SPD - Selecting Protective
Devices
Based
on the
2008 NEC
®
Value
INSIDE FRONT
COVER
3
©2008 Cooper Bussmann
Selecting Protective Devices Handbook (SPD)
Based on the 2008 NEC
®
Welcome to the Cooper Bussmann
®Selecting Protective Devices Handbook (SPD). This is a
comprehensive guide to electrical overcurrent protection and electrical design considerations.
Information is presented on numerous applications as well as the requirements of codes and
standards for a variety of electrical equipment and distribution systems.
How to Use:
The SPD is comprised of major sections which are arranged by topic. There are three methods for
locating specific information contained within:
1. Table of Contents: The table of contents sequentially presents the major sections and their
contents. New or revised sections are noted in red text.
2. Index: The index, found on page 265, is more detailed than the table of contents and is
organized alphabetically by topic with corresponding page number references.
3. 2008 NEC
®Section Index: The NEC
®Section Index, found on page 264, makes it easy to find
information associated with specific National Electrical Code
®section references.
For other technical resources and product information visit
www.cooperbussmann.com
.
This handbook is intended to clearly present product data and technical information that will help the end user with design applications. Cooper Bussmann reserves the right, without notice, to change design or construction of any products and to discontinue or limit their distribution. Cooper Bussmann also reserves the right to change or update, without notice, any technical information contained in this handbook. Once a product has been selected, it should be tested by the user in all possible applications. Further, Cooper Bussmann takes no responsibility for errors or omissions contained in this handbook, or for mis-application of any Cooper Bussmann product. Extensive product and application information is available online at: www.cooperbussmann.com.
National Electrical Code®is a trademark of the National Fire Protection Association, Inc., Batterymarch Park, Quincy, Massachusetts, for a triennial electrical publication. The term, National Electrical Code, as used herein means the triennial publication constituting the National Electrical Code and is used with permission of the National Fire Protection Association, Inc.
4 ©2008 Cooper Bussmann
2008 SPD Introduction . . . .3
Benefits Offered By Fuses . . . .5
Fuseology . . . .6 - 31 Overcurrents and Voltage Ratings . . . .6
Voltage Ratings & Slash Voltage Ratings . . . .7
Amp Rating and Interrupting Rating . . . .8 - 10 Selective Coordination & Current Limiting . . . .11
Non Time-Delay Fuse Operation . . . .12
Dual-Element, Time-Delay Fuse Operation . . . .13
Dual-Element Fuse Benefits . . . .14 - 15 Branch Circui & Application Limited Overcurrent Protective Devices . . . .16 - 20 Branch Circuit Fuse Selection Chart (600V or less) 21 Branch Circuit Fuse Dimensions . . . .22 - 23 Cooper Bussmann Branch Circuit, Power Distribution Fuses . . . .24 - 25 Disconnects, Panelboards, & One-Time Fuses . . . .26
Fuse Holders, Fuse Blocks, Power Distribution Blocks & Surge Suppression . . . .27
High Speed Fuses . . . .28 - 29 Medium Voltage Fuses . . . .30 - 31 Applying Interrupting Rating: Circuit Breakers . . . .32 - 57 Interrupting Rating Vs. Interrupting Capacity . .32 - 34 Single-Pole Interrupting Capability . . . .35 - 40 Series Rating: Protecting Circuit Breakers . . . .41 - 46 - Square D Series Rating Chart . . . .47 - 48 - Cutler-Hammer Series Rating Chart . . . .49 - 51 - General Electric Series Rating Chart . . . .52 - 55 - Siemens Series Rating Chart . . . .56 - 57 Conductor Protection — General . . . . .58 - 60 General . . . .58
Small Conductors . . . .58
Tap Conductors . . . .58-59 Cable Limiters — Applications . . . .61
Conductors & Terminations — Applications . . . .62 - 64 Equipment Protection . . . .65 - 75 General . . . .64 - 68 Transformers — 600V or Less . . . .69 - 70 Transformers — Over 600V . . . .71 - 72 Photovoltaic (PV) Systems . . . .73 - 75 Component Protection . . . .76 - 86 Introduction and Current-Limitation . . . .76
How To Use Current-Limitation Charts . . . .77 - 78 Wire & Cable . . . .79 - 81 Tap Conductor Sizing . . . .81
Small Wire (16 & 18 AWG) . . . .81
Busway . . . .82-83 HVAC and Refrigeration Equipment . . . .84
Transfer Switches . . . .85
Ballasts . . . .86
Industrial Control Panels . . . .87 - 107 Short-Circuit Current Rating Marking Requirements . . . .87-88 Determining Assembly SCCR: Two Sweep Method . . . .89 - 90 Umbrella Fuse Limits . . . .91 - 93 Determining Assembly SCCR: Example . . . .94 - 104 Increasing Assembly SCCR . . . .105-107 Selective Coordination . . . .108 - 146 Introduction . . . .108
Fuses . . . .109 - 111 Fuse Selectivity Ratio Guide . . . .112 -113 Fusible Lighting Panels . . . .114
Fuse Selective Coordination Example . . . .115
Circuit Breakers: Operation Basics . . . .116 -118 Circuit Breakers: Achieving Selective Coordination . . . .119 -125 Fuse & Circuit Breaker Mixture . . . .126
Mandatory Selective Coordination Requirements .127 Why Selective Coordination is Mandatory . . .128 -130 System Considerations . . . .131 -135 OCPD Choices for Selective Coordination . .136 -137 Inspection Checklist . . . .138
Objections & Misunderstandings . . . .139-144 Elevator Circuits . . . .145 -146 Ground Fault Protection . . . .147 - 157 Introduction & Requirements . . . .147 - 149 Overcurrent Protective Devices . . . .150
GFPR Considerations . . . .151 - 155 Current-Limitation . . . .156 - 157 Electrical Safety . . . .158 - 170 Introduction . . . .158 -159 Arc-Flash Protection . . . .160 -162 Maintenance Considerations . . . .163
Arc-Flash Hazard Analysis . . . .164
Arc-Flash Incident Energy Calculator . . . .165 -167 Arc-Flash Hazard Analysis . . . .168 -169 Arc-Flash Protection Marking . . . .168 -170 Devices for Motor Circuits . . . .171 - 180 Branch Circuit Devices and Disconnect Selection Tables: . . . .171 - 173 Motor Branch Circuit Devices . . . .174 -179 Motor Circuit Branch Circuit Protection — Is Resettability of Value? . . . .180
Motor Protection . . . .181 - 189 Voltage Unbalance & Single-Phasing . . . .181 - 186 Basic Explanation . . . .187 - 189 Motor Branch Circuit Protection — NEC®430.52 Explanation . . . .190
Motor Circuit Notes . . . .191
Motor Circuits — Group Switching . . . .192
Motor Circuit Protection Tables . . . .193 - 207 NEC®Article 430 and Tables Explanation . .193 - 194 200Vac Three-Phase Motors & Circuits . . . .194 - 195 208Vac Three-Phase Motors & Circuits . . . .195 - 196 230Vac Three-Phase Motors & Circuits . . .197 - 198 460Vac Three-Phase Motors & Circuits . . .198 - 199 575Vac Three-Phase Motors & Circuits . . .200 - 201 115Vac Single-Phase Motors & Circuits . . . .202
230Vac Single-Phase Motors & Circuits . . . .203
90Vdc Motors & Circuits . . . .204
120Vdc Motors & Circuits . . . .205
180Vdc Motors & Circuits . . . .206
240Vdc Motors & Circuits . . . .206 - 207 Motor Protection — Tips For Electricians & Maintenance Crews . . . .208
Motor Starter Protection . . . .209 - 212 Graphic Explanation . . . .209 - 210 Low Voltage Motor Controllers . . . .211
Type 1 Versus Type 2 Protection . . . .212
Table of Contents
(
Red indicates NEW or significantly REVISED information)
Motor Controller & Fuse Selection For Type 2 Protection . . . .213 - 227 General Electric Company — IEC . . . .213General Electric Company — NEMA . . . .214 - 216 Rockwell Automation, Allen-Bradley — IEC . . . .217
