s&c power fuses types smd-1a, smd-2b, smd-2c, · pdf files&c power fuses typ es...

S&C Power FusesTypes SMD-1A, SMD-2B,

SMD-2C, SMD-3, and SMD-50

Outdoor Transmission (34.5 kV through 138 kV)

Descriptive Bulletin 212-30April 26, 2004

Supersedes Descriptive Bulletin 212-30 dated 3-15-93 ©2004

S&C ELECTRIC COMPANYSpecialists in Electric Power Switching and Protection

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S&C SMD® Power Fuses Set the Standard of Excellence forTransformer and Capacitor-Bank Protection in Utility and IndustrialSubstations

S & C E L E C T R I C C O M P A N Y

S&C Power Fuses—Types SMD-1A, SMD-2B, SMD-2C, SMD-3, and SMD-50 provide reliable and eco-nomical protection for transformers and capacitorbanks in outdoor substations served at voltages of34.5 kV through 138 kV. Like other S&C power fuses,SMD Power Fuses incorporate precision-engineerednondamageable silver or nickel-chrome fusible ele-ments. Consequently, the SMD’s time-current char-acteristics are precise and permanently accurate—assuring not only dependable performance, but alsothe continued reliability of system coordinationplans. The precise time-current characteristics andnondamageability of these power fuses permitsource-side protective devices to be set for fasteroperation than may be practical with other powerfuses or power circuit breakers, thereby providingbetter system protection without compromisingcoordination.

SMD Power Fuses are offered with maximumcontinuous current ratings from 100 amperes to 300amperes (depending on fuse type and voltage rat-ing), and are available with fault-interrupting ratingsas summarized in the tables on pages 18 and 19.Fuse units are available in a wide range of ampereratings and in three different speeds: S&C Standard,Slow, and Very Slow (SMD-50 Power Fuses areoffered in the S&C Standard and Slow Speeds only). . . permitting coordination with protective relays,circuit reclosers, and other fuses. The broad selec-tion of ampere ratings and speeds permits close fus-ing to achieve maximum protection and optimumcoordination.

SMD Power Fuses are available in a total of sixmounting configurations designed to accommodatethe space and bus configuration requirements ofmany different station layouts. Fuse mountings areillustrated on pages 12 through 14.

APPLICATION

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Transformer Protection with SMD Power FusesHigh-voltage power fuses provide a reliable and eco-nomical means to protect small- to medium-sizedload transformers installed in utility and industrialsubstations. The considerable economies inherentin power-fuse protection are possible, first, becausethe fuse itself is much less costly than other types ofprotective equipment and, second, because there isno need for auxiliary equipment such as station bat-teries, motor-driven operators, and protective

relays. Further benefits of a compact fuse-protec-tion package are low installation costs and a space-saving design that will fit on almost any structure. Inaddition, unlike relay-actuated protective devicessuch as circuit breakers and reclosers, power fuseshave maintenance-free time-current characteristicsand require only minimal physical maintenance—such as periodic checking of the condition of thefuse-unit bore and occasional refinishing of fusetubes exposed to severe weathering.


Transformer protection using SMD Power Fuses.

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The transmission-voltage power fuse should beselected to provide system protection as well astransformer protection. For system protection, thefuse should operate promptly in response to poten-tially damaging overcurrent conditions in order tominimize short-circuit stresses on source-side con-ductors and equipment, and also to limit the extentof service interruption to the smallest possible por-tion of the system. For transformer protection, theprimary-side fuse should operate promptly inresponse to a bus or cable fault located between thetransformer and the nearest secondary-side over-current protective device and, also, provide backupprotection for the transformer in the event the sec-ondary-side overcurrent protective device eitherfails to operate due to malfunction or operates tooslowly due to incorrect ratings or settings.

S&C SMD Power Fuses provide full-fault-spec-trum protection for transformers: that is, these fuseswill detect and interrupt all faults—large, medium,and small (even down to minimum melting current);whether the fault is on the primary or secondaryside; with line-to-line or line-to-ground voltageacross the fuse; whether the transformer is adjacentto the fuse or cable-connected to it from a remote

location; and regardless of transformer winding con-nections. SMD Power Fuses are capable of handlingthe full range of transient recovery voltages associ-ated with these conditions. And they develop a pos-itive internal gap of high dielectric strength after cir-cuit interruption, thereby preventing destructivereignitions when exposed to full system voltage. The“dropout” action of these power fuses provides theadditional benefit of visible air gap isolation for thetransformer after fuse operation.

