Current Limitation

Component Protection Introduction and Current-Limitation

This issue analyzes the protection of electrical system components from fault Current-Limitation Defined currents. It gives the specifier the necessary information regarding the short- Today, most electrical distribution systems are capable of delivering very high circuit current or withstand rating of electrical circuit components, such as short-circuit currents, some in excess of 200,000A. Many circuit components wire, bus, motor starters, etc. Proper protection of circuits will improve have relatively low short circuit withstandability of a few thousand amps. If the reliability and reduce the possibility of injury. Electrical systems can be components are not capable of handling these short-circuit currents, they destroyed if the overcurrent devices do not limit the short-circuit current to could easily be damaged or destroyed. The current-limiting ability of today’s within the withstand rating of the system’s components. Merely matching the modern fuses allows components with low short-circuit withstand ratings to be amp rating of a component with the amp rating of a protective device will not specified in spite of high available fault currents. assure component protection under short circuit conditions. NEC® 240.2 offers the following definition of a current-limiting device: The National Electrical Code® covers Component Protection in several sections. The first section to note is 110.10. Current-Limiting Overcurrent Protective Device: A device that, when interrupting currents in its current-limiting range, reduces the current flowing Component Protection and in the faulted circuit to a magnitude substantially less than that obtainable in The National Electrical Code® the same circuit if the device were replaced with a solid conductor having comparable impedance. 110.10 Circuit Impedance and Other Characteristics: The overcurrent protective devices, the total impedance, the component short-circuit current The concept of current-limitation is pointed out in the following graph, where ratings, and other characteristics of the circuit to be protected shall be the prospective available fault current is shown in conjunction with the limited selected and coordinated to permit the circuit-protective devices used to current resulting when a current-limiting fuse clears. The area under the clear a fault to do so without extensive damage to the electrical current curve is representative of the amount of short circuit energy being components of the circuit. This fault shall be assumed to be either between dissipated in the circuit. Since both magnetic forces and thermal energy are two or more of the circuit conductors or between any circuit conductor and directly proportional to the square of the current, it is important to limit the the grounding conductor or enclosing metal raceway. Listed products short-circuit current to as small a value as possible. The maximum magnetic applied in accordance with their listing shall be considered to meet the forces vary as the square of the “PEAK” current and thermal energy varies as requirements of this section. the square of the “RMS” current. This requires that overcurrent protective devices, such as fuses and circuit Current-Limiting Effect of Fuses breakers be selected in such a manner that the short-circuit current (withstand) ratings of the system components will not be exceeded should a Prospective available short-circuit short circuit occur. 100,000 current that would flow when a fuse is not used. The “short-circuit withstand rating” is the maximum short- circuit current that a component can safely withstand. Failure to provide adequate protection may result in component destruction under short circuit conditions. After calculating the fault levels throughout the electrical system, the next step is to check the withstand rating of wire and cable, circuit breakers, transfer switches, starters, etc., under short circuit conditions. Current Note: The let-through energy of the protective device must be equal to or less than the short-circuit withstand rating of the component being protected. 10,000 Peak Let-Through Current of Fuse

CAUTION: Choosing overcurrent protective devices strictly on the basis of 0 Time voltage, current, and interrupting rating alone will not assure component tc

protection from short-circuit currents. High interrupting capacity electro- Total Clearing Time of Fuse mechanical overcurrent protective devices (circuit breakers), especially those that are not current-limiting, may not be capable of protecting wire, Thus, the current-limiting fuse in this example (above waveform) would limit cable or other components within high short circuit ranges. The interrupting the let-through energy to a fraction of the value which is available from the rating of a protective device pertains only to that device and has absolutely system. In the first major loop of fault current, standard non-current-limiting, no bearing on its ability to protect connected downstream components. electro-mechanical protective devices would let-through approximately 100 Quite often, an improperly protected component is completely destroyed times* as much destructive energy as the fuse would let-through. under short circuit conditions while the protective device is opening the faulted circuit. 100,000 2 * = 100 Before proceeding with the study of component withstandability, the (10,000 ) technology concerning “current-limitation” will be reviewed.