Rockwell Automation, Allen-Bradley — NEMA . . .218
Square D Company — IEC . . . .219 - 221 Square D Company — NEMA . . . .222
Siemens — IEC . . . .223
Siemens — NEMA . . . .224
Cutler Hammer Freedom Series — IEC . . . .225 - 226 Cutler Hammer Freedom Series — NEMA . . . .227
Motor Circuits With Power Electronic Devices — Power Electronic Device Circuit Protection . . . .228 - 230 Motor Circuit Protection — Group Motor Protection . . . .231
Motor Control Circuit Protection . . . .232 - 235 Medium Voltage Motor Circuits — R-Rated Medium Voltage Fuses . . . .236
Cost of Ownership . . . .237 - 238 Fusible Equipment vs. Circuit Breaker Equipment 237 Preventive Maintenance . . . .238
Short Circuit Current Calculations . .239 - 245 Introduction . . . .239
Three-Phase Short Circuits . . . .241
Single-Phase Short Circuits . . . .242 - 243 Impedance & Reactance Data . . . .244
Conductors & Busways "C" Values . . . .245
Voltage Drop Calculations . . . .246 - 248 Ratings of Conductors and Tables to Determine Volt Loss . . . .246
Copper Conductors — Ratings & Volt Loss . . . .247
Aluminum Conductors — Ratings & Volt Loss . . .248
Fuse Diagnostic Sizing Charts . . . . .249 - 253 Ballasts . . . .249
Capacitors (NEC®460) . . . .249
Electric Heat (NEC®424) . . . .250
Mains, Feeders, Branches . . . .250
Motor Loads (NEC®430) . . . .251
Solenoids (Coils) . . . .251
Transformers 600V Nominal or Less . . . .252
Transformers Over 600V Nominal . . . .253
Solid State Devices . . . .253
Fuse Sizing Guide — Building Electrical Systems . . . .254
Fuse Specifications . . . .255 - 256 Suggestions . . . .255
Suggested Fuse and Fusible Equipment Specifications . . . .256
Cooper Bussmann Current-Limiting Fuse Let-Through Data . . . .257 - 261 Glossary . . . .262 - 263 Electrical Formulas . . . .263
2008 NEC®Section Index . . . .264 Index . . . .265-266 Fuse Cross Reference & Low-Peak
©2008 Cooper Bussmann 5
Benefits Offered By Fuses
High Interrupting Rating of 200,000 Amps or More
Modern current-limiting fuses have high interrupting ratings at no extra cost. Whether for the initial installation or system updates, a fusible system can maintain a sufficient interrupting rating. This helps with achieving high assembly short-circuit current ratings. See pages 6 to 8 for Fuseology Interrupting Rating details.
Type 2 Protection
Type 2 “No Damage” protection of motor starters when applied properly. See page 164 for details on Type 1 versus Type 2 protection.
High SCCR
High assembly short-circuit current ratings can be achieved. See pages 78 to 88 for Industrial Control Panels.
Rejection Features
Modern current-limiting fuses have rejection features which assure replacement with a device of the same voltage rating and equal or greater interrupting rating. In addition, rejection features restrict the fuses used for replacement to ones of the same class or type.
Flexibility
Increased flexibility in panel use and installation. Valuable time that was spent gathering information for proper application is drastically reduced with fuses since:
• Fuses can be installed in systems with available fault currents up to 200kA or 300kA covering the majority of installations that exist. • Fuses can handle line-to-ground fault currents up to their marked
interrupting rating where mechanical devices often have single pole interrupting capabilities far less than their marked IR (typically 8,660 amps for any marked IR) See pages 6 to 8 and 33 to 34 for Fuse Single Pole Interrupting Ratings and pages 29 to 34 for Circuit Breaker Single Pole Interrupting Capabilities. • Fuses have a straight voltage rating instead of a slash voltage
rating. A device with a slash voltage rating is limited to installation in ONLY a solidly grounded wye type system. Fuses can be installed in any type of installation independent of the grounding scheme used. See pages 5 to 6 for Slash Voltage Rating.
Reliability
Fuses provide reliable protection throughout the life of the installation. After a fault occurs, fuses are replaced with new factory calibrated fuses assuring the same level of protection that existed previous to the fault. Resettable devices may not provide the same level of protection following a fault and need to be tested for calibration and possibly replaced.
No Venting
Fuses do not VENT. Therefore fuses will not affect other components in the panel while clearing a fault. Additional guards or barriers which isolate devices that vent from other components are not required.
Helps Regulation Compliance
Eliminates invitation to reset into a fault and potential OSHA violation. Resetting or manually reenergizing a circuit without investigating the cause is prohibited in OSHA CFR29:1910-334. Fuses are not resettable and eliminate the invitation to reset. See page 132 for Is Resettability of Value?
Workplace Safety
Superior current limitation provides enhanced workplace safety. See pages 116 to 126 for Electrical Safety.
Component Protection Via Current Limitation
Superior current limitation provides protection of circuit components for even the most susceptible components such as equipment grounding conductors. See pages 67 to 77 for Component Protection and pages 78 to 88 for Industrial Control Panels.
Selective Coordination
Achieving selective coordination is simple. Typically selective coordination can be achieved between current-limiting fuses by simply maintaining a minimum amp ratio between upstream and downstream fuses. This can aid in diagnostics within the building electrical system or machine panel as only the effected circuit is isolated. Selective coordination helps isolate faulted circuits from the rest of the system and prevents unnecessary power loss to portions of a building. See pages 9 and 88 to 105 for Selective Coordination.
Specify the Cooper Bussmann
Low-Peak
®System
• Safety Built-in rejection features • Selective coordination with a
minimum 2:1 ratio
• Maximum current-limiting protection for distribution equipment
• Type "2" Protection for motor starters • Only one type of fuse throughout building • Reduces inventory
• 300,000A interrupting rating • May reduce arc-flash hazard
M M KRP-C_SP KRP-C_SP KRP-C_SP LP-CC LPS-RK_SP LPS-RK_SP LPS-RK_SP LPJ_SPI LPJ_SP LPJ_SP Feeder For MCC Branch For Large Motor Feeder For MLO Lighting Panel Branch For Resistance Load Resistance Load 20A Circuit Breakers Reduced Voltage Starter For Large Motor LP1
6
Electrical distribution systems are often quite complicated. They cannot be absolutely fail-safe. Circuits are subject to destructive overcurrents. Harsh environments, general deterioration, accidental damage or damage from natural causes, excessive expansion or overloading of the electrical distribution system are factors which contribute to the occurrence of such overcurrents. Reliable protective devices prevent or minimize costly damage to transformers, conductors, motors, and the other many components and loads that make up the complete distribution system. Reliable circuit protection is essential to avoid the severe monetary losses which can result from power blackouts and prolonged downtime of facilities. It is the need for reliable protection, safety, and freedom from fire hazards that has made the fuse a widely used protective device.
field stresses. The magnetic forces between bus bars and other conductors can be many hundreds of pounds per linear foot; even heavy bracing may not be adequate to keep them from being warped or distorted beyond repair.
