UL198D - Standard for Safety for Class K Fuses

Underwriters Laboratories Inc.333 Pfingsten RoadNorthbrook, IL 60062UL Standard for SafetyforClass K Fuses, UL 198DFifth Edition, Dated April 10, 1995UL provides this publication "as is" without warranty of any kind, either expressed or implied, including but not limited to, the implied warranties of merchantability or fitness. Revisions of UL Standards for Safety are issued from time to time. A UL Standard for Safety is current only if it incorporates the most recently adopted revisions.UL's Standards-on-Diskette are copyrighted by UL, and neither the distribution diskette nor the file for the Standard on the distribution diskette should be altered in any way.If a single-user version Standards-on-Diskette was purchased, one copy of the Standard may be stored on the hard disk of a single personal computer, or one back-up copy of the diskette may be made as a working diskette. 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Only one copy of the Standard may be printed for each authorized user of a multiple-user version of a Standards-on-Diskette.The requirements in this Standard are now in effect, except for those paragraphs, sections, tables, and figures having future effective dates as indicated in the note following the affected item. The prior text for requirements that have been revised and that have a future effective date are located after the Standard and are preceded by a "SUPERSEDED REQUIREMENTS" notice.Copyright 1995 Underwriters Laboratories Inc.APRIL 10, 1995ANSI/UL 198D - 19941UL 198DStandard forClass K FusesFirst Edition —June, 1970Second Edition —August, 1978Third Edition —August, 1987Fourth Edition —July, 1988The first, second, and third editions of the Standard were titled High-Interrupting-Capacity Class K Fuses. The first edition of the Standard was numbered UL 198.3.Fifth EditionApril 10, 1995Approval as an American National Standard (ANSI) covers the numbered paragraphs on pages dated April 10, 1995. These pages should not be discarded when revised or additional pages are issued if it is desired to retain the ANSI approved text.An effective date included as a note immediately following certain requirements is one established by Underwriters Laboratories Inc.Approved as ANSI/UL 198D - 1982, May 17, 1982Approved as ANSI/UL 198D - 1987, August 14, 1987Approved as ANSI/UL 198D - 1994, November 17, 1994Revisions of this standard will be made by issuing revised or additional pages bearing their date of issue. A UL Standard is current only if it incorporates the most recently adopted revisions, all of which are itemized on the transmittal notice that accompanies the latest published set of revision pages.ISBN 1-55989-813-5©COPYRIGHT 1978, 1995 UNDERWRITERS LABORATORIES INC.APRIL 10, 1995CLASS K FUSES UL 198D 3CONTENTSFOREWORDINTRODUCTION1 Scope .......................................................................52 Glossary .....................................................................53 Units of Measurement ..........................................................64 Undated References ...........................................................6CONSTRUCTION5 General ..................................................................... 7PERFORMANCE6 General (10)7 Current-Carrying Capacity Test (12)8 Temperature Test (12)9 Clearing-Time Current Test (14)10 200-Percent Current at Rated Voltage Test ........................................ 1511 Interrupting-Ability and I and I t Tests .. (15)p 212 Blade Alignment Test ......................................................... 18HIGH-CURRENT TEST-CIRCUIT CALIBRATION AND INSTRUMENTATION13 Galvanometers (19)14 Circuit Calibration (20)15 Power-Factor Determination (22)16 Available Current ............................................................. 2417 I (Peak Let-Through Current).. (25)p 18 Recovery Voltage (25)19 Comparison of Instrumentation (27)20 Determination of I t (27)221 Final Calibration .............................................................. 29RATINGS22 Details ..................................................................... 29MARKINGS23 Details (29)4CLASS K FUSES UL 198D APRIL 10, 1995FOREWORDA. This Standard contains basic requirements for products covered byUnderwriters Laboratories Inc. (UL) under its Follow-Up Service for this categorywithin the limitations given below and in the Scope section of this Standard. Theserequirements are based upon sound engineering principles, research, records of testsand field experience, and an appreciation of the problems of manufacture,installation, and use derived from consultation with and information obtained frommanufacturers, users, inspection authorities, and others having specializedexperience. They are subject to revision as further experience and investigation mayshow is necessary or desirable.B. The