Analysis of Current-Limiting Fuse Let-Through Charts Prior to using the Fuse Let-Through Charts, it must be determined what let- The degree of current-limitation of a given size and type of fuse depends, in through data is pertinent to equipment withstand ratings. general, upon the available short-circuit current that can be delivered by the Equipment withstand ratings can be described as: How Much Fault Current electrical system. Current-limitation of fuses is best described in the form of a can the equipment handle, and for How Long? Based on standards presently let-through chart that, when applied from a practical point of view, is useful to available, the most important data that can be obtained from the Fuse Let- determine the let-through currents when a fuse opens. Through Charts and their physical effects are the following: Fuse let-through charts are plotted from actual test data. The test circuit that A. Peak let-through current: mechanical forces establishes line A-B corresponds to a short circuit power factor of 15%, that is B. Apparent prospective RMS symmetrical let-through current: heating effect associated with an X/R ratio of 6.6. The fuse curves represent the cutoff value 1 of the prospective available short-circuit current under the given circuit C. Clearing time: less than ⁄2 cycle when fuse is in it’s current-limiting range (beyond conditions. Each type or class of fuse has its own family of let-through curves. where fuse curve intersects A-B line). This is a typical example showing the short-circuit current available to an 800A The let-through data has been generated by actual short- circuit tests of circuit, an 800A Low-Peak current-limiting time-delay fuse, and the let-through current-limiting fuses. It is important to understand how the curves are data of interest. generated, and what circuit parameters affect the let-through curve data. Typically, there are three circuit parameters that can affect fuse let-through 800 Amp Low-Peak® Current-Limiting Time-Delay performance for a given available short-circuit current. These are: Fuse and Associated Let-Through Data 1. Short-circuit power factor 2. Short-circuit closing angle 3. Applied voltage Current-limiting fuse let-through curves are generated under worst case conditions, based on these three variable parameters. The benefit to the user is a conservative resultant let-through current (both Ip and IRMS). Under actual field conditions, changing any one or a combination of these will result in lower let-through currents. This provides for an additional degree of reliability when applying fuses for equipment protection. Current-Limiting Let-Through Charts for Cooper Bussmann fuses are near the back of this book. Analysis of a Current-Limiting Fuse

400,000 Available Peak Short- Circuit Current = 198,000A 300,000 I 200,000 Available RMS Short- Circuit Current = 86,000A 100,000 80,000 Peak Let-Through Current 60,000 800A of Fuse= 49,000A

RMS Let-Through Current 30,000 of Fuse = 21,000A 20,000 TIME tm ta t = Fuse Melt Time 10,000 tc m 8000 ta = Fuse Arc Time 6000 tc = Fuse Clearing Time

4000 3000 A 2000 AMP RATING

1000 INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPS PEAK LET-THROUGH INSTANTANEOUS 1000 2000 3000 4000 6000 8000 10,000 20,000 30,000 40,000 60,000 80,000 100,000 200,000

PROSPECTIVE SHORT-CIRCUIT CURRENT – SYMMETRICAL RMS AMPS

How to Use the Let-Through Charts Most electrical equipment has a withstand rating that is defined in terms of an Using the example given, one can determine the pertinent let-through data for RMS symmetrical-short-circuit current, and in some cases, peak let-through the KRP-C-800SP amp Low-Peak fuse. The Let-Through Chart pertaining to current. These values have been established through short circuit testing of the 800A Low-Peak fuse is illustrated. that equipment according to an accepted industry standard. Or, as is the case with conductors, the withstand rating is based on a mathematical calculation A. Determine the PEAK let-through CURRENT. and is also expressed in an RMS short-circuit current. Step 1. Enter the chart on the Prospective Short-Circuit current scale If both the let-through currents (I and I ) of the current-limiting fuse at 86,000 amps and proceed vertically until the 800A fuse RMS p curve is intersected. and the time it takes to clear the fault are less than the withstand rating Step 2. Follow horizontally until the Instantaneous Peak Let-Through of the electrical component, then that component will be protected from Current scale is intersected. short circuit damage. Step 3. Read the PEAK let-through CURRENT as 49,000A. (If a fuse The following Table shows typical assumed short-circuit current ratings for had not been used, the peak current would have been various unmarked components. 198,000A.) B. Determine the APPARENT PROSPECTIVE RMS Typical Short-Circuit Current Ratings For Unmarked SYMMETRICAL let-through CURRENT. Components* Step 1. Enter the chart on the Prospective Short-Circuit current scale Component Short- Circuit at 86,000A and proceed vertically until the 800A fuse curve is Rating, kA intersected. Industrial Control Equipment: Step 2. Follow horizontally until line A-B is intersected. a. Auxiliary Devices 5 Step 3. Proceed vertically down to the Prospective Short-Circuit b. Switches (other than Mercury Tube Type) 5 c. Mercury Tube Switches Current. Rated over 60 amperes or over 250 volts 5 Step 4. Read the APPARENT PROSPECTIVE RMS SYMMETRICAL Rated 250 volts or less, 60 amperes or less, and over 2kVA 3.5 let-through CURRENT as 21,000A. (The RMS Rated 250 volts or less and 2kVA or less 1 SYMMETRICAL let-through CURRENT would be 86,000A if Meter Socket Base 10 there were no fuse in the circuit.) Photoelectric Switches 5 Receptacle (GFCI Type) 10 Current-Limitation Curves — Cooper Bussmann Receptacle (other than GFCI Type) 2 Low-Peak Time-Delay Fuse KRP-C-800SP Snap Switch 5 Terminal Block 10 Thermostat 5 *Based upon information in UL 891 (Dead-Front Switchboards)