Fuses
The fuse is a reliable overcurrent protective device. A “fusible” link or links encapsulated in a tube and connected to contact terminals comprise the fundamental elements of the basic fuse. Electrical resistance of the link is so low that it simply acts as a conductor. However, when destructive currents occur, the link very quickly melts and opens the circuit to protect conductors and other circuit components and loads. Modern fuses have stable characteristics. Fuses do not require periodic maintenance or testing. Fuses have three unique performance characteristics:
1. Modern fuses have an extremely “high interrupting” rating–can open very high fault currents without rupturing.
2. Properly applied, fuses prevent “blackouts.” Only the fuse nearest a fault opens with-out upstream fuses (feeders or mains) being affected–fuses thus provide “selective coordination.” (These terms are precisely defined in subsequent pages.) 3. Fuses provide optimum component protection by keeping fault currents to a low
value…They are said to be “current- limiting.”
©2008 Cooper Bussmann
Overcurrents and Voltage Ratings
Fuses are constructed in an almost endless variety of configurations. These photos depict the internal construction of Cooper Bussmann Dual-Element, Semi-Tron®and
Low-Peak®Class L fuses.
Overcurrents
An overcurrent is either an overload current or a short-circuit current. The overload current is an excessive current relative to normal operating current, but one which is confined to the normal conductive paths provided by the conductors and other components and loads of the distribution system. As the name implies, a short-circuit current is one which flows outside the normal conducting paths.
Overloads
Overloads are most often between one and six times the normal current level. Usually, they are caused by harmless temporary surge currents that occur when motors start up or transformers are energized. Such overload currents, or transients, are normal occurrences. Since they are of brief duration, any temperature rise is trivial and has no harmful effect on the circuit components. (It is important that protective devices do not react to them.)
Continuous overloads can result from defective motors (such as worn motor bearings), overloaded equipment, or too many loads on one circuit. Such sustained overloads are destructive and must be cut off by protective devices before they damage the distribution system or system loads. However, since they are of relatively low magnitude compared to short-circuit currents, removal of the overload current within a few seconds to many minutes will generally prevent equipment damage. A sustained overload current results in overheating of conductors and other components and will cause deterioration of insulation, which may eventually result in severe damage and short circuits if not interrupted.
Short Circuits
Whereas overload currents occur at rather modest levels, the short-circuit or fault current can be many hundred times larger than the normal operating current. A high level fault may be 50,000A (or larger). If not cut off within a matter of a few thousandths of a second, damage and destruction can become rampant–there can be severe insulation damage, melting of conductors, vaporization of metal, ionization of gases, arcing, and fires. Simultaneously, high level short-circuit currents can develop huge
magnetic-The Louisiana Superdome in New Orleans is the world’s largest fully enclosed stadium. The overall electrical load exceeds 30,000,000 VA. Distribution circuits are protected with Cooper Bussmann Low-Peak fuses.
Voltage Rating - General
This is an extremely important rating for overcurrent protective devices (OCPDs). The proper application of an overcurrent protective device according to its voltage rating requires that the voltage rating of the device be equal to or greater than the system voltage. When an overcurrent protective device is applied beyond its voltage rating, there may not be any initial indicators. Adverse consequences typically result when an improperly voltage rated device attempts to interrupt an overcurrent, at which point it may self-destruct in an unsafe manner. There are two types of OCPD voltage ratings: straight voltage rated and slash voltage rated.
The proper application is straightforward for overcurrent protective devices with a straight voltage rating (i.e.: 600V, 480V, 240V) which have been evaluated for proper performance with full phase-to-phase voltage used during the testing, listing and marking. For instance, all fuses are straight voltage rated and there is no need to be concerned about slash ratings. However, some mechanical overcurrent protective devices are slash voltage rated (i.e.: 480/277, 240/120, 600/347). Slash voltage rated devices are limited in their applications and extra evaluation is required when they are being
considered for use. The next section covers fuse voltage ratings followed by a section on slash voltage ratings for other type devices.
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Fuses are a universal protective device. They are used in power distribution systems, electronic apparatus, vehicles…and as illustrated, our space program. The Space Shuttle has over 600 fuses installed in it protecting vital equipment and circuits.
Voltage Rating-Fuses
Most low voltage power distribution fuses have 250V or 600V ratings (other ratings are 125, 300, and 480 volts). The voltage rating of a fuse must be at least equal to or greater than the circuit voltage. It can be higher but never lower. For instance, a 600V fuse can be used in a 208V circuit. The voltage rating of a fuse is a function of its capability to open a circuit under an overcurrent condition. Specifically, the voltage rating determines the ability of the fuse to suppress the internal arcing that occurs after a fuse link melts and an arc is produced. If a fuse is used with a voltage rating lower than the circuit voltage, arc suppression will be impaired and, under some overcurrent conditions, the fuse may not clear the overcurrent safely. 300V rated fuses can be used to protect single-phase line-to-neutral loads when supplied from three-phase, solidly grounded, 480/277V circuits, where the single-phase line-to-neutral voltage is 277V. This is permissible because in this application, a 300V fuse will not have to interrupt a voltage greater than its 300V rating. Special consideration is necessary for semiconductor fuse applications, where a fuse of a certain voltage rating is used on a lower voltage circuit.
Slash Voltage Ratings
Some multiple-pole, mechanical overcurrent protective devices, such as circuit breakers, self-protected starters, and manual motor controllers, have a slash voltage rating rather than a straight voltage rating. A slash voltage rated overcurrent protective device is one with two voltage ratings separated by a slash and is marked such as 480Y/277V or 480/277V. Contrast this to a straight voltage rated overcurrent protective device that does not have a slash voltage rating limitation, such as 480V. With a slash rated device, the lower of the two ratings is for overcurrents at line-to-ground voltages, intended to be cleared by one pole of the device. The higher of the two ratings is for overcurrents at line-to-line voltages, intended to be cleared by two or three poles of the circuit breaker or other mechanical overcurrent device. Slash voltage rated overcurrent protective devices are not intended to open phase-to-phase voltages across only one pole. Where it is possible for full phase-to-phase voltage to appear across only one pole, a full or straight rated overcurrent protective device must be utilized. For example, a 480V circuit breaker may have to open an overcurrent at 480V with only one pole, such as might occur when Phase A goes to ground on a 480V, B-phase, corner grounded delta system.
The NEC®addresses slash voltage ratings for circuit breakers in 240.85 restricting their use to solidly grounded systems where the line to ground voltage does not exceed the lower of the two values and the line voltage does not exceed the higher value.