The close fusing necessary to provide superiorprotection for secondary-side faults is possible withSMD Power Fuses because they utilize silver or pre-tensioned nickel-chrome fusible elements that arenot damaged by transient surges that may heat theelement nearly to the severing point; they are avail-able in a variety of speeds that provide time-currentcharacteristics especially suited to protecting trans-formers for very-low-magnitude fault currents; andbecause they possess substantial peak-load capabil-ities and surge capacity more than adequate to with-stand transformer magnetizing-inrush currents aswell as severe hot- and cold-load pickup currents.Close fusing with SMD Power Fuses, coupled withthe exceptional low-current fault-interrupting


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performance, assures maximum protection for thetransformer for a broad range of secondary-sidefault currents, thus minimizing the life-shorteningthermal and mechanical stresses associated withprolonged transformer through-faults. In addition,the ability to fuse closer to the transformer full-loadcurrent facilitates coordination with source-sideprotective devices by permitting the use of lowerampere ratings or time-dial settings for fasterresponse.

Capacitor Bank Protection with SMD PowerFusesS&C SMD Power Fuses are suitable for fusing of sta-tion capacitor banks, particularly where availablefault currents are high. These power fuses have asubstantial continuous peak-load capability whichpermits the use of smaller ampere ratings than maybe possible with other makes of power fuses—with-out risking nuisance fuse operations due to capaci-tor-bank inrush or outrush currents. Close fusingwith SMD Power Fuses ensures rapid isolation offaulted capacitor banks . . . thereby protecting thesystem from unnecessary outages.

Other Application ConsiderationsSMDs Are Not Voltage Critical

S&C SMD Power Fuses are not “voltage critical,”and therefore may be applied at any system voltageequal to or less than the rated voltage of the fuse.Moreover, these fuses operate without producingovervoltages that can cause spurious operation ofsurge arresters or contribute to failure of trans-former insulation.

SMDs Permit Reduced Phase Spacing and

Ground-Clearance Requirements

The exhaust of Type SMD Power Fuses is noncon-ducting—unlike the highly ionized blast associatedwith operation of expulsion-type fuses that usefiber-lined fuse tubes. Consequently, it is possible tomake use of standard electrical clearances toground and between adjacent phases . . . a definiteplus for many station applications where space maybe at a premium. Without stretched-out phase spac-ings, structures can be smaller, simpler, and—ofcourse—less expensive. (For recommended mount-ing clearances, refer to the appropriate S&C DataBulletin.) Moreover, because of the selection ofmounting configurations, a wide variety of stationlayouts can be accommodated using SMD PowerFuses.SMDs Are Not Disruptive to Source-Side

Loads

S&C SMD Power Fuses provide prompt, reliableoperation without disrupting service to source-sideloads. Other protection alternatives involving trans-fer-trip relaying, automatic grounding switches, orsacrificial switches result in extensive interruptionof service to other loads in the event of a fault at thetransformer. Moreover, grounding-switch and sacri-ficial-switching schemes result in severe bolted-faultstress to the system and source transformers, plusundue fault testing of line-terminal circuit breakersfor all faults—even transformer secondary-sidefaults.

For additional detailed application recommenda-tions and technical information on SMD PowerFuses, including minimum melting and total clearingtime-current characteristics, preloading and ambi-ent temperature adjustment factors, and loadingcapabilities, consult your nearest S&C Sales Office.

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CONSTRUCTION AND OPERATIONThe Fusible ElementS&C Type SMD Power Fuses possess the perfor-mance characteristics and quality that make themespecially suitable for fault protection on 34.5-kVthrough 138-kV transmission and subtransmissionsystems. The fuses are available in a wide variety ofampere ratings and time-current characteristics,permitting close fusing to achieve maximum protec-tion and optimum coordination for both the source-side system and downstream equipment. The initialand sustained accuracy of their melting time-currentcharacteristics assures that these fuses can bedepended upon to operate exactly when they shouldand—equally important—not to operate when theyshouldn’t. This permanent accuracy is achievedprincipally in the design and construction of thefusible element.