The following components will be analyzed by establishing the short-circuit withstand data of each component and then selecting the proper current- limiting fuses for protection: • Wire and Cable • Bus (Busway, Switchboards, Motor Control Centers and Panelboards) • Transfer Switches • HVAC Equipment • Ballasts • Circuit Breakers A detailed analysis of motor circuit component protection is provided later in the section on motor circuits. C. Clearing time If the RMS Symmetrical available is greater than the point where the fuse characteristic curve intersects with the diagonal A-B line, then the fuse 1 clearing time is ⁄2 cycle or less. In this example, the intersection is approximately 9500A; so for short-circuit currents above approximately 9500A, this KRP-C-800SP fuse is current-limiting. The current-limiting charts and tables for Cooper Bussmann fuses are in the rear of this book under “Current-Limiting Let-Through Charts.” Refer to these tables when analyzing component protection in the following sections.

The Need for Current-Limitation The National Electrical Code® requires ground fault protection for intermediate If ground fault protection is required, then the best protection is a switch and high ground faults as well as low grade ground faults. For high magnitude equipped with a ground fault relay scheme, a shunt trip mechanism, and ground faults, ground fault relay schemes operate too slowly to prevent current-limiting fuses. The reason is that this system will offer protection for extensive equipment damage. The main or feeder overcurrent devices, such high magnitude ground faults as well as low magnitude ground faults. Ground as fuses or circuit breakers must clear the circuit. Current-limiting fuses fault relay schemes and shunt trip mechanisms on switches or circuit breakers substantially limit the energy let-through for higher magnitude ground faults can protect equipment against extensive damage from low magnitude ground and thereby offer a higher degree of protection. Conventional circuit breakers faults - this is their intended purpose. However, burn downs still occur in are not current-limiting protective devices and during higher magnitude ground switchboards, large motor control centers, and large distribution panels faults can let-through large amounts of damaging energy. generally located in equipment rooms where high available ground fault 1,000 currents are present. 800 600 400 300

200 1600A CB 100 80 60 40 30

S ND

CO 10 E

S 8 6 4 TIME IN 3

2 TIME IN SECONDS

1 .8 .6 .4 .3

.1 .08 .06 .04 .03

.02

.01 00 100 200 300 400 600 8 ,000 1,000 2,000 3,000 4,000 6,000 8 0,000 10,000 20,000 30,000 40,000 60,000 8

100,000 CURRENT IN AMPS CURRENT IN AMPS Clearing characteristic for 1600A circuit breaker. A 20,000A fault is cleared by the 1600A circuit breaker in 0.05 seconds. The circuit breaker has a fixed operating time Clearing characteristic for a 1600A fuse. A 20,000 amp fault is cleared by the KRP-C- for high values of current. This time is approximately 0.05 seconds (three cycles). 1600SP fuse in 0.019 to 0.039 seconds (between one and two cycles). For currents Therefore, high magnitude ground faults and short circuits are permitted to flow for at greater than 25,000A the fuse enters its current-limiting range. Then the clearing time least three cycles. is less than one half cycle (less than 0.008 seconds).