430.83(E) was revised for the 2005 NEC®to address the proper application of motor controllers marked with a slash voltage rating. The words "solidly grounded" were added to emphasize that slash voltage rated devices are not appropriate for use on corner grounded delta, resistance grounded and ungrounded systems.
Slash voltage rated OCPDs must be utilized only on solidly grounded systems. This automatically eliminates their usage on impedance-grounded and ungrounded systems. They can be properly utilized on solidly grounded, wye systems, where the voltage to ground does not exceed the device’s lower voltage rating and the voltage between any two conductors does not exceed the device’s higher voltage rating. Slash voltage rated devices cannot be used on corner-grounded delta systems whenever the voltage to ground exceeds the lower of the two ratings. Where slash voltage rated devices will not meet these requirements, straight voltage rated overcurrent protective devices are required.
Overcurrent protective devices that may be slashed rated include, but are not limited to:
• Molded case circuit breakers – UL489 • Manual motor controllers – UL508
• Self protected Type E combination starters – UL508
• Supplementary protectors – UL1077 (Looks like a small circuit breaker and sometimes referred to as mini-breaker. However, these devices are not a circuit breaker, they are not rated for branch circuit protection and can not be a substitute where branch circuit protection is required.)
What about fuses, do they have slash voltage ratings? No, fuses do not have this limitation. Fuses by their design are full voltage rated devices; therefore slash voltage rating concerns are not an issue when using fuses. For instance, Cooper Bussmann Low-Peak®LPJ (Class J) fuses are rated at 600V. These fuses could be utilized on systems of 600V or less, whether the system is solidly grounded, ungrounded, impedance grounded, or corner grounded delta.
If a device has a slash voltage rating limitation, product standards require these devices, such as circuit breakers, manual motor controllers, self protected starters, or supplementary protectors to be marked with the rating such as 480Y/277V. If a machine or equipment electrical panel utilizes a slash voltage rated device inside, it is recommended that the equipment nameplate or label designate this slash voltage rating as the equipment voltage rating. UL508A industrial control panels requires the electrical panel voltage marking to be slash rated if one or more devices in the panel are slash voltage rated.
©2008 Cooper Bussmann
Voltage Ratings and Slash Voltage Ratings
A B C 480Y/277 Volt three phase, four wire, solidly grounded, wye system Circuit breaker
480Y/277 slash voltage rating 480 volts Line-to-line
Ground N
277 volts Line-to-ground
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Slash voltage devices are limited in application to solidly grounded, wye systems due to the nature of the way that these devices are tested, listed and labeled. Any piece of equipment that utilizes a slash voltage rated overcurrent protective device is therefore, limited to installation only in a solidly grounded, wye system and should require marking that notes this limitation.
Equipment that utilizes straight voltage rated overcurrent protective devices provides more value and utilization to the owner or potential future owners than equipment that utilizes slash voltage rated devices. In today’s business environment, machinery and equipment may be moved several times during its useful life. Equipment utilizing slash voltage rated overcurrent devices is not suitable for many electrical systems found in industrial environments.
Amp Rating
Every fuse has a specific amp rating. In selecting the amp rating of a fuse, consideration must be given to the type of load and code requirements. The amp rating of a fuse normally should not exceed the current carrying capacity of the circuit. For instance, if a conductor is rated to carry 20A, a 20A fuse is the largest that should be used. However, there are some specific circumstances in which the amp rating is permitted to be greater than the current carrying capacity of the circuit. A typical example is motor circuits; dual-element fuses generally are permitted to be sized up to 175% and non-time-delay fuses up to 300% of the motor full-load amps. As a rule, the amp rating of a fuse and switch combination should be selected at 125% of the continuous load current (this usually corresponds to the circuit capacity, which is also selected at 125% of the load current). There are exceptions, such as when the fuse-switch combination is approved for continuous operation at 100% of its rating.
Testing Knife-Blade Fuses
A common practice when electricians are testing fuses is to touch the end caps of the fuse with their probes. Contrary to popular belief, fuse manufacturers do not generally design their knife-blade fuses to have electrically energized fuse caps during normal fuse operation. Electrical inclu-sion of the caps into the circuit occurs as a result of the coincidental mechanical contact between the fuse cap and terminal extending through it. In most brands of knife-blade fuses, this mechanical contact is not guaranteed; therefore, electrical contact is not guaranteed. Thus, a resistance reading taken across the fuse caps is not indicative of whether or not the fuse is open. In a continuing effort to promote safer work environments, Cooper Bussmann has introduced newly designed versions of knife-blade Fusetron®fuses (Class RK5) and knife-blade Low-Peak fuses (Class RK1) for some of the amp rat-ings. The improvement is that the end caps are insulated to reduce the possi-bility of accidental contact with a live part. With these improved fuses, the informed electrician knows that the end caps are isolated. With older style non-insulated end caps, the electrician doesn’t really know if the fuse is “hot” or not. A portion of all testing-related injuries could be avoided by proper test-ing procedures. Cooper Bussmann hopes to reduce such injuries by informtest-ing electricians of proper procedures.
Interrupting Rating
A protective device must be able to withstand the destructive energy of short-circuit currents. If a fault current exceeds a level beyond the capability of the protective device, the device may actually rupture, causing additional damage. Thus, it is important when applying a fuse or circuit breaker to use one which can sustain the largest potential short-circuit currents. The rating which defines the capacity of a protective device to maintain its integrity when reacting to fault currents is termed its “interrupting rating”. The interrupting rating of most branch-circuit, molded case, circuit breakers typically used in residential service entrance panels is 10,000A. (Please note that a molded case circuit breaker’s interrupting capacity will typically be lower than its interrupting rating.) Larger, more expensive circuit breakers may have interrupting ratings of 14,000A or higher. In contrast, most modern, current-limiting fuses have an interrupting rating of 200,000 or 300,000A and are commonly used to protect the lower rated circuit breakers. The National Electrical Code®110.9, requires equipment intended to break current at fault levels to have an interrupting rating sufficient for the current that must be interrupted. The subjects of interrupting rating and interrupting capacity are treated later in more detail.
©2008 Cooper Bussmann
Amp Rating and Interrupting Rating
Always Test at the Blade
Insulated
Caps A continuity test across
any knife-blade fuse should be taken ONLY along the fuse blades.
Do NOT test a knife-blade fuse with meter probes
to the fuse caps.
This photograph vividly illustrates the effects of overcurrents on electrical components when protective devices are not sized to the amp rating of the component.
Non-Insulated
©2008 Cooper Bussmann 9
Interrupting Rating
The following series of images from high-speed film demonstrate the destructive power associated with short-circuit currents.
The first group of photos depicts a test conducted on a 480V circuit breaker. The breaker has an interrupting rating of 14,000A, however, the test circuit was capable of delivering 50,000A of short-circuit current at 480V. The results can be seen below.
1
2
4
4
3
2
1
Before Fault
During Interruption
After Interruption
3
This second group of photos uses the same test circuit as the previous test, however, the test subjects are a pair of 600V, one-time fuses with an
interrupting rating of 10,000A. Notice in this test, as well as the circuit breaker test, the large amount of destructive energy released by these devices. Misapplying overcurrent protective devices in this manner is a serious safety hazard as shrapnel and molten metal could strike electricians or maintenance personnel working on equipment, or anyone who happens to be nearby.