Nondamageable Construction

S&C Power Fuses have silver or pretensionednickel-chrome current-responsive elements withthese characteristics: (1) they are drawn throughprecision dies to very accurate diameters; and (2)they are of solderless construction, brazed into theirterminals. Their melting time-current characteristicsare precise, with only 10% total tolerance in meltingcurrent compared to the 20% tolerance of manyfuses (20% and 40% respectively, in terms of time).And their design and construction features assure that they will conform to their time-currentcharacteristics not only initially, but on a sustainedbasis . . . they are corrosion resistant and nondam-ageable . . . neither age, vibration, nor surges thatheat the element nearly to the severing point willaffect the characteristics of S&C Power Fuses.

Arcing rod

Series of leversreduces springforce to correctamount for pre-tensioning offusible element

Fusible elementof nickel-chrome wire, pretensioned

Lower terminal

Nondamageable low-current, nickel-chrome fusible element for SMD Fuse Units rated 1 and 3E amperes. In these ratings, thenickel-chrome wire is too fine to withstand the full force of the spring. An assembly of levers in effect multiplies the tensile strength of the wire to permit the desired pretensioningwithout jeopardizing the security of the fusible element.

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The nondamageability of S&C Power Fuses—which is made possible by the construction featuresillustrated below—provides these advantages:

1. Superior transformer protection. SMD PowerFuses make it possible to fuse close to the trans-former full-load current, thus providing protec-tion against a broad range of secondary-sidefaults.

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

3. Close coordination with other overcurrent pro-tective devices . . . attainable because of the ini-tial and sustained precision of the fusible ele-ments, and because no “safety zones” or “setbackallowances” need be applied to the publishedtime-current characteristics to protect the ele-ment against damage.

4. Operating economies. There is no need to replaceunblown companion fuses on suspicion of dam-age following a fuse operation.

Nondamageable nickel-chrome fusible element for SMDFuse Units rated 5E and 7E amperes. When called upon to operate, the pretensioned nickel-chrome wire weakensabruptly and separates before its cross-section changes.

Nondamageable silver fusible element for SMD FuseUnits rated 10E amperes and larger. These ratingsemploy the silver fusible element, strain-wire construc-tion, which is not damaged by overloads or transientfaults approaching the minimum melting current.

Fusible elementof nickel-chromewire, pretensioned

Fusible element of silver wire, helicallycoiled to absorbmechanical vibrationand thermal shock

Arcingrod

Silver-brazedjoints

Lowerterminal

Arcingrod

Lowerterminal

Silver-brazedjoints

Strain wire

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The SMD Fuse UnitAn SMD Fuse Unit consists principally of a fusibleelement, a spring-driven arcing rod, and a solid-material arc-extinguishing medium contained withina filament-wound glass-epoxy tube.

The fusible element is connected at one end—through a current-transfer bridge and copper con-ducting tube—to the fuse-unit lower ferrule. Theother end of the fusible element is swaged and sil-ver-brazed to the silver-clad copper arcing rod whichextends upward through the solid-material medium.A spring-loaded tulip contact near the upper end ofthe fuse provides electrical continuity between thearcing rod and the fuse-unit upper ferrule, therebycompleting the load-current path through the fuse. Astainless-steel drive spring provides the storedenergy to drive the arcing rod upward through thearc-extinguishing medium during a fuse operation,and also to trip the latch mechanism on the fuseupper live parts so that the fuse unit can automati-cally “drop” to the open position.

The figure to the right shows construction detailsof a typical SMD Power Fuse Unit.

Red brassupper ferrule

Copper conductingtube

Drive spring

Tulip contact—spring loaded

Insulating tube—filament-woundglass-epoxy

Solid-material arc-extinguishing medium

Arcing rod—silver-clad copper

Strain wire

Fusible element

Current-transfer bridge(not visible)

Red brass lower ferrule

SMD-2C Fuse Unit illustrated.