The previous two figures illustrate the time-current characteristics for a 1600A Current-Limitation current-limiting fuse and a 1600A circuit breaker. The higher the fault current The effect of a fuse protecting the circuit is to limit the instantaneous peak the faster the fuse operates. Notice, the mechanical overcurrent protective current and thermal or heating effect current to a value less than that which device reaches an irreducible operating time. For large conventional service would flow in the first half cycle had the fuse not been in the circuit. Current- 1 entrance circuit breakers this fixed operating time varies from 1 ⁄2 cycles to five limitation for high level ground faults can substantially reduce the damaging cycles depending on the type and size. (If short-time-delay trip settings are effect. used, the operating time can be as long as 30 cycles.) The large conventional mechanical overcurrent protective device reaches an Of importance is the fact that modern, rejection type fuses are current-limiting irreducible minimum clearing time and therefore permits the full fault current protective devices. For faults above approximately 25,000A, the 1600A fuse flow for several cycles. The damaging peak current and thermal or heating 1 operates in its current-limiting range; clearing the circuit in less than ⁄2 cycle effect current flow unrestricted without limitation for several cycles. At higher and limiting the peak current and energy let-through to the circuit components. magnitude fault currents, large amounts of heating energy and magnetic forces are permitted to flow and the equipment must absorb the full available fault current energy.

Current-Limitation No Current-Limitation

Available current that would flow Available current flows for without a fuse. operating time of circuit breaker.

3 Cycle Clearing Time Fuse Current- Limiting Effect

Compare the Difference

Arc-flash when circuit protected by 601A Class L current-limiting fuses. Arc-flash when circuit protected by a 1600A non-current limiting circuit breaker set at 640A with short time delay: circuit interrupted in six cycles.

See pages 67 to 69 for current-limiting definition and how to analyze these charts.

Low-Peak Class L Time-Delay Fuses KRP-C_SP Fuse – RMS Let-Through Currents (kA) KRP-C_SP Fuse Size Prosp. 1000000 601 800 1200 1600 2000 2500 3000 4000 5000 6000 900000 Short 800000 B 700000 C.C. IRMS IRMS IRMS IRMS IRMS IRMS IRMS IRMS IRMS IRMS 600000 500000 5,000 5 5 5 5 5 5 5 5 5 5 400000 6000A 10,000 8 10 10 10 10 10 10 10 10 10 300000 5000A 4000A 15,000 9 12 15 15 15 15 15 15 15 15 200000 3000A 2500A 20,000 10 13 17 20 20 20 20 20 20 20 2000A 1600A 25,000 11 14 19 22 25 25 25 25 25 25 100000 1200A 90000 80000 30,000 11 14 20 24 27 30 30 30 30 30 70000 800A 60000 601A 35,000 12 15 21 25 29 35 35 35 35 35 50000 40000 40,000 13 16 22 26 30 35 40 40 40 40 30000 50,000 14 17 23 28 32 37 50 50 50 50

20000 AMPERE RATING 60,000 15 18 25 30 34 40 49 60 60 60 70,000 15 19 26 32 36 42 52 62 70 70

10000 9000 80,000 16 20 27 33 38 44 54 65 76 80 8000 7000 6000 90,000 17 21 29 34 39 45 56 67 79 90 5000

INSTANTANEOUS PEAK LET THRU CURRENT IN AMPERES THRU PEAK LET INSTANTANEOUS 100,000 17 22 30 36 41 47 58 70 81 100 4000

3000 150,000 20 25 34 41 47 54 67 80 93 104 A 200,000 22 27 37 45 51 59 73 87 102 114 2000 250,000 24 29 40 49 55 64 79 94 110 123

1000 300,000 25 31 43 52 59 68 84 100 117 30 1000 2000 5000 3000 4000 6000 7000 8000 9000 Note: For I value at 300,000 amperes, consult Factory. 70000 10000 20000 30000 40000 50000 60000 80000 90000 RMS 100000 200000 300000 PROSPECTIVE SHORT CIRCUIT CURRENT - SYMMETRICAL RMS AMPERES

Low-Peak Class J, Dual-Element Time-Delay Fuses LPJ_SP Fuse – RMS Let-Through Currents (kA) LPJ_SP Fuse Size Prosp. B 15 30 60 100 200 400 600 100000 Short 90000 80000 70000 C.C. IRMS IRMS IRMS IRMS IRMS IRMS IRMS 60000 600A 50000 1,000 1 111111 40000 400A

30000 200A 3,000 1 112233

20000 100A 5,000 1 112355

60A 10000 10,000 1 122468 9000 8000 7000 30A 15,000 1 123479 6000 5000 4000 15A 20,000 1 1234710 3000 25,000 1 1235810 2000 30,000 1 1235811 AMPERE RATING