This last group depicts the same test circuit as the previous two tests, which is 50,000A available at 480V. This time the test was performed with modern current-limiting fuses. These happen to be Cooper Bussmann Low-Peak fuses with a 300,000A interrupting rating. Notice that the fault was cleared without violence.
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The table below depicts four different situations involving an overcurrent device with a normal current rating of 100A and an interrupting rating of only 10,000A.
As depicted in the diagram that follows, when using overcurrent protective devices with limited interrupting rating, it becomes necessary to determine the available short-circuit currents at each location of a protective device. The fault currents in an electrical system can be easily calculated if sufficient
information about the electrical system is known. (See the Point-to-Point Method for Short Circuit Calculations, pages 192 to 198.) With modern fuses, these calculations normally are not necessary since the 200,000A or 300,000A interrupting rating is sufficient for most applications.
Also, if using circuit breakers or self-protected starters, it may be necessary to evaluate the devices’ individual pole interrupting capability for the level of fault current that a single pole of a multi-pole device may have to interrupt. This is covered in-depth in the “Single-Pole Interrupting Capability” section on pages 29 to 34.
©2008 Cooper Bussmann
Interrupting Rating
P r ote ctive De v ic
In the first three instances above, the circuit current condition is within the safe operating capabilities of the overcurrent protective device. However, the fourth case involves a misapplication of the overcurrent device. A short circuit on the load side of the device has resulted in a fault current of 50,000A flowing through the overcurrent device. Because the fault current is well above the interrupting rating of the device, a violent rupture of the protective device and resulting damage to equipment or injury to personnel is possible. The use of high interrupting rated fuses (typically rated at 200,000 or 300,000A) would prevent this potentially dangerous situation.
The first paragraph of NEC®110.9 requires that the overcurrent protective device be capable of interrupting the available fault current at its line terminals.
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Selective Coordination — Prevention of Blackouts
The coordination of protective devices prevents system power outages or blackouts caused by overcurrent conditions. When only the protective device nearest a faulted circuit opens and larger upstream fuses remain closed, the protective devices are “selectively” coordinated (they discriminate). The word “selective” is used to denote total coordination…isolation of a faulted circuit by the opening of only the localized protective device.
Current-Limitation — Component Protection
©2008 Cooper Bussmann
Selective Coordination & Current Limitation
Fuse opens and clears short-circuit in less than 1/ 2 cycle Initiation of short-circuit current Normal load current
Areas within waveform loops represent destructive energy impressed upon circuit components
Circuit breaker trips and opens short-circuit in about 1 cycle 2:1 (or more) LPS-RK 600SP LPS-RK 200SP KRP-C 1200SP 2:1 (or more)
This diagram shows the minimum ratios of amp ratings of Low-Peak fuses that are required to provide “selective coordination” (discrimination) of upstream and down-stream fuses.
Unlike electro-mechanical inertial devices (circuit breakers), it is a simple matter to selectively coordinate fuses of modern design. By maintaining a minimum ratio of fuse-amp ratings between an upstream and downstream fuse, selective coordination is achieved. Minimum selectivity ratios for Cooper Bussmann fuses are presented in the Selectivity Ratio Guide in “Fuse Selective Coordination” section. Adherence to the tabulated selectivity ratios normally proves adequate.
This book has an indepth discussion on coordination. See sections “Fuse Selective Coordination” and “Circuit Breaker Coordination.”
This burnt-out switchboard represents the staggering monetary losses in equipment and facility downtime that can result from inadequate or deteriorated protective devices. It emphasizes the need for reliable protective devices that properly function without progressive deterioration over time.
A non-current-limiting protective device, by permitting a short-circuit current to build up to its full value, can let an immense amount of destructive short circuit heat energy through before opening the circuit.
In its current-limiting range, a current-limiting fuse has such a high speed of response that it cuts off a short circuit long before it can build up to its full peak value.
If a protective device cuts off a short-circuit current in less than one-half cycle, before it reaches its total available (and highly destructive) value, the device limits the current. Many modern fuses are current-limiting. They restrict fault currents to such low values that a high degree of protection is given to circuit components against even very high short-circuit currents. They permit breakers with lower interrupting ratings to be used. They can reduce bracing of bus structures. They minimize the need of other components to have high short-circuit current “withstand” ratings. If not limited, short-circuit currents can reach levels of 30,000 or 40,000A or higher (even above 200,000A) in the first half cycle (0.008 seconds, 60Hz) after the start of a short circuit. The heat that can be produced in circuit components by the immense energy of short-circuit currents can cause severe insulation damage or even explosion. At the same time, huge magnetic forces developed between conductors can crack insula-tors and distort and destroy bracing structures. Thus, it is important that a protective device limit fault currents before they reach their full potential level. See Current-Limitation section and Fuse Let-Through Charts Analysis section for in-depth discussion. See Fuse Current-Limiting Let-Through Charts section for Cooper Bussmann fuse data.
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The principles of operation of the modern, current-limiting Cooper Bussmann fuses are covered in the following paragraphs.
Non-Time-Delay Fuses
The basic component of a fuse is the link. Depending upon the amp rating of the fuse, the single-element fuse may have one or more links. They are electrically connected to the end blades (or ferrules) (see Figure 1) and enclosed in a tube or cartridge surrounded by an arc quenching filler material. Cooper Bussmann Limitron®and T-Tron®fuses are both single-element fuses. Under normal operation, when the fuse is operating at or near its amp rating, it simply functions as a conductor. However, as illustrated in Figure 2, if an overload current occurs and persists for more than a short interval of time, the temperature of the link eventually reaches a level that causes a restricted segment of the link to melt. As a result, a gap is formed and an electric arc established. However, as the arc causes the link metal to burn back, the gap becomes progressively larger. Electrical resistance of the arc eventually reaches such a high level that the arc cannot be sustained and is
extinguished. The fuse will have then completely cut off all current flow in the circuit. Suppression or quenching of the arc is accelerated by the filler material.
Single-element fuses of present day design have a very high speed of response to overcurrents. They provide excellent short circuit component protection. However, temporary, harmless overloads or surge currents may cause nuisance openings unless these fuses are oversized. They are best used, therefore, in circuits not subject to heavy transient surge currents and the temporary overload of circuits with inductive loads such as motors, transformers, solenoids, etc. Because single-element, fast-acting fuses such as Limitron and T-Tron fuses have a high speed of response to short-circuit currents, they are particularly suited for the protection of circuit breakers with low interrupting ratings.
Whereas an overload current normally falls between one and six times normal current, short-circuit currents are quite high. The fuse may be subjected to short-circuit currents of 30,000 or 40,000A or higher. Response of current-limiting fuses to such currents is extremely fast. The restricted sections of the fuse link will simultaneously melt (within a matter of two or three-thousandths of a second in the event of a high-level fault current).
The high total resistance of the multiple arcs, together with the quenching effects of the filler particles, results in rapid arc suppression and clearing of the circuit. (Refer to Figures 4 & 5) Short-circuit current is cut off in less than a half-cycle, long before the short-circuit current can reach its full value (fuse operating in its current-limiting range).