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Principal Parts of an SMD Power FuseAn SMD Power Fuse consists of a mounting and thereplaceable fuse unit. The mounting includes a gal-vanized steel base (except SMD-2B Power Fusesrated 115 kV and 138 kV, which include an aluminumbase as illustrated on page 12), insulators, upper andlower contact assemblies, and fuse-unit end fittings.Upper and lower contact assemblies for SMD-1A,SMD-2B, SMD-2C, and SMD-3 Power Fuses includerugged bronze-alloy castings and silver-clad con-tacts. SMD-50 Power Fuse contact assembliesinclude aluminum-alloy castings and silver-clad con-tacts. All styles of fuse-unit end fittings feature sil-

ver-clad contact surfaces as well. The fuse unit isfurnished separately. Live parts consisting of upperand lower contact assemblies and fuse-unit end fit-tings are available separately with all SMDTransmission Power Fuses for the convenience ofpurchasers who wish to make up their own mount-ings. Fuse-unit end fittings are also sold as separateitems to assist users in keeping their spare fuse unitsready for quick replacement in case of an emer-gency.

The illustration below shows the principal partsof a typical SMD Power Fuse, including fuse unitand fuse-unit end fittings.


Base

Insulators

Latch-and-upper-contact assembly

Hinge-and-lower-contact assembly

Fuse-unit upperend fitting

Fuse-unit lowerend fitting

Fuse unit

Live parts

Mounting

Power fuse components (SMD-1A Power Fuse, Vertical 180° Opening Style illustrated).

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The Live PartsSilver-Clad Contacts

Superb current transfer between the SMD Fuse Unitand the upper and lower fuse-mounting contacts isassured by the wiping action of the silver-clad con-tact surfaces. As the fuse unit is closed into theupper contact assembly, silver-clad contact fingersfirst engage and “wipe” across the silver-clad con-tact surfaces of the fuse-unit upper end fitting. Then,during latching, a high-pressure, low-resistance con-tact is established by compressive flexing of thecontact fingers.

The lower contacts feature silver-clad surfacesand compressive loading for built-in wiping actionand efficient current transfer between the lowercontact assembly and the fuse-unit lower end fitting.

Articulated Latch

The spring-biased articulated latching mechanismof Type SMD Power Fuses is designed to compen-sate for variations in insulator spacing or contactalignment that may result from possible slight dis-tortion of a fuse base bolted to an irregular struc-ture. The automatic leveling action provided by themechanism ensures positive latching even if theinsulators have moved from the position of nominalcontact alignment.

Latching is accomplished by the roller in thespring-biased “floating” latch assembly riding overand dropping in behind the nose projection on theupper end fitting. Due to the “floating” action of thelatch assembly, the fuse cannot be dropped out byvibration or shock that may jar the insulator stacks.While normally preventing any accidental opening,the roller-type latch releases without resistancewhen deliberately tripped for dropout action.

Icing does not interfere with dropout action. Asdescribed to the right, the force of the drive springpropels the release tube upward . . . breaking anyaccumulation of ice and actuating the latch-releasemechanism.

Fault Interruption in SMD Fuse UnitsFast, positive fault interruption (as shown in thesequence of illustrations to the right) is achieved inSMD Fuse Units by the following means:

• High-speed elongation of the arc in the solid-material-lined bore by rapid movement of thespring-driven arcing rod, and

• The efficient deionizing action of the gases gen-erated through thermal reaction of the solidmaterial due to the heat of the confined arc.

The resultant high rate of dielectric recovery morethan matches the transient-recovery-voltage sever-ity of any circuit where SMD Power Fuses areapplied.

Positive Dropout Action

When the fuse unit is blown, the force of the drivespring causes the arcing rod to drive the releasetube upward and disengage the latch on the upper-contact assembly. After the latch is fully disengaged,the compression-loaded contact fingers thrust thefuse unit outward, permitting it to swing to the fullyopen position. Tripping of the latch and the start ofthe dropout action during fault interruption areillustrated to the right.


11 Overcurrent melts the fusible element, then transfers to thestrain wire, which volatilizes instantly. Arcing is initiated asillustrated.

22 Released force of the drive spring accelerates the arcing rodupward, causing rapid elongation of the arc in the solid-mate-rial-lined bore of the fuse unit. Under maximum fault condi-tions, heat from the confined arc causes the solid material inthe large-diameter section of the arc-extinguishing chamberto undergo a thermal reaction—generating turbulent gasesand effectively enlarging the bore diameter so that the arcenergy is released with a mild exhaust. Under low-to-moder-ate-fault conditions, the arc is extinguished in the upper sec-tion of the arc-extinguishing chamber where the small-diam-eter bore effectively concentrates the deionizing gases forefficient arc extinction.

