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Chapter 17 Overcurrent Protection: Fuses and Circuit Breakers Objectives • List and identify the types, classes, and ratings of fuses and circuit breakers • Describe the operation of fuses and circuit breakers • Develop an understanding of switch sizes, ratings, and requirements Objectives (cont'd.) • Define interrupting rating, short-circuit currents, I2t, Ip, RMS, and current limitation • Apply the National Electrical Code to the selection and installation of overcurrent protective devices • Use the time-current characteristics curves and peak let-through charts Overcurrent Protection • NEC® Article 240 states the requirements for overcurrent protection – NEC® 240.1 (FPN): use if the current reaches a value that will cause excessive temperature in conductors or conductor insulation • Two types of overcurrent protective devices are commonly used: – Fuses and circuit breakers Overcurrent Protection (cont'd.) • Underwriters Laboratories, Inc. (UL) and National Electrical Manufacturers Association (NEMA) establish standards for ratings, types, classifications, and testing procedures for fuses and circuit breakers Disconnect Switches • Fused switches are available in ratings of several amperes in both 250 and 600 volts – Used with copper conductors unless marked to indicate suitability for use with aluminum – Rating, unless otherwise marked, is based on: • 140°F (60°C) wire (14 AWG through 1 AWG) and 167°F (75°C) for wires 1/0 AWG and larger – May be equipped with ground-fault sensing and labels that indicate their intended use Disconnect Switches (cont'd.) • Accessibility of overcurrent devices – Occupants of must have access to overcurrent devices for their circuits, NEC® 240.24(B) • How high should disconnect switches be mounted? – NEC® 240.24(A) requires that overcurrent devices be readily accessible – Same rule applies to a circuit breaker Fuses and Circuit Breakers • Voltage rating – Equal to or greater than the voltage of the circuit in which they are to be used • Continuous current rating – Amperes that the device can continuously carry – Rating is usually based on the ampacity of circuit conductors Fuses and Circuit Breakers (cont'd.) • Protection of conductors – When overcurrent device is rated at 800 amperes or less • The Code permits the use of next higher standard ampere-rated fuse or circuit breaker Fuses and Circuit Breakers (cont'd.) – When overcurrent device is rated above 800ampere • Conductor ampacity must be equal to or greater than the rating of the fuse or circuit breaker • Interrupting rating – Highest current where a device is intended to interrupt under standard test conditions Fuses and Circuit Breakers (cont'd.) • Short-circuit current rating – Ability to withstand fault current equal to or less than the short-circuit rating for the length of time it takes the overcurrent device to react • Speed of response – Time required for a fuse to open varies inversely with current that flows through fuse – Circuit breaker also has this characteristic Types of Fuses • Dual-element, time-delay fuse – Provides a time delay in low-overload range to eliminate unnecessary opening of the circuit because of harmless overloads • Using fuses for motor overload protection – Sizing dual-element fuses slightly larger than the overload relay provides backup protection – If overload relays fail to operate, dual-element fuses will provide backup overload protection Types of Fuses (cont'd.) • Applying fuses and breakers on motor circuits – High starting currents of motors can cause nuisance opening of fuses and nuisance tripping of circuit breakers – Check time-current curves of fuses and breakers to make sure that they will handle the momentary motor starting inrush currents without nuisance opening or tripping Types of Fuses (cont'd.) • Using fuses for motor branch-circuit, shortcircuit, and ground-fault protection – NEC® Table 430.52: maximum size permitted for dual-element fuses is based on a maximum of 175 percent of full-load current of the motor • Dual-element, time-delay, current-limiting fuses – Can handle currents five times their ampere rating for at least 10 seconds Types of Fuses (cont'd.) • Fast-acting, current-limiting fuses (nontimedelay) – Extremely fast response in both low-overload and short-circuit ranges – Has the lowest energy let-through values – Provides better protection to mains, feeders and subfeeders, circuit breakers, bus duct, switchboards, and other circuit components Types of Fuses (cont'd.) • Types of cartridge fuses – According to the Code, all cartridge fuses must be marked to show: • Ampere rating • Voltage rating • Interrupting rating when greater than 10,000 amperes • Current-limiting type, if applicable • Trade name or name of manufacturer Types of Fuses (cont'd.) • Plug fuses – Requirements in NEC® Article 240, Part V – Opening characteristics available in three types: • Standard link type does not have much time delay • Loaded link type has a metal bead element that gives it time delay to hold motor inrush starting currents • Dual-element, time-delay has a spring-loaded shortcircuit element plus an overload element connected in series with short-circuit element Testing Fuses • OSHA: electrical equipment must not be worked on when it is energized • When power is turned on: – Exercise extreme caution when checking fuses – Using a voltmeter, first step is to set the scale to its highest voltage setting and then change to a lower scale after you are within the range of the voltmeter Testing Fuses (cont'd.) – Reading from line-to-load side of a good fuse should show zero to small voltage • When power is turned off: – Remove fuse from switch, then use an ohmmeter to take a resistance reading – Good fuse: zero or a very minimal resistance; open (blown) fuse will show a high reading Testing Fuses (cont'd.) • Cable limiters – Used where parallel cables are used on service entrances and feeders – Devices that can isolate a faulted cable