©2008 Cooper Bussmann
Non Time-Delay Fuse Operation
With continued growth in electrical power generation, the higher levels of short-circuit currents made available at points of consumption by electrical utilities have greatly increased the need for protective devices with high short-circuit interrupting ratings. The trend is lower impedance transformers due to better efficiencies, lower costs, and utility deregulation. Utilities routinely replace transformers serving customers. These transformers can have larger kVA ratings and/or lower impedance, which results in higher available short-circuit currents. Devices that can interrupt only moderate levels of short-circuit currents are being replaced by modern fuses having the ability to cut-off short-circuit currents at levels up to 300,000 amps.
Figure 1. Cutaway view of typical single-element fuse.
Figure 2. Under sustained overload, a section of the link melts and an arc is established.
Figure 3. The “open” single-element fuse after opening a circuit overload.
Figure 4. When subjected to a short-circuit current, several sections of the fuse link melt almost instantly.
Figure 5. The “open” single-element fuse after opening a shorted circuit.
13 There are many advantages to using these fuses. Unlike single-element fuses,
the Cooper Bussmann dual-element, time-delay fuses can be sized closer to provide both high performance short circuit protection and reliable overload protection in circuits subject to temporary overloads and surge currents. For AC motor loads, a single-element fuse may need to be sized at 300% of an AC motor current in order to hold the starting current. However, dual-element, time-delay fuses can be sized much closer to motor loads. For instance, it is generally possible to size Fusetron dual-element fuses, FRS-R and FRN-R and Low-Peak dual-element fuses, LPS-RK_SP and LPN-RK_SP, at 125% and 130% of motor full load current, respectively. Generally, the Low-Peak dual-element fuses, LPJ_SP, and CUBEFuse™, TCF, can be sized at 150% of motor full load amps. This closer fuse sizing may provide many advantages such as: (1) smaller fuse and block, holder or disconnect amp rating and physical size, (2) lower cost due to lower amp rated devices and possibly smaller required panel space, (3) better short circuit protection – less short-circuit current let-through energy, and (4) potential reduction in the arc-flash hazard.
When the short-circuit current is in the current-limiting range of a fuse, it is not possible for the full available short-circuit current to flow through the fuse – it’s a matter of physics. The small restricted portions of the short circuit element quickly vaporize and the filler material assists in forcing the current to zero. The fuse is able to “limit” the short-circuit current.
Overcurrent protection must be reliable and sure. Whether it is the first day of the electrical system or thirty, or more, years later, it is important that overcurrent protective devices perform under overload or short circuit conditions as intended. Modern current-limiting fuses operate by very simple, reliable principles.
©2008 Cooper Bussmann
Dual-Element, Time-Delay Fuse Operation
Short circuit element
Overload element
Spring
Filler quenches the arcs
Small volume of metal to vaporize Filler material
Insulated end-caps to help prevent accidental contact with live parts.
Before
After
Figure 6. This is the LPS-RK100SP, a 100A, 600V Low-Peak, Class RK1,
dual-element fuse that has excellent time-delay, excellent current-limitation and a 300,000A interrupting rating. Artistic liberty is taken to illustrate the internal portion of this fuse. The real fuse has a non-transparent tube and special small granular, arc-quenching material completely filling the internal space.
Figure 7. The true dual-element fuse has distinct and separate overload element and
short circuit element.
Figure 8. Overload operation: Under sustained
overload conditions, the trigger spring fractures the calibrated fusing alloy and releases the “connector.” The insets represent a model of the overload element before and after. The calibrated fusing alloy connecting the short circuit element to the overload element fractures at a specific temperature due to a persistent overload current. The coiled spring pushes the connector from the short circuit element and the circuit is interrupted.
Figure 9. Short circuit operation: Modern fuses are designed with minimum metal in
the restricted portions which greatly enhance their ability to have excellent current-limiting characteristics – minimizing the short circuit let-through current. A short-circuit current causes the restricted portions of the short circuit element to vaporize and arcing commences. The arcs burn back the element at the points of the arcing. Longer arcs result, which assist in reducing the current. Also, the special arc quenching filler material contributes to extinguishing the arcing current. Modern fuses have many restricted portions, which results in many small arclets – all working together to force the current to zero.
Figure 10. Short circuit operation: The special small granular, arc-quenching material
plays an important part in the interruption process. The filler assists in quenching the arcs; the filler material absorbs the thermal energy of the arcs, fuses together and creates an insulating barrier. This process helps in forcing the current to zero. Modern current-limiting fuses, under short circuit conditions, can force the current to zero and complete the interruption within a few thousandths of a second.
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Advantages of Cooper Bussmann
Dual-Element, Time-Delay Fuses
Cooper Bussmann dual-element, time-delay fuses have four distinct advantages over single-element, non-time-delay fuses:
1. Provide motor overload, ground fault and short circuit protection. 2. Permit the use of smaller and less costly switches.
3. Give a higher degree of short circuit protection (greater current limitation) in circuits in which surge currents or temporary overloads occur.
4. Simplify and improve blackout prevention (selective coordination).
Motor Overload and Short Circuit Protection
provides ground fault and short-circuit protection, requiring separate overload protection per the NEC®. In contrast, the 40A dual-element fuse provides ground fault, short circuit and overload protection. The motor would be protected against overloads due to stalling, overloading, worn bearings, improper voltage, single-phasing, etc.
In normal installations, Cooper Bussmann dual-element fuses of motor-running, overload protection size, provide better short circuit protection plus a high degree of back up protection against motor burnout from overload or single-phasing should other overload protective devices fail. If thermal overloads, relays, or contacts should fail to operate, the dual-element fuses will act independently and thus provide “back-up” protection for the motor. When secondary single-phasing occurs, the current in the remaining phases increases to a value of 173% to 200% of rated full-load current. When primary single-phasing occurs, unbalanced voltages that occur in the motor circuit also cause excessive current. Dual-element fuses sized for motor overload protection can help protect motors against the overload damage caused by single-phasing. See the section “Motor Protection–Voltage Unbalance/Single-Phasing” for discussion of motor operation during single-phasing.
©2008 Cooper Bussmann
Dual-Element Fuse Benefits
When used in circuits with surge currents such as those caused by motors, transformers, and other inductive components, the Cooper Bussmann Low-Peak and Fusetron dual-element, time-delay fuses can be sized close to full-load amps to give maximum overcurrent protection. Sized properly, they will hold until surges and normal, temporary overloads subside. Take, for example, a 10 HP, 200 volt, three-phase motor with a full-load current rating of 32.2A.
The preceding table shows that a 40A, dual-element fuse will protect the 32.2A motor, compared to the much larger, 100A, single-element fuse that would be necessary. It is apparent that if a sustained, harmful overload of 200% occurred in the motor circuit, the 100A, single-element fuse would never open and the motor could be damaged. The non-time-delay fuse, thus, only
Permit the Use of Smaller and Less Costly Switches
Aside from only providing short-circuit protection, the single-element fuse also makes it necessary to use larger size switches since a switch rating must be equal to or larger than the amp rating of the fuse. As a result, the larger switch may cost two or three times more than would be necessary were a dual-element Low-Peak or Fusetron fuse used. The larger, single-dual-element fuse itself could generate an additional cost. Again, the smaller size switch that can be used with a dual-element fuse saves space and money. (Note: where larger switches already are installed, fuse reducers can be used so that fuses can be sized for motor overload or back-up protection.)