33 Continued upward travel of the arcing rod after arc extinctioncauses the arcing rod to drive the release tube upward,thereby tripping the latch mechanism and initiating positivedropout of the blown SMD Fuse Unit.

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11 22 33Releasetube

Spring-biasedlatch assembly

Drivespring

Arcingrod

Fusibleelement

Silver-cladfuse-unitend fitting

Silver-cladcontact finger

Silver-cladcontact finger

Silver-cladfuse-unit end fitting

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12

Vertical 180° Opening Style† (34.5-kV SMD-1A Power Fuse illustrated)

Vertical 45° Opening Style† (115-kV SMD-2B Power Fuseillustrated)

Vertical-Offset Style† (69-kVSMD-2B Power Fuse illustrated)

1 Refer to tables on pages 18 and 19 for additional, detailed informationon interrupting ratings.

▲ SMD-3 Mountings are not available in vertical-offset style.

FUSE MOUNTINGSVertical and Vertical-Offset Styles

† Complete mounting shown; live parts can be furnished separately.

StyleFuseType

Ratings

kV Amperes, RMS

Nom. Max BIL MaxInterrupting1111

(Sym.)

Vertical180° Opening

(34.5 kV through 138 kV),

Vertical 45° Opening

(115 kV and 138 kV), and

Vertical-Offset (34.5 kV through 69 kV)

SMD-5034.54669

3848.372.5

200250350

100E100E100E

6 7005 0003 350

SMD-1A34.54669

3848.372.5

200250350

200E200E200E

17 50013 1008 750

SMD-2C34.546

3848.3

200250

300E300E

33 50031 500

SMD-2B

69115138138

72.5121145145

350550650750

300E250E250E250E

17 50010 5008 7508 750

SMD-3▲ 69 72.5 350 300E 25 000

AVAILABLE MOUNTING STYLES AND RATINGS

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Upright Styles

Upright Style† (34.5-kV SMD-1A Power Fuse illustrated)


StyleFuseType

Ratings

kV Amperes, RMS


(Sym.)

Upright

SMD-1A34.54669

3848.372.5

200250350

200E200E200E

17 50013 1008 750

SMD-2C34.546

3848.3

200250

300E300E

33 50031 500

SMD-2B

69115138138

72.5121145145

350550650750

300E250E250E250E

17 50010 5008 7508 750

SMD-3 69 72.5 350 300E 25 000



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Right-Angle Style



Right-Angle Style† (34.5-kV SMD-1A Power Fuse illustrated)


StyleFuseType

Ratings

kV Amperes, RMS


(Sym.)

Right-Angle

SMD-5034.54669

3848.372.5

200250350

100E100E100E

6 7005 0003 350

SMD-1A34.54669

3848.372.5

200250350

200E200E200E

17 50013 1008 750

SMD-2C34.546

3848.3

200250

300E300E

33 50031 500

SMD-2B 69 72.5 350 300E 17 500SMD-3 69 72.5 350 300E 25 000

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Inverted Style



Inverted Style† (34.5-kV SMD-1A Power Fuse illustrated)


StyleFuseType

Ratings

kV Amperes, RMS


(Sym.)

Inverted

SMD-1A34.54669

3848.372.5

200250350

200E200E200E

17 50013 1008 750

SMD-2C34.546

3848.3

200250

300E300E

33 50031 500

SMD-2B

69115138138

72.5121145145

350550650750

300E250E250E250E

17 50010 5008 7508 750

SMD-3 69 72.5 350 300E 25 000

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FUSE HANDLINGOpening and Closing SMD Power FusesSMD Power Fuses rated 34.5 kV through 69 kV in theVertical, Vertical-Offset, Right-Angle, and InvertedStyles are easily opened (or closed) using a univer-sal pole equipped with the appropriate S&CHandling Tool; i.e., the S&C Distribution Prong forSMD-50 Power Fuses, or the S&C Station Prong forSMD-1A, SMD-2B, SMD-2C, and SMD-3 PowerFuses. During a closing operation, the fuse unit isrestrained from tilting in the hinge by trunnions andcams on the fuse-unit lower end fitting; it is self-guiding . . . so the fuse can be closed from nearly anyangle.