observance of the requirements of this Standard by a manufacturer is oneof the conditions of the continued coverage of the manufacturer's product.C. A product which complies with the text of this Standard will not necessarily bejudged to comply with the Standard if, when examined and tested, it is found to haveother features which impair the level of safety contemplated by these requirements.D. A product employing materials or having forms of construction differing fromthose detailed in the requirements of this Standard may be examined and testedaccording to the intent of the requirements and, if found to be substantially equivalent,may be judged to comply with the Standard.E. UL, in performing its functions in accordance with its objectives, does notassume or undertake to discharge any responsibility of the manufacturer or any otherparty. The opinions and findings of UL represent its professional judgment given withdue consideration to the necessary limitations of practical operation and state of theart at the time the Standard is processed. UL shall not be responsible to anyone forthe use of or reliance upon this Standard by anyone. UL shall not incur any obligationor liability for damages, including consequential damages, arising out of or inconnection with the use, interpretation of, or reliance upon this Standard.F. Many tests required by the Standards of UL are inherently hazardous andadequate safeguards for personnel and property shall be employed in conductingsuch tests.APRIL 10, 1995CLASS K FUSES UL 198D 5INTRODUCTION1 Scope 1.1 These requirements cover Class K fuses rated 250 or 600 V ac, 600 A or less for use in accordance with the National Electrical Code, NFPA 70.1.2 Class K fuses are of the nonrenewable cartridge type and may have an interrupting rating of 50,000,100,000, or 200,000 rms symmetrical A, with I (peak let-through current) and total I t (clearing amperes-p 2squared-seconds) specified for the individual subclasses.1.3 Class K fuses are divided into three subclasses, depending upon I and I t characteristics. These p 2subclasses are identified by the designations K1, K5, or K9.1.4 A product that contains features, characteristics, components, materials, or systems new or different from those covered by the requirements in this standard, and that involve a risk of fire, electric shock, or injury to persons, shall be evaluated using the appropriate additional component and end-product requirements as determined necessary to maintain the acceptable level of safety as originally anticipated by the intent of this standard. A product whose features, characteristics, components, materials, or systems conflict with specific requirements or provisions of this standard cannot be judged to comply with this standard. Where considered appropriate, revision of requirements shall be proposed and adopted in conformance with the methods employed for development, revision, and implementation of this standard.2 Glossary2.1 CASE SIZE — Fuses of all ratings for which the size of the cartridge is the same — for example fuses rated in the range of 31 — 60 A are 60 A case size and fuses rated in the range of 61 — 100 A are 100 A case size.2.2 CLEARING TIME —The time elapsing from the beginning of an overcurrent condition to the final circuit interruption. The clearing time is equal to the sum of the melting time and the arcing time.2.3 CURRENT-LIMITING FUSE — A fuse that:a) interrupts all available currents within its interrupting rating,b) limits the clearing time at rated voltage to an interval equal to or less than the first major or symmetrical current-loop duration when tested within its current-limiting range, andc) limits I to a value less than the peak current that would be possible with the fuse replaced by a p solid conductor of the same impedance. (Class K fuses are not permitted to be designated as current-limiting since the current-limiting range is not determined, although current limitation does occur in order to meet maximum values of I ).p 2.4 CURRENT-LIMITING RANGE —That range of rms symmetrical available current, equal to and less than the interrupting rating, in which the total clearing time at rated voltage and frequency is less than 1/2 cycle.6CLASS K FUSES UL 198D APRIL 10, 1995 2.5 INTERRUPTING RATING —The rating specifying the highest rms symmetrical alternating current that the fuse is required to interrupt under the conditions of these requirements.2.6 I (PEAK LET-THROUGH CURRENT) —The maximum instantaneous current through the fuse duringpthe total clearing time.22.7 I t (AMPERES-SQUARED-SECONDS) —An expression of the energy available as a result of current flow. The term is usually applied during the melting, arcing, or total clearing time of the fuse; in the latter instance it is often designated as Clearing I t.22.8 MAXIMUM ENERGY —The test condition under