1000 35,000 1 1245912 900 800 700 600 40,000 1 2346912 500 400 50,000 1 23461013 INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPERES PEAK LET-THROUGH INSTANTANEOUS 300 A 60,000 1 23461114 200 80,000 1 23571215

100 100,000 1 24581217 100 200 300 400 500 600 700 800 900 3000 4000 6000 7000 8000 9000 1000 2000 5000 70000 10000 20000 30000 40000 50000 60000 80000 90000 150,000 1 24691419 100000 200000 300000 PROSPECTIVE SHORT-CIRCUIT CURRENT - SYMMETRICAL RMS AMPERES 200,000 2 34691621 250,000 2 3 5 7 10 17 23 300,000 2 3 5 7 11 18 24

Note: For IRMS value at 300,000 amperes, consult Factory.

See pages 67 to 69 for current-limiting definition and how to analyze these charts.

Low-Peak Class RK1 Dual-Element Time-Delay Fuses LPN-RK_SP – RMS Let-Through Currents (kA) LPN-RK_SP Fuse Size Prosp. B 30 60 100 200 400 600 400,000 Short C.C. IRMS IRMS IRMS IRMS IRMS IRMS 300,000 1,000 1 1 1 1 1 1 200,000 2,000 1 1 2 2 2 2 3,000 1 1 2 3 3 3 100,000 90,000 80,000 5,000 1 2 2 3 5 5

70,000 AMPERE RATING 60,000 600A 10,000 1 2 3 4 7 9 50,000 40,000 400A 15,000 1 2 3 5 8 11

30,000 200A 20,000 1 3 3 5 8 11

20,000 25,000 1 3 3 5 9 12 100A 30,000 2 3 4 6 9 12 60A

10,000 35,000 2 3 4 6 10 13 9,000 8,000 30A 7,000 40,000 2 3 4 6 10 13 6,000 5,000 50,000 2 3 4 7 11 14 4,000 60,000 2 3 4 7 11 16 3,000 70,000 2 3 4 7 12 16 A INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPERES IN CURRENT PEAK LET-THROUGH INSTANTANEOUS 2,000 80,000 2 4 5 8 12 16 90,000 2 4 5 7 13 17 1,000 100,000 2 4 5 8 13 17 1,000 2,000 3,000 4,000 6,000 8,000

10,000 20,000 30,000 40,000 60,000 80,000 150,000 2 4 6 9 15 19 300,000 100,000 200,000 200,000 3 5 6 11 16 20 PROSPECTIVE SHORT-CIRCUIT CURRENT - SYMMETRICAL RMS AMPERES 250,000 3 5 7 11 17 21 300,000 3 6 7 12 18 22

Low-Peak Class RK1 Dual-Element Time-Delay Fuses LPS-RK_SP – RMS Let-Through Currents (kA) LPS-RK_SP Fuse Size Prosp. B Short 30 60 100 200 400 600 400,000 C.C. IRMS IRMS IRMS IRMS IRMS IRMS 300,000 1,000 1 1 1 1 1 1 200,000 2,000 1 1 2 2 2 2 3,000 1 1 2 3 3 3 100,000 90,000 5,000 1 2 2 3 5 5

80,000 AMPERE RATING 70,000 60,000 600A 10,000 1 2 3 4 7 10 50,000 400A 40,000 15,000 1 2 3 5 8 11

30,000 200A 20,000 2 3 3 5 9 12

20,000 25,000 2 3 4 6 9 12 100A 60A 30,000 2 3 4 6 10 13

10,000 35,000 2 3 4 6 10 13 9,000 8,000 30A 7,000 40,000 2 3 4 6 10 14 6,000 5,000 50,000 2 3 5 7 11 15 4,000 60,000 2 4 5 7 12 15 3,000 70,000 2 4 5 8 13 16

INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPERES IN CURRENT PEAK LET-THROUGH INSTANTANEOUS A 2,000 80,000 2 4 5 8 13 16 90,000 2 4 5 8 13 17 1,000 100,000 2 4 6 9 14 17 1,000 2,000 3,000 4,000 6,000 8,000

10,000 20,000 30,000 40,000 60,000 80,000 150,000 3 5 6 10 15 19 100,000 200,000 300,000

PROSPECTIVE SHORT-CIRCUIT CURRENT - SYMMETRICAL RMS AMPERES 200,000 3 5 7 11 16 21 250,000 3 6 7 12 17 22 300,000 3 6 7 12 18 23

See pages 67 to 69 for current-limiting definition and how to analyze these charts.