Note: SMD-1A, SMD-2B, SMD-2C, SMD-3, andSMD-50 Power Fuses must not be opened underload. However, SMD-50 Power Fuses are equippedwith arcing horns which permit switching trans-former magnetizing current only. Moreover, SMD-50Power Fuses in the Vertical-Offset and Right-AngleStyles are suitable for live closing operations sincethe fuse’s mild exhaust is vented in a direction awayfrom the operator in the event of fuse closure into afaulted circuit.

Installing or Removing Fuse UnitsFuse Units for SMD-50 Power Fuses may beinstalled in (or removed from) a mounting using theS&C Distribution Prong.

Fuse units for SMD-1A Power Fuses rated 34.5 kVthrough 69 kV in all styles except the Upright Style

may be installed or removed using a universal poleequipped with the S&C Small Round Socket. A simi-lar handling tool, the S&C Large Round Socket, maybe used with SMD-2B, SMD-2C, and SMD-3 PowerFuses rated 34.5 kV through 69 kV (in all stylesexcept the Upright Style). Both the small and thelarge round sockets are slightly oversize so that theoperator can engage the release tube on the fuseunit easily. When removing the fuse unit from themounting, the operator should be positioneddirectly beneath the hinge. Then, a simple liftingmotion is all that is required to remove the fuse unit.Because the fuse unit is securely seated in thesocket, it effectively becomes part of the pole . . .there’s no cantilever loading to contend with, andthe fuse unit can’t jiggle or fall out. The fuse unitshould be lowered to ground level by planting thebase of the universal pole firmly on the groundagainst a fence or other fixed object and carefully“walking” the pole down until the fuse unit can begrasped and removed from the socket by hand.Although SMD-1A, SMD-2B, SMD-2C, and SMD-3Power Fuses rated 34.5 kV through 69 kV can beinstalled and removed using a universal poleequipped with the S&C Station Prong, use of theappropriate round socket is recommended to pro-vide complete, positive control. Moreover, S&CRound Sockets include a straight prong that may beused for fuse opening and closing operations—thereby eliminating any need for a separate S&CStation Prong.

Opening (or closing) SMD Power Fuses rated 34.5 kV through 69 kV.

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Additional Handling RecommendationsFuse units for SMD Transmission Power Fuses inthe Upright Style (in all voltage ratings) must beinstalled and removed by hand. Power fuses rated115 kV and 138 kV are generally serviced by hand aswell; however, SMD-1A, SMD-2B, SMD-2C, and SMD-3 Power Fuses in the Vertical 180° Opening

and Inverted Styles—in all available voltage ratings—may be serviced using the S&C Fuse Hoistwhenever fuse mountings are equipped with theoptional fuse-hoist hook (Catalog Number Suffix “-H”). Contact your nearest S&C Sales Office fordetails.

Installing (or removing) SMD-50 Fuse Unit using an S&C Distribution Prong.

Installing (or removing) SMD-1A, SMD-2B, SMD-2C,or SMD-3 Fuse Unit using an S&C Round Socket.

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INTERRUPTING RATINGSShort-Circuit Interrupting RatingsThe ratings shown below and on page 19 are themaximum interrupting ratings of the fuses basedupon full line-to-line voltage across a single fuse.Obviously, this is only one criterion of fuse perfor-mance. These fuses have also been rigorously testedthrough the full spectrum of fault currents, from thelowest to the highest fault—not only primary faultsbut also secondary-side faults as seen from the pri-mary side of the transformer—and under all realisticconditions of circuitry. In all S&C testing, specialattention is given to establishing and controlling cir-cuit parameters to duplicate conditions as severe asthose which will be encountered in the field. Thisinvolves testing at all degrees of asymmetry andmatching the rate of rise of the transient recoveryvoltage of the test circuit to that found in actual field

applications. This rate of rise depends, in turn, oncarefully established laboratory test conditions toobtain realistic natural frequencies and typicalamplitudes of transient recovery voltage.