which, in a specified maximum time (1/2 cycle), the maximum amount of heat possible is generated in the fuse before clearing.2.9 MELTING TIME —The time between the initiation of a current large enough to cause the current-responsive element to melt and the instant when arcing starts.2.10 NORMAL-FREQUENCY RECOVERY VOLTAGE —The normal-frequency rms voltage impressed upon the fuse after the circuit has been interrupted and after high-frequency transients have subsided.2.11 RATING TESTS —Current-carrying-capacity, temperature, and clearing time-current tests.2.12 RECOVERY VOLTAGE —The voltage impressed upon the fuse after a circuit has been interrupted.2.13 THRESHOLD CURRENT —The rms symmetrical available current at the threshold of the current-limiting range, where melting of the current-responsive element has occurred at approximately 90 degrees on the symmetrical current wave and total clearing time is less than 1/2 cycle under specific conditions.2.14 THRESHOLD RATIO —The ratio obtained by dividing the threshold current by the current rating of the fuse.2.15 TIME DELAY FUSE — A fuse which does not interrupt the circuit before a minimum specified time when carrying 500 percent of its rated current in the Clearing-Time Current Test, Section 9. See Table 9.1.3 Units of Measurement3.1 If a value for measurement is followed by a value in other units in parentheses, the second value may be only approximate. The first stated value is the requirement.4 Undated References4.1 Any undated reference to a code or standard appearing in the requirements of this standard shall be interpreted as referring to the latest edition of that code or standard.APRIL 10, 1995CLASS K FUSES UL 198D7CONSTRUCTION5 General5.1 Iron or steel, plated or unplated, shall not be used for parts that are depended upon to carry current.5.2 A ferrule shall be of brass or copper and shall be circular in cross section, as shown in Figure 5.1. 5.3 The ferrules of a fuse shall be aligned so the fuse passes through a cylindrical gauging tube having a length not less than that of the fuse and an inside diameter as indicated in Table 5.1.Table 5.1Diameters of gauging tubesInside diameter of gauging tube, inches (mm)Case size,A250 V fuses600 V fuses300.580 (14.73)0.830 (21.08)600.830 (21.08) 1.080 (27.43)5.4 The blades of a fuse shall be of copper or brass and shall be in substantial alignment.5.5 Alignment of the blades shall be provided and maintained by means other than friction between surfaces.Exception: A press-fitted assembly of materials not subject to shrinkage or warping due to heat or moisture is considered acceptable for maintaining blade alignment provided the construction complies with the requirements in the Blade Alignment Test, Section 12.5.6 A cap shall be of metal of a thickness that provides acceptable physical strength and, if depended upon to carry current, shall be of brass or copper. Caps, assembly rings, washers, and the like, of iron or steel shall be protected against corrosion.5.7 A connection between the fusible element and a contact of a fuse shall provide positive and permanent electrical connection. The connection shall be soldered, brazed, or welded, or shall otherwise be made equivalently secure.5.8 The tube of a fuse shall be of ceramic, melamine-impregnated glass fiber, or the equivalent. Exception: A tube of a fuse may be hard fiber or other material that has been shown by investigation to be acceptable for the application. See 11.6.3.5.9 An adhesive employed in a fuse shall be acceptable for the application and shall function as intended. See6.5 — 6.7.5.10 The dimensions of a Class K fuse shall be in accordance with Figure 5.1 and Table 5.2.8CLASS K FUSES UL 198D APRIL 10, 1995Figure 5.1Class K cartridge fusesFerrule-type fuses0 —60 AKnife-blade type fuses61 —600 ANote —The dash line represents the limit of the maximum projection of a screw, rivet head, or the like. It becomes a circle for a fuse rated more than 200 A.APRIL 10, 1995CLASS K FUSES UL 198D 9Table 5.2Dimensions of Class K cartridge fuses in inches (mm)Caselength outside ferrule diameter Thick-Width nearest cylin-parallel at right size,of diameter or of ness of of live drical to angles A fuse of tube blade ferrule blade blade part body blade to blade OverallMaximum of Outside fuse to of Measured Measured aMinimumfrom mid-overall lengthpoint of length b c d Maximum dimensions over projections e Minimumdistance Minimum f A B C D EF G H I JFuses rated 250V302.000.530.500.562(50.8)(13.5)(12.7)(14.27) 603.000.780.6250.812(76.2)(19.8)(15.9)(20.62) 1005.87 1.000.1250.7500.660.59 1.03(149.2)(25.4)(3.18)(19.05)(16.7)(15.1)(26.2)2007.12 1.370.188 1.1250.940.84 1.19 4.12(181.0)(34.9)(4.78)(28.58)(23.8)(21.4)(30.2)(104.8)4008.62 1.870.250 1.625 1.20 1.20 1.19 4.62(291.1)(47.6)(6.35)(41.28)(30.6)(30.6)(30.2)(117.5)60010.372.250.250 2.000 1.45 1.45 1.53 