumns 3, 4, and 7 of the table below have beendetermined in accordance with the procedures des-cribed in ANSI Standard C37.41-1981. Moreover, withrespect to the requirement in this standard for testingwith circuits having an X/R ratio of at least 15(corresponding to an asymmetry factor of 1.55), S&C’stests were performed under the more severe conditionof X/R = 20, corresponding to an asymmetry factorof 1.6. Based upon the recognition that there are manyapplications where the X/R ratio is less severe than thevalue of 15 specified by the standard, higher symmet-rical interrupting ratings are listed in columns 5 and 6for X/R = 10 and 5, respectively.

S OF TYPE SMD-20 POWER FUSES

Documents

Descriptive Bulletin 242-32 - BimexC Power Fuses - Type SMD-20 With SMU-20 Fuse Units Outdoor Distribution (14.4 kv through 34.5 kv) current without risk of unwanted operation due