The short-circuit interrupting ratings listed incolumns 3, 4, and 7 of these tables have been deter-mined in accordance with the procedures describedin the latest issue of ANSI Standard C37.41.Moreover, with respect to the requirement in thisstandard for testing with circuits having an X/R ratioof at least 15 (corresponding to an asymmetry factorof 1.55), S&C’s tests were performed under the moresevere condition of X/R = 20, corresponding to anasymmetry factor of 1.6. Based upon the recognitionthat there are many applications where the X/R ratiois less severe than the value of 15 specified by thestandard, higher symmetrical interrupting ratingsare listed in columns 5 and 6 for X/R = 10 and 5,respectively.

kV, Nominal Amperes, RMS, InterruptingMVA, Interrupting, Three-

Phase Symmetrical, Based on

SMD-1A System Asymmetrical

Symmetrical

Based on Based on Based on

34.52327.634.5

28 00028 00028 000

17 50017 50017 500

19 25019 25019 250

22 40022 40022 400

700840

1 000‡

4627.634.546

24 00024 00021 000

15 00015 00013 100

——

14 500

——

16 800

——

1 000‡

6934.54669

16 00016 00014 000

10 00010 000

8 750

——

9 600

——

11 200

——

1 000‡

11569

1158 0008 000

5 0005 000

—5 500

—6 400

—1 000‡

138115138

8 0006 700

5 0004 200

5 5004 600

6 4005 400

1 0001 000‡

X— = 15R

X— = 5R

X— = 15R

X— = 10R

SMD-1A POWER FUSES—50/60-Hertz Short-Circuit Interrupting Ratings

SMD-2B POWER FUSES—50/60-Hertz Short-Circuit Interrupting RatingskV, Nominal Amperes, RMS, Interrupting

MVA, Interrupting, Three-Phase Symmetrical,

Based onSMD-2B System Asymmetrical

Symmetrical


69

2327.634.54669

————

28 000

21 90021 90021 90021 90017 500

————

17 500

————

17 500

————

2 000‡

11569

11516 80016 800

10 50010 500

—10 500

—10 500

—2 000‡

138115138

16 80014 000

10 5008 750

—8 750

—8 750

—2 000‡

X— = 15R

X— = 5R

X— = 15R

X— = 10R

‡ Nominal rating.

‡ Nominal rating.

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SMD-3 System Asymmetrical

Symmetrical


69

2327.634.54669

————

40 000

25 00025 00025 00025 00025 000

————

25 000

————

25 000

————

3 000‡

X— = 15R

X— = 5R

X— = 15R

X— = 10R



SMD-50 System Asymmetrical

Symmetrical


34.52327.634.5

10 60010 60010 600

6 7006 7006 700

7 3007 3007 300

8 5008 5008 500

265320400‡

4627.634.546

9 6009 6008 000

6 0006 0005 000

——

5 500

——

6 400

——

400‡

6934.54669

6 4006 4005 300

4 0004 0003 350

——

3 700

——

4 300

——

400‡

X— = 15R

X— = 5R

X— = 15R

X— = 10R

SMD-3 POWER FUSES—50/60-Hertz Short-Circuit Interrupting Ratings

‡ Nominal rating.

SMD-50 POWER FUSES—50/60-Hertz Short-Circuit Interrupting Ratings

‡ Nominal rating.



SMD-2C System Asymmetrical

Symmetrical


34.52327.634.5

53 50053 50053 500

33 50033 50033 500

36 800a36 800a36 800a

42 800a42 800a42 800a

1 3001 6002 000‡

46

2327.634.546

50 50050 50050 50050 500

31 50031 50031 50031 500

—— —

34 500a

—— —

40 200a

—— —

2 500‡

X— = 15R

X— = 5R

X— = 15R

X— = 10R

SMD-2C POWER FUSES—50/60-Hertz Short-Circuit Interrupting Ratings

a These ratings apply only to fuses rated 40E amperes and above. For fuse units rated30E amperes and below, refer to ratings for X/R = 15.

‡ Nominal rating.

212-30 04-05-2004 02:21 PM

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.S.A

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S&C ELECTRIC COMPANYSpecialists in Electric Power Switching and Protection

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212-30 04-05-2004 02:21 PM

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