Overcurrent Protection— Fuses and Circuit Breakers Chapter Twenty Eight Overcurrent Protection— Fuses and Circuit Breakers

Objectives After studying this chapter, you should be able to: Understand the important NEC® requirements for fuses and circuit breakers Discuss the five possible circuit conditions Understand the various types and operation of fuses and circuit breakers

Objectives (cont’d.) Know when to use single-pole and 2-pole circuit breakers Understand the term interrupting rating for fuses and circuit breakers Calculate available short-circuit current using a simple formula Understand series-rated panelboards Understand the meaning of selective coordination and nonselective coordination

The Basics Overcurrent protection Covered in NEC® Article 240 For residential services, branch circuits, and feeders is provided by circuit breakers or fuses Fuses and circuit breakers are sized by matching ampere ratings to conductor ampacities and connected load currents

Key NEC® Requirements for Overcurrent Protection Some key NEC® references are: NEC® Table 210.24, NEC® 240.4, 240.4(B), 240.4(D) NEC® 240.20(A), NEC® 240.21, NEC® 240.22 NEC® 240.24(A), 240.24(D), 240.24(E), 240.24(F) NEC® 230.70(A)(2), 230.79(C), 230.90(A), 230.91

Five Circuit Conditions Normal: when the current flowing is within the capability of the circuit and/or the connected equipment Overload: a condition where the current flowing is more than the circuit and/or connected equipment is designed to safely carry

Five Circuit Conditions (cont’d.) Short circuit: a condition when two or more normally insulated circuit conductors come in contact with one another, resulting in a current flow that bypasses the connected load Ground fault: a condition when a “hot” or ungrounded conductor comes in contact with a grounded surface

Five Circuit Conditions (cont’d.) Open circuit: a condition where the circuit is not closed somewhere in the circuit

FIGURE 28-1 A normally loaded circuit. © Cengage Learning 2015

FIGURE 28-2 An overloaded circuit. © Cengage Learning 2015

FIGURE 28-3 Note that the connected load is short-circuited. © Cengage Learning 2015

FIGURE 28-4 The insulation on the “hot” conductor has come in contact with the metal conduit. This is termed a “ground fault”. © Cengage Learning 2015

FIGURE 28-5 The circuit is “open” where marked “X FIGURE 28-5 The circuit is “open” where marked “X.” No current flows in the circuit so no heat is created. © Cengage Learning 2015

Fuses Plug fuses are: Limited to circuits that do not exceed 125 volts Used in circuits having a grounded neutral where no conductor operates at over 150 volts to ground Rated between 0 and 30 amperes

Fuses (cont’d.) All new plug fuse installations are required to be Type S fuses, 240.52 Classified at 0 through 15 amperes, 16 through 20 amperes, and 21 through 30 amperes In the 0- to 15-ampere range, there are many ampere ratings to choose from Excellent for protecting motors

Fuses (cont’d.) Time-current: how long it takes a fuse or circuit breaker to open under different current values Three types of plug fuses: Non time-delay (W) Time-delay, loaded link (TL) time-delay, dual element (T)

Fuses (cont’d.) Cartridge fuses are available with the three basic types of time-current characteristics Most common is the dual-element cartridge fuse, available in 250 and 600 volt ratings with ampere ratings from 0 to 600 Class H and Class R types Other classes of fuses: CC, G, and T

Circuit Breakers Installations in dwellings normally use thermal-magnetic circuit breakers NEC® 240.80 through 240.86 give the requirements for circuit breakers Thermal-magnetic circuit breakers are temperature sensitive The NEC® permits 100 percent continuous loading on an overcurrent device only if it is listed for that

Circuit Breakers (cont’d.) Single-pole breakers are used on 120-volt circuits; they control one hot wire Two-pole breakers are used on 240-volt circuits; they control two hot wires on different phases A two-pole breaker may be used on a dedicated split-wired receptacle such as the disposer/dishwasher receptacles

Circuit Breakers (cont’d.) Handle ties connect two adjacent single-pole breakers together, thus permitting both breakers to be switched on or off simultaneously Handle ties must be tested and listed for use on a specific manufacturers circuit breaker

Interrupting Ratings for Fuses and Circuit Breakers This rating indicates the maximum current that a device can safely de-energize; NEC® 110.9 Panelboards are available in two types Fully rated Series rated Series-rated systems are less costly

FIGURE 28-11 A fully rated system using main fuses and branch-circuit fuses. The entire assembly (fuses and panelboard) is tested, listed, and marketed with a maximum short-circuit rating. Look for the marking on the panelboard. This type of combination can be found in commercial and industrial facilities. © Cengage Learning 2015

FIGURE 28-12 A series-rated system using main fuses and branch-circuit breakers. The entire assembly (main fuses, branch-circuit breakers, and panelboard) is tested, listed, and marked with a maximum short-circuit rating. Look for the marking on the panelboard. The lower interrupting rating of the branch-circuit breakers is acceptable because the combination has been tested and listed as a series-rated system. This type of combination can be found in light commercial installations such as multimetering service equipment. © Cengage Learning 2015

Short-Circuit Currents The ratings required to determine the maximum available short-circuit current delivered by a transformer are: The kVA Impedance values of the transformer Transformers used in modern electrical installations are efficient and have very low impedance values

How to Calculate Short-Circuit Current Short-circuit current is also referred to as fault current Please refer to the text for an example of fault-current calculations using a simple formula

Panelboards . . . What Are They? Panel, panelboard, and load center mean more or less the same thing Panelboard is used by NEC® and the UL Standards Load center Is not used by either NEC® or the UL Standards Is a panelboard with less gutter space as well as less depth and width

Panelboards . . . What Are They? (cont’d.) Under overload, short circuit, or ground-fault conditions Selective coordination: only the overcurrent device nearest the fault opens Nonselective coordination: the branch breaker or fuses might open, the main breaker or fuses might open, or both might open