5.19(263.5)(57.1)(6.35)(50.80)(36.9)(36.9)(38.9)(131.8)Fuses rated 600V305.000.780.500.812(127.0)(19.8)(12.7)(20.62)605.50 1.030.62 1.062(139.7)(26.2)(15.9)(26.97)1007.87 1.000.1250.7500.780.72 1.75(200.0)(25.4)(3.18)(19.05)(19.8)(18.3)(44.4)2009.62 1.370.188 1.125 1.060.98 2.25 6.12(244.5)(34.9)(4.78)(28.58)(27.0)(25.0)(57.1)(155.6)40011.62 1.870.250 1.625 1.45 1.45 2.507.12(295.3)(47.6)(6.35)(41.28)(36.9)(36.9)(63.5)(181.0)60013.372.250.250 2.000 1.72 1.72 2.698.19(339.7)(57.1)(6.35)(50.80)(43.7)(43.7)(68.3)(208.0)Tolerances: 0 —60 A, ±0.03 inch (±0.8 mm); 61 — 200 A, ±0.06 inch (±1.6 mm); 201 — 600 A, ±0.09 inch (±2.4 mm).a Tolerance: ±0.008 inch (±0.20 mm).b Tolerance: ±0.003 inch (±0.08 mm).c Tolerance: ±0.035 inch (±0.89 mm).d The maximum overall dimension of a screw ring, the position of which with respect to the position of the knife blade cannot be e predetermined, shall not be greater than the value specified for dimension H.The length of the cylindrical body may be less than the indicated value if other acceptable interference means such as pins through the f blades or collars, are provided to prevent mounting the fuse in a fuseholder that will accommodate a fuse rated in the next lower bracket of current ratings.5.11 Solder or any other projection on the end of a ferrule-type fuse shall not cause the overall length to exceed the dimension in Table 5.2.5.12 No part of a knife-blade type fuse shall extend farther from the axis of the fuse than the distance given in Column G of Table 5.2.Exception: A handle or similar attachment constructed and located so that it will not interfere with the installation of a fuse in a standard cartridge fuseholder may extend for a longer distance.PERFORMANCE 6 General 6.1Tests are to be performed with the number of samples specified in Table 6.1.Table 6.1Class K fuses —tests and number of samplesCase testrated and Inter-and10 kAsize,at 500voltagetemper-rupting Maximum I and I tinter-135200A percenttest ature ability energy testsruptingpercent percent 50 kA 25 kA Rating tests Interrupting ability and I and I t tests a Time current percent delay at current100 kAb200percent 110c p 2dClearing time —Intermediatecurrentinterruptinge p 2fg3033221111115 603322111111410033211111113200322111111124003221111111260032211111112If the manufacturer so elects, fuses that successfully complete the temperature and 110-percent current tests may be used for the a 135- or 200-percent current tests, in which case the temperature of such fuses shall not be higher than that of the ambient air when the 135- or 200-percent current test is started.For 30 and 60 A case sizes, the two samples required are to consist of one sample of the maximum rating and one sample of the bminimum rating for that case size, for example 30 and 8 ampere or 60 or 35 ampere ratings.This test may be combined with the 200-percent current test if conditions of both tests are met.c Fuses having interrupting ratings less than 200 kA will require fewer samples.dSee the exception to 11.3.1.e Assumes same sample used for let-through measurement and interrupting ability at 100 kA. Sample and test circuit must meet fconditions of both tests. See 11.4.4.For paper tube fuses, two sets of samples are needed. See 11.6.3.g6.2 Table 6.1 is based on the assumption that the same internal features, such as fusible element material and shape, are constant for any case size. If the internal features, such as fusible element material, change at some intermediate rating in a particular case size, this fact is to be considered during selection of test samples, and sufficient samples are to be selected so that constructions involving each different type of material or construction are subjected to the complete test program specified in Table 6.1.6.3 If the overall results of all tests of one of the ratings enumerated in Table 6.1 or the largest rating in a construction group are acceptable, all fuses of the case size or construction group represented by those samples are considered to be acceptable.6.4 The required markings on a fuse shall remain legible and attached after each test.6.5 With reference to 5.9, samples of the fuse are to be subjected to the interrupting-ability, low interrupting-ability, and overload clearing at rated voltage tests. Before being tested, samples are to be conditioned in an oven at the temperature and time prescribed in 6.6 and other samples are to be conditioned for a minimum of 5 days in a humidity cabinet having a relative humidity of 93 ±2 percent at 32.0 ±2.0E C (89.6 ±3.6E F).Exception: Conditioning tests on the adhesive may be omitted if the fuse performs acceptably during the interrupting-ability, low interrupting-ability, and overload clearing at rated voltage tests with the adhesive omitted.6.6 The oven conditioning prescribed in 6.5 is to be conducted for 1000 hours. The oven