Better Short Circuit Component Protection
(Current-Limitation)
The non-time-delay, fast-acting fuse must be oversized in circuits in which surge or temporary overload currents occur. Response of the oversized fuse to short-circuit currents is slower. Current builds up to a higher level before the fuse opens…the current-limiting action of the oversized fuse is thus less than a fuse whose amp rating is closer to the normal full-load current of the circuit. Therefore, oversizing sacrifices some component protection.
15 In the table above, it can be seen that the 40A Low-Peak dual-element fuse
used to protect a 10Hp (32.2 FLA) motor keeps short-circuit currents to approximately half the value of the non-time-delay fuse.
Better Selective Coordination (Blackout Prevention)
The larger an upstream fuse is relative to a downstream fuse (for example, feeder to branch), the less possibility there is of an overcurrent in the downstream circuit causing both fuses to open (lack of selective coordination). Fast-acting, non-time-delay fuses require at least a 3:1 ratio between the amp rating of a large upstream, line-side Low-Peak time-delay fuse and that of the downstream, loadside Limitron fuse in order to be selectively coordinated. In contrast, the minimum selective coordination ratio necessary for Low-Peak dual-element fuses is only 2:1 when used with Low-Peak loadside fuses.
Better Motor Protection in Elevated Ambients
The derating of dual-element fuses based on increased ambient temperatures closely parallels the derating curve of motors in an elevated ambient. This unique feature allows for optimum protection of motors, even in high temperatures.
©2008 Cooper Bussmann
Dual-Element Fuse Benefits
The use of time-delay, dual-element fuses affords easy selective
coordination–coordination hardly requires anything more than a routine check of a tabulation of required selectivity ratios. As shown in the preceding illustration, close sizing of Cooper Bussmann dual-element fuses in the branch circuit for motor overload protection provides a large difference (ratio) in the amp ratings between the feeder fuse and the branch fuse, compared to the single-element, non-time-delay Limitron fuse.
Affect of ambient temperature on operating characteristics of Fusetron®and
Low-Peak dual-element fuses.
Below is a rerating chart for single element fuses or non dual element fuses.
16 ©2008 Cooper Bussmann
Branch-Circuit & Application Limited OCPDs
Branch-Circuit OCPDs & Application Limited OCPDs
In most cases, branch circuit overcurrent protective devices (OCPD) are the only type of overcurrent protective devices permitted to be used to protect electrical building system mains, feeders and branch circuits, and in utilization equipment mains, feeders and branch circuits. Yet, too often OCPDs which are not branch circuit rated are misapplied where a branch circuit rated OCPD is required. However, the “branch circuit overcurrent protective device” term can be difficult to grasp due to the multiple ways the electrical industry uses the phrase “branch circuit”, and since most manufacturers do not identify their overcurrent protective devices with the specific wording “branch circuit overcurrent protective device.”
Not using a branch circuit OCPD where required could result in potentially serious electrical safety hazards to people or damage to property. In addition National Electrical Code violations could be tagged by the authority having jurisdiction (AHJ), resulting in project delays and unplanned costs. There are three types of overcurrent protective devices discussed in this section:
1. Branch circuit overcurrent protective devices: can be used for protection of the entire circuit on a main, feeder or branch of an electrical system
2. Application limited: the device is suitable for specific branch circuit applications under limited conditions per the NEC®
(often listed or recognized for the specific use)
3. Application limited: supplementary protective device (cannot be used for branch circuit applications under any circumstances)
NEC®Article 100 offers the following definition for a branch circuit overcurrent device:
With the definition, it becomes clear that a branch circuit overcurrent protective device is suitable for use at any point in the electrical system to protect branch circuits, as well as feeder circuits and mains. The definition also illustrates that a branch circuit overcurrent device must be capable of protecting against the full range of overcurrents which includes overloads and short-circuits as well as have an interrupting rating sufficient for the application (this reflects the interrupting rating requirements of 110.9). In addition to the traits described in the definition, branch circuit overcurrent devices meet minimum common standardized requirements for spacings and operating time-current characteristics.
Branch-Circuit Overcurrent Device. A device capable of providing
protection for service, feeder, and branch-circuits and equipment over the full-range of overcurrents between its rated current and its interrupting rating. Branch-circuit overcurrent protective devices are provided with interrupting ratings appropriate for the intended use but no less than 5,000 amperes.
Table 1 lists acceptable branch circuit overcurrent device types along with Cooper Bussmann®fuse part numbers. Branch circuit OCPDs can be used in
any circuit, unlike the application limited OCPDs.
Listed Branch Circuit OCPDs
Product standards establish the minimum level of safety for a given product type by having certain minimum product performance criteria and physical specifications. The commercial products listed to a product standard must meet the minimum performance criteria of that specific product standard. However, in addition, commercial products may have performance better than the minimum of the standard or other performance criteria not addressed in the product standard.
Fuses and circuit breakers each have their own separate product standards. There are significant differences in the minimum level of safety performance incorporated in the product standards for current-limiting fuses versus product standards for circuit breakers. Specifically, the discussion will focus on current-limiting fuses and molded case circuit breakers. The safety system of each of these product technologies differs considerably. Table 2 identifies several key characteristics of the electrical safety system offered by fuses and molded case circuit breakers.
Device Type Acceptable Devices Cooper Bussmann Branch Circuit Fuses
Class J Fuse LPJ_SP, JKS, DFJ, TCF*
Class RK1 Fuse LPN-RK_SP, LPS-RK_SP
KTN-R, KTS-R
Class RK5 Fuse FRN-R, FRS-R
UL 248 Fuses Class T Fuse JJN, JJS
Class CC Fuse LP-CC, KTK-R, FNQ-R
Class L Fuse KRP-C_SP, KLU, KTU
Class G Fuse SC
Class K5 Fuse NON, NOS (0-60A)
Class H Fuse NON, NOS (61-600A)
UL 489 Molded Case CBs
Circuit Breakers Insulated Case CBs
UL 1066 Low Voltage
Circuit Breakers Power CBs
*TCF fuse have Class J performance and special finger-safe dimensions
Table 1
Acceptable Branch Circuit Overcurrent
Protective Device Types
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©2008 Cooper Bussmann
Branch-Circuit & Application Limited OCPDs
Equipment Has Rejects Replacement Rejects Replacement Rejects Replacement Rejects Replacement Fuse Mounting of Other UL Fuse of Lower Voltage of Fuses with IR Less of Fuses Classes with
for UL Fuse Fuse Classes Rated Fuses Than 200kA Greater Short-Circuit
Class Below Energy Let Through
L, J, CC, T, G Yes Yes Yes Yes
Class R
Yes Yes Yes Yes
(RK1 and RK5)
Equipment Has Rejects Replacement Rejects Replacement Rejects Replacement Rejects Replacement CB Mounting with Other CB Type with Lower Voltage with Lower Interrupting of CB Having Higher
for Type Below Rated CB Rated CB Short-Circuit
Energy Let Through Molded Case No No No No Circuit Breaker Molded Case Circuit Breaker No No No No Marked Current-limiting
Fuse Safety System per
Product Standards
Molded-Case Circuit Breaker Safety System per Product
Standards
Molded-Case Circuit Breakers (MCCB) & Fuse Safety Systems
Table 2
18 ©2008 Cooper Bussmann
Listed Branch Circuit Fuses: Current-Limiting
UL248 Standards cover distinct classes of low-voltage (600 volts or less) fuses. Of these, modern current-limiting fuse Classes J, CC, L, R, T and G are the most important. The rejection feature of current-limiting fuses ensures a safety system for the life of the electrical system. Listed current-limiting fuses have physical size rejection features that help prevent installation of fuses that cannot provide a comparable minimum level of protection for critical ratings and performance. This is inherent in all current-limiting fuse classes. Each fuse class found in Table 1 on page 14 has certain criteria that must be met. These include
1. Maximum let-through limits (Ip and I2t ) during fault conditions
2. Minimum voltage ratings
3. Minimum interrupting ratings (200kA for Class J, T, R, CC and L) 4. Physical rejection of larger fuse amperages*
5. Physical rejection of non-current limiting fuses
*Amperages greater than fuse holder rating (i.e. 30A fuse holder will not accept 35A fuse)
By meeting these product standard requirements, the fuse industry provides branch circuit fuses that ensure a minimum specific level of circuit protection, when current-limiting fuses and equipment are used. Using a given fuse class will secure the voltage rating, interrupting rating and degree of current limitation for the life of the electrical system. This can be thought of as a “safety system” since the physical mounting configuration only permits the same specific fuse class to be installed. Each class of current-limiting fuses has its own unique physical dimensions so that fuses of a different class are not interchangeable. For instance, Class R fuses cannot be installed in Class J fuse equipment. Modern Class J, CC, L, R, T, and G fuse equipment rejects the installation of any other fuse class. The fuse class dimensions on page 21 exhibit the dimensional rejection characteristic of modern current-limiting fuses. Class R has two categories: Class RK5 and RK1 which are interchangeable, but no other fuse class can be installed. Class H, an older style fuse class, is not considered current-limiting and is not recommended for new installations. Class R fuses can be installed in Class H fuse equipment as an upgrade. However, Class H fuses cannot be installed in Class R fuse equipment. Class R equipment physically rejects the installation of Class H fuses.
