Chapter Twenty Eight Overcurrent Protection Fuses and Circuit

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

® NEC Key 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. ” 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 singlepole 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 k. VA – 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 groundfault 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