rather than having the fault open the entire phase – Selected on the basis of conductor size – Are available for cable-to-cable or cable-to-bus installation for either aluminum or copper hot phase conductors Figure 17-18 Use of cable limiters in a service entrance Delta, 3-Phase, CornerGrounded “B” Phase System • Fuses shall be installed in series with ungrounded conductors for overcurrent protection, NEC® 240.15(A) – Sometimes called a corner ground • There are certain instances where a 3-pole, 3-phase switch may be installed, where it is permitted to install fuses in two poles only Figure 17-20 Threephase, 3-wire delta system with grounded “B” phase Delta, 3-Phase, Corner-Grounded “B” Phase System (cont'd.) • Solid neutrals – Made of copper bar that has exactly the same dimensions as a fuse for a given ampere rating and voltage rating – Generally used in retrofit situations Time-Current Characteristic Curves and Peak Let-Through Charts • Fuse manufacturers furnish information for: – Time-current characteristic curves, including total clearing and minimum melting curves – Peak let-through charts • Time-current characteristic curves can be used to answer questions about fuse capabilities Peak Let-Through Charts (cont'd.) • The use of peak let-through charts – Properly matches short-circuit current rating of electrical equipment with let-through current values of overcurrent protective devices • Peak let-through charts give a good indication of current-limiting effects of a current-limiting fuse or circuit breaker under “bolted fault” conditions Circuit Breakers • NEC® Article 100 definition – Device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent • Types of circuit breakers: – Molded-case circuit breakers – Power circuit breakers – Insulated-case circuit breakers Figure 17-28 Molded-case circuit breakers Circuit Breakers (cont'd.) • NEC® 240.80 through 240.86 state the basic requirements for circuit breakers • Thermal-magnetic circuit breakers – Contain a bimetallic element that, on continuous overload, moves until it unlatches the inner tripping mechanism of the breaker – Time required for the breaker to open the circuit depends upon the fault current and mechanical condition of circuit breaker Circuit Breakers (cont'd.) • Ambient-compensated circuit breakers – Some circuit breakers are ambient (surrounding temperature) compensated – If installing thermal circuit breakers in extremely hot or extremely cold temperatures, consult manufacturers’ literature – Ambient factors can affect proper operation of a circuit breaker, such as dust, fumes, etc. Circuit Breakers (cont'd.) • Common misapplication – Common violation of NEC® 110.9 and 110.10: • Installation of a main circuit breaker that has a high interrupting rating while making the assumption that branch-circuit breakers are protected adequately against short circuit – Standard molded case circuit breakers with high interrupting ratings cannot protect standard enduse equipment having lower interrupting rating Series-Rated Applications • Series-rated equipment – Main overcurrent device and branch-circuit overcurrent devices are connected in series • Series-rated systems – Less costly than fully-rated systems – Available fault current does not exceed interrupting rating of the line-side overcurrent device but does exceed interrupting rating of the load-side overcurrent device Figure 17-33 Series-rated circuit breakers. In this example, both the 20-ampere breaker and the 100-ampere main breaker trip off under high-level fault conditions Series-Rated Applications (cont'd.) • Where high available fault currents indicate the need for high interrupting breakers or fuses, fully rated system is generally used • Another less costly way to safely match main circuit breaker or main fuses ahead of branch-circuit breakers is to use listed series-rated equipment Series-Rated Systems Where Electric Motors are Connected • NEC® 240.86(C) sets forth two requirements – Do not connect electric motors between load side of higher rated overcurrent device and line side of lower rated overcurrent device – Sum of connected motor full-load currents shall not exceed one percent of the interrupting rating of lower rated circuit breaker Current-Limiting Breakers • Current-limiting circuit breaker limits the letthrough energy (I2t) to something less than the I2t of a one-half cycle symmetrical wave • When installing circuit breakers, it is important to ensure that: – Circuit breakers have the proper rating – All circuit components can withstand the letthrough current of the breaker Cost Considerations • There are a number of different types of circuit breakers to choose from • Selection of the type to use depends upon a number of factors, including interrupting rating, selectivity, space, and cost Motor Circuits • NEC® Table 430.52 – Shows that maximum setting of a conventional inverse-time circuit breaker must not exceed 250 percent of full-load current of the motor – For instantaneous-trip circuit breaker, maximum setting is 800 percent of motors’ full-load current • Design B motors, the maximum setting is 1100 percent Motor Circuits (cont'd.) • If an “engineering evaluation” can demonstrate the need to exceed percentages shown in NEC® Table 430.52, then: – 800 percent setting may be increased to a maximum of 1300 percent – 1100 percent setting may be increased to a maximum of 1700 percent Heating, Air-Conditioning, and Refrigeration Overcurrent Protection • Check the nameplate carefully—and do what it says – Nameplate on HVAC equipment might indicate “maximum size fuse,” “maximum size fuse or circuit breaker,” or “maximum size fuse or HACR circuit breaker” Summary • NEC® Article 240 sets the requirements for overcurrent protection • Two types of overcurrent protective devices commonly used: fuses and circuit breakers • Factors that must be considered when selecting proper fuses and circuit breakers: – Voltage rating, continuous current rating, interrupting rating, and speed of response