Listed Branch Circuit Molded-Case Circuit Breakers
The safety system for circuit breakers is not near as stringent. The initial thought of many people is that this is acceptable, since circuit breakers are resettable. However, that is faulty thinking. Circuit breakers frequently need to be replaced and in addition, circuit breakers are often added to existing equipment in spare spaces for new circuits. Or in the industrial control panel market, during the procurement process, a lower cost circuit breaker is mistakenly thought to be equivalent to an existing specified circuit breaker. It is easy to mistakenly substitute a lesser rated molded case circuit breaker for a higher rated molded case circuit breaker since there is not a physical rejection protocol in the product standard to prevent the installation of the lesser rated device. This can create a serious safety hazard. It can negate the voltage rating protection, the interrupting rating protection and the short-circuit protection of equipment. In existing installations this may not be realized until the protective device fails to operate properly. For a given frame size circuit breaker, there are various part numbers for different interrupting ratings which are physically interchangeable. So it’s possible to install a 10kA interrupting rated molded case circuit breaker in a panel that is listed for 65kA and which requires only circuit breakers with a 65kA interrupting rating be installed. Similarly it may be possible to replace a higher voltage rating circuit breaker with a circuit breaker of the same physical dimensions with a lower voltage rating. A listed current-limiting molded case circuit breaker has to be tested and marked as current-limiting. Most molded case circuit breakers are not current-limiting. Yet a current-limiting circuit breaker and a standard (non-current-limiting) circuit breaker can be physically interchangeable.
Here is an example of how simple it is: use Class J fuses and equipment, and only Class J fuses can be installed. This ensures the voltage rating is 600V (whether the system is 120, 208, 480, or 575V), the interrupting rating is at least 200kA, and the short-circuit protection is provided by the current-limiting let-through characteristics of the Class J. If the fuse has to be replaced, only a Class J fuse physically fits into the equipment.
The illustration above shows Class R type fuse rejection clips, which accept only the Class R rejection type fuses.
Three circuit breakers of the same frame size which are physically interchangeable. The 240V CB could be substituted for the 600V CB, the 100A CB could be substituted for the 20A CB, and the 10kA CB could be substituted for the 65kA CB.
Branch-Circuit & Application Limited OCPDs
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©2008 Cooper Bussmann
Branch circuit overcurrent protective devices can also be used to provide the additional protection that a supplementary overcurrent protective device provides: see Figure 2. Rather than using a supplementary overcurrent protective device for supplementary protection of the luminaire, a branch-circuit overcurrent protective device is used. The fact that a branch-branch-circuit overcurrent device (KTK-R-3) is used where a supplementary device is permitted does not turn the circuit between the lighting panel and the fixture from a branch-circuit to a feeder. In the case of Figure 2, the branch circuit starts on the loadside of the 20A fuse in the lighting panel.
Application Limited OCPDs
The preceding paragraphs covered branch circuit overcurrent protective devices. There are two other categories to be considered:
(1) Permitted for specific branch circuit applications under limited conditions per the specific reference in the NEC®: These OCPDs have
some limitation(s) and are not true branch circuit devices, but may be permitted, if qualified for the use in question. For example, most high speed fuses are not branch circuit OCPDs, however high speed fuses are allowed to be used for short circuit protection on motor circuits utilizing power electronic devices by 430.52(C)(5). Motor Circuit Protectors (MCPs) are recognized devices (not listed) and can be used to provide short-circuit protection for motor branch circuits, if used in combination with a listed combination starter with which the MCP has been tested and found acceptable [per 430.52(C)(3)]. Self protected starters are another application limited OCPD; they are listed only for use as protection of motor branch circuits. These examples are only suitable for use on motor branch circuits; they cannot be used on other branch circuit types or for main or feeder protection. When considering the use of application specific devices, special attention must be paid to the circuit type/application, NEC®requirements, and the device’s product listing or recognition. In other words, these types of overcurrent devices are only acceptable for use under special conditions.
(2)Supplementary overcurrent protective devices: These devices have limited applications and must always be in compliance with 240.10
The NEC®definition for a supplementary overcurrent protective device is shown below. Supplementary protective devices can only be used as additional protection when installed on the load side of a branch circuit overcurrent device. Supplementary devices must not be applied where branch circuit overcurrent protective devices are required; unfortunately this unsafe misapplication is prevalent in the industry. Supplementary devices are properly used in appliance applications and for additional, or supplementary protection where branch circuit overcurrent protection is already provided. In appliance applications, the supplementary devices inside the appliance provide protection for internal circuits and supplement the protection provided by the branch circuit protective devices.
The use of supplementary overcurrent protective devices allowed by 240.10 is for applications such as lighting and appliances shown in Figure 1. The supplementary protection is in addition to the branch circuit overcurrent protection provided by the device protecting the branch circuit (located in the lighting panel in Figure 1).
240.10 Supplementary Overcurrent Protection. Where supplementary
overcurrent protection is used for luminaires, appliances, and other equipment...it shall not be used as a substitute for required branch-circuit overcurrent devices or in place of the required branch-circuit protection…
NEC®Article 100
Supplementary Overcurrent Protective Device.
A device intended to provide limited overcurrent protection for specific applications and utilization equipment such as luminaires (lighting fixtures) and appliances. This limited protection is in addition to the protection provided in the required branch circuit by the branch-circuit overcurrent protective device.