temperature is to be determined from Figure 10.1. The temperature is to be the temperature at the intersection of a selected temperature-index line in Figure 10.1 with the 1000-hour line, where the temperature index T is, at the manufacturer's option, either the maximum observed operating temperature of the adhesive, or the maximum acceptable operating temperature of any part in contact with the adhesive. Intermediate temperature indices are to be interpolated. The maximum observed operating temperature is that attained with the fuse carrying 110 percent of rated current, while in a 25E C (77E F) ambient. Measurement may be made in ambients of 18 —32E C (65 —90E F) and the reading adjusted to 25E C. The maximum acceptable operating temperature is the sum of two values: the rise acceptable in the temperature test, and 25E C.Exception: On the same selected temperature-index line in Figure 10.1, a shorter or longer time at the corresponding higher or lower temperature may be employed if agreeable to all concerned and provided that the time is not less than 300 hours.Figure 10.1Aging time versus aging temperature for adhesive6.7 After the conditioning, a fuse is to be returned to room temperature before testing.7 Current-Carrying Capacity Test7.1 A fuse shall carry 110 percent of its rated current for not less than 15 minutes for 200 A or less case sizes, and not less than 30 minutes for larger than 200 A case sizes after constant temperature has been obtained. The test may be conducted as part of the temperature test. When the fuse is carrying this current, no external soldered connection shall melt, and a casing or enclosure shall not char or rupture.8 Temperature Test8.1 The temperature rise on the exterior surface of a fuse shall not exceed the applicable values in Table 8.1 when tested according to 8.2 —8.9.Table 8.1Maximum temperature risesCase size,A E C E C E C Casing Ferrules Blades 305050— 605050—10050—5020050—6040050—6560050—758.2 A 48 — 60 hertz alternating-current test circuit of any convenient voltage is to be used. The fuse is tocarry 110 percent of its rated current until temperatures are constant.Exception: A direct-current test circuit may be used if no ferrous metal other than bolts, nuts, or other smallassembly parts are employed in the fuse.8.3 Each fuse is to be supported in the intended manner in an appropriate single-pole fuseholder. The fuseholder is to be mounted horizontally on a bench or test board of nonconducting material so that the fuse under test will be held in a horizontal position above the board.8.4 For a test board constructed for the testing of two or more fuses in series, the fuseholders are to be located so that there is a spacing of not less than 6 inches (150 mm) between any two fuses under test. The fuseholders and the ammeter are to be connected to each other and to the power supply by general-use copper building wire as specified in Table 8.2.Table 8.2Wire for temperature-test connectionsCase size,A Minimum length , feet (m)Size, AWG or kcmil (mm )Wirea 2 30 2 (0.6) 8 (8.4) 60 2 (0.6) 4 (21.2)100 2 (0.6) 1 (42.4)200 2 (0.6) 4/0 (107)400 4 (1.2) 500 (253)6004 (1.2)1000 (507)Any connection to the source of supply is not to be less than 4 feet (1.2 m) long.a8.5A connection to a conductor larger than No. 8 AWG (8.4 mm ) is to be made by means of a soldering2lug or pressure wire connector.8.6 For a fuse rated at 60 A or less, a pressure-type fuseholder —such as the setscrew or the spring-reinforced type — is to be used to provide proper contact between the clips and the ferrules of the fuse.8.7 For a fuse rated more than 60 A, c-clamps or ordinary clip clamps are to be used to press the fuseholderterminals against the knife-blade terminals of the fuse. A c-clamp is to be of iron or steel, weigh not more than 3 ounces (85 grams), and neither of the two faces of the clamp making contact with the fuseholder terminals is to have an area of more than 1/2 square inch (323 mm ).28.8 The temperature of the enclosure or casing and terminals of a fuse is to be determined by using an all-glass mercury thermometer. The bulb of the thermometer is to be placed against the center of the top or theupper surface of the fuse, midway between the caps or ferrules. The temperature of the terminals is to be determined with a thermometer bulb placed against the top or upper surface of each ferrule or blade. A thermometer is to be supported so its stem is at an angle of not more than 60 degrees with the vertical, and contact between the bulb and the surface of the fuse is to be provided by using fresh glaziers' putty, or the equivalent, that completely covers the bulb.8.9 The test is to be conducted with the ambient air at any temperature within the range of 25 ±7E C (77 ±13E F). During the test the air temperature is to remain within these limits and is not to vary more than 5E C (9E F). The test is to be continued until the temperatures on the fuse are constant. A temperature is considered constant when three successive thermometer readings, taken at 5-minute intervals, indicate no temperature rise change.9 Clearing-Time Current Test9.1 A fuse shall clear within the time specified in Table 9.1 when tested according to 9.2 —9.4. No externalsoldered connections shall melt, and a casing or enclosure shall not char or rupture.Table 9.1 Clearing timeCase size,A 135-percent ratingNontime delayTime delay500-percent ratingMaximum clearing time, minutesMinimum clearing time fortime delay fuse,seconds 200-percent ratinga 30 60 2 410b60 60 4 610 100120 6 810 200120 81010 400120101210 600120121410This test is conducted if the fuse is marked with the letter "D." "Time Delay," "Dual Element," or an equivalent designation. See 9.4a and 23.1(g).This clearing time may be reduced to 8 seconds for a fuse rated in the range of 0 — 30 A, 250 V.b9.2 A fuse is to be tested in open air, starting at room temperature, according to 8.2 —8.7. The test is to be conducted with the ambient air at any temperature within the range of 25 ±7E C (77 ±13E F). During the test the air temperature is to remain within these limits and is not to vary more than 5E C (9E F).9.3 The 135- and 200-percent tests are to be conducted on a preset circuit or the current is to be adjusted at a uniform rate to the required test current within 3 seconds.9.4 The 500-percent time-delay test is to be conducted on a preset circuit adjusted to the required test current with a copper bar in the circuit. The copper bar is then to be replaced with the test fuse, and the circuit turned on and readjusted at a uniform rate to the required test current within 3 seconds.10 200-Percent Current at Rated Voltage Test10.1 A fuse with filler is to be tested on a circuit adjusted to obtain 200-percent current at rated voltage witha power factor of 80 percent or less. An external soldered connection shall not melt, and the casing of the enclosure shall not char or rupture in any manner. The recovery voltage is to be applied continuously and without interruption for 1 minute after interruption of the current by the fuse. If there has been no reestablishment of current or any indication of a tendency to restrike —smoking, unusual heating, internal arcing, and the like —before the end of that interval, the performance is acceptable. If evidence of a tendency to restrike has been noted, application of the recovery voltage is to be continued until it is definitely determined that complete and permanent interruption of the circuit has occurred or until the ultimate effect on the fuse is determined.211 Interrupting-Ability and I and I t Testsp11.1 General11.1.1 Each fuse is to be tested one at a time according to the test procedure described in 11.1.2 —11.6.3. The fuse shall remain intact and permanently clear the circuit. There shall be no hole in the ferrule, nor an increase in overall length of the cylindrical portion of the fuse of more than 1/8 inch (3.2 mm).11.1.2 The fuse is to be clamped in place or mounted, with or without clamps, in fuse clips spaced to fit the fuse.11.1.3 The test circuit is to have an open-circuit voltage not less than the fuse rating. Other than for the low interrupting-ability test, the power factor is to be 20 percent or less. The power factor for the test at 10,000 amperes is to be 50 percent or less. The frequency for the test circuit is to be 48 —60 hertz. The test circuit is to comply with the calibration requirements for test circuits as specified in Sections 13 —21.11.1.4 The start-to-arc point on an oscillogram is that point where the voltage across the fuse first starts to rise suddenly or abruptly after circuit closure.11.1.5 The recovery voltage across the terminals of a fuse is to be as described under Recovery Voltage, Section 18.11.1.6 The recovery voltage is to be applied continuously across a fuse for 30 seconds after completion of the short-circuit interruption by the fuse in each of the interrupting-ability and let-through measurement tests. Acceptable performance is indicated if, at the end of that time, there is no evidence of a tendency to restrike—smoking, unusual heating, internal arcing, and the like. If there is such evidence the test results are considered unacceptable or, at the request of the manufacturer, the application of the voltage may be continued until it is definitely established that the fuse is dormant. Intermittent current flow at voltage peaks after initial current interruption (minor restrike) is acceptable. See 18.4.11.1.7Other than for the low interrupting-ability test, the maximum peak voltage occurring during the interruption is not to exceed 3000 volts. This voltage is to be measured with an instrument having a frequency response that is linear from 50 to 3000 hertz.。