Fusetron Class RK5 Dual-Element Time-Delay Fuses FRN-R – RMS Let-Through Currents (kA) FRN-R Fuse Size Prosp. B 400000 Short 30 60 100 200 400 600 300000 C.C. IRMS IRMS IRMS IRMS IRMS IRMS

200000 5,000 1 2 3 5 5 5 10,000 2 3 4 7 10 10

100000 AMPERE RATING 15,000 2 3 5 8 11 15 90000 80000 70000 600A 20,000 2 4 5 8 12 16 60000 400A 50000 25,000 2 4 6 9 13 17 200A 40000 30,000 2 4 6 10 14 18 30000 100A 35,000 2 4 6 10 15 19

20000 60A 40,000 2 5 7 11 15 20 50,000 3 5 7 11 17 21 10000 9000 30A 60,000 3 5 8 12 18 22 8000 7000 70,000 3 6 8 13 19 23 6000 5000 80,000 3 6 8 13 19 24 4000 INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPERES IN CURRENT PEAK LET-THROUGH INSTANTANEOUS 90,000 3 6 9 14 20 25 3000 A 100,000 3 6 9 14 21 26 2000 150,000 4 7 10 16 24 29 200,000 4 8 11 18 26 32 1000 3000 4000 6000 7000 8000 9000 1000 2000 5000 30000 60000 80000 90000 70000 10000 20000 40000 50000 100000 200000

PROSPECTIVE SHORT-CIRCUIT CURRENT - SYMMETRICAL RMS AMPERES

Fusetron Class RK5 Dual-Element Time-Delay Fuses FRS-R – RMS Let-Through Currents (kA) FRS-R Fuse Size Prosp. 30 60 100 200 400 600 B Short 400000 C.C. IRMS IRMS IRMS IRMS IRMS IRMS 300000 5,000 1 1 3 4 5 5 200000 10,000 1 2 4 5 9 10 15,000 1 2 4 6 10 14

100000

90000 AMPERE RATING 20,000 2 2 5 7 11 15 80000 70000 600A 25,000 2 2 5 7 12 17 60000 400A 50000 30,000 2 3 5 8 13 18 40000 200A 35,000 2 3 5 8 13 18 30000 40,000 2 3 6 9 14 19 100A 20000 50,000 2 3 6 9 14 20

60A 60,000 2 3 6 10 15 22 10000 9000 30A 70,000 3 4 7 11 17 23 8000 7000 6000 80,000 3 4 7 12 17 23 5000 90,000 3 4 7 12 17 24 4000 INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPERES IN CURRENT PEAK LET-THROUGH INSTANTANEOUS 100,000 3 4 8 13 18 25 3000 A 150,000 3 5 9 14 21 27 2000 200,000 4 6 9 16 23 32

1000 3000 4000 6000 7000 8000 9000 1000 2000 5000 70000 30000 60000 80000 90000 10000 20000 40000 50000 100000 200000

PROSPECTIVE SHORT-CIRCUIT CURRENT - SYMMETRICAL RMS AMPERES

See pages 67 to 69 for current-limiting definition and how to analyze these charts.

Tron Class T Fast-Acting Fuses JJN – RMS Let-Through Current (kA) JJN Fuse Size Prosp. B 15 30 60 100 200 400 600 800 1200 400,000 Short 300,000 C.C. IRMS IRMS IRMS IRMS IRMS IRMS IRMS IRMS IRMS 200,000 500 1 1 1 1 1 1 1 1 1

100,000 1,000 1 1 1 1 1 1 1 1 1 80,000 AMPERE RATING 5,000 1 1 1 1 2 3 5 5 5 60,000 1200 800 10,000 1 1 1 2 2 4 6 7 9 40,000 600 30,000 15,000 1 1 1 2 3 4 6 9 10 20,000 400 200 20,000 1 1 1 2 3 5 7 10 11 100 10,000 25,000 1 1 2 2 3 5 7 10 12 8,000 60 30,000 1 1 2 2 3 5 8 11 13 6,000 30 4,000 15 35,000 1 1 2 3 4 6 8 11 13 3,000 40,000 1 1 2 3 4 6 9 11 13 2,000 50,000 1 1 2 3 4 7 9 12 15 1,000 60,000 1 1 2 3 4 7 10 13 16 800 600 70,000 1 1 2 3 5 7 10 14 17 400 80,000 1 2 2 3 5 8 11 15 17

INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPS 300 A 90,000 1 2 2 3 6 8 11 15 18 200

100 200 300 400 600 800 100,000 1 2 2 4 6 8 12 16 19 1,000 2,000 3,000 4,000 6,000 8,000 10,000 20,000 30,000 40,000 60,000 80,000

100,000 200,000 150,000 1 2 3 4 6 9 13 17 22 PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS 200,000 2 2 3 4 7 9 15 19 23

Tron Class T Fast-Acting Fuses JJS – RMS Let-Through Current (kA) JJS Fuse Size Prosp. B 400,000 Short 15 30 60 100 200 400 600 800 300,000 C.C. IRMS IRMS IRMS IRMS IRMS IRMS IRMS IRMS 200,000 500 1 1 1 1 1 1 1 1

100,000 AMPERE RATING 1,000 1 1 1 1 1 1 1 1 80,000 800 5,000 1 1 1 2 3 4 5 5 60,000 600 40,000 400 10,000 1 1 1 2 3 6 8 9 30,000 15,000 1 1 2 3 4 7 10 11 20,000 200 100 20,000 1 1 2 3 4 7 10 12 10,000 60 25,000 1 1 2 3 5 7 11 13 8,000 30 6,000 30,000 1 1 2 3 5 8 12 14 15 4,000 35,000 1 1 2 3 5 9 13 15 3,000 40,000 1 2 2 4 5 9 13 15 2,000 50,000 1 2 2 4 6 10 14 17 1,000 60,000 1 2 3 4 6 10 16 18 800 600 70,000 1 2 3 4 7 11 17 19 400 80,000 1 2 3 4 7 11 17 20

INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPS 300 A 200 90,000 1 2 3 4 7 12 18 21

100 200 300 400 600 800 100,000 2 2 3 5 7 12 19 22 1,000 2,000 3,000 4,000 6,000 8,000 10,000 20,000 30,000 40,000 60,000 80,000 100,000 200,000 150,000 2 3 4 6 8 14 22 25 PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS 200,000 2 3 4 6 9 16 24 28

See pages 67 to 69 for current-limiting definition and how to analyze these charts.

Low-Peak Class CC Time-Delay Fuses LP-CC – RMS Let-Through Currents (A) LP-CC Fuse Size Prosp. 1 8 Short 1 /4 2 /10 15 20 25 30 C.C. IRMS IRMS IRMS IRMS IRMS IRMS 1,000 100 135 240 305 380 435 3,000 140 210 350 440 575 580 5,000 165 255 420 570 690 710 10,000 210 340 540 700 870 1000 20,000 260 435 680 870 1090 1305 30,000 290 525 800 1030 1300 1520 40,000 315 610 870 1150 1390 1700 50,000 340 650 915 1215 1520 1820 60,000 350 735 1050 1300 1650 1980 80,000 390 785 1130 1500 1780 2180 100,000 420 830 1210 1600 2000 2400 200,000 525 1100 1600 2000 2520 3050 INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPS

PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS

Limitron Class J Fast-Acting Fuses JKS – RMS Let-Through Currents (kA) JKS Fuse Size Prosp. B 400,000 Short 30 60 100 200 400 600 300,000 C.C. IRMS IRMS IRMS IRMS IRMS IRMS

200,000 5,000 1 1 2 3 4 5 10,000 1 2 3 4 6 9

100,000 15,000 1 2 3 4 7 10 AMPERE RATING 80,000 20,000 1 2 3 5 8 11 60,000 600 50,000 25,000 2 3 3 6 9 12 40,000 400 30,000 2 3 3 6 9 13 30,000 200 35,000 2 3 4 6 9 13 20,000 40,000 2 3 4 7 10 14 100 60 50,000 2 3 4 7 10 15 10,000 8,000 60,000 2 3 5 7 11 16 6,000 30 70,000 2 3 5 8 11 17 5,000 4,000 80,000 2 3 5 8 12 17 3,000 90,000 2 4 6 9 13 18 A 2,000 100,000 2 4 6 9 13 18

INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPS 150,000 2 5 6 9 14 22 1,000 200,000 3 5 7 10 16 24 1,000 2,000 3,000 4,000 5,000 6,000 8,000 10,000 20,000 30,000 40,000 50,000 60,000 80,000 100,000 200,000 PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS