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Overcurrent Protection:

Fuses & Circuit Breakers

ELECTRICAL SAFETY

Safe Design & Const.Safe Design & Const.Safe Design & Const.Safe Design & Const.

2Safe Work PracticeSafe Maintenance

Safe Equipment

OVERCURRENT PROTECTION

� NEC240 states the requirements for OCPD

� NEC® 240.1 (FPN): use if the current reaches a

value that will cause excessive temperature in

conductors or conductor insulation

OVERCURRENT PROTECTION

� UL and NEMA establish standards for ratings,

types, classifications, and testing procedures

for fuses and circuit breakers

DISCONNECT SWITCHES

� Fused switches are available in ratings of 30-6000A 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 (NEC110.14):

� 140°F (60°C)------ wire (14 -1 AWG)

� 167°F (75°C)------wires 1/0 AWG & larger

� May be equipped with ground-fault sensing and labels that indicate their intended use

DISCONNECT SWITCHES

� Occupants must have access to OCPD for their

circuits, NEC240.24(B)

� OCPD must be readily accessible NEC

240.24(A)

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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

FUSES AND CIRCUIT BREAKERS

� Protection of conductors

�OCPD is rated at 800 A or less: next higher

standard

�OCPD is rated above 800-ampere: must be equal

to or greater than the rating of the fuse or circuit

breaker

FUSES AND CIRCUIT BREAKERS (CONT'D.)

� Interrupting rating: Highest current where a

device is intended to interrupt under standard

test conditions

� 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

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

TYPES OF FUSES

� 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

� Using fuses for motor branch-circuit, short-circuit, 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

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TYPES OF FUSES

� Fast-acting, current-limiting fuses (nontime-

delay)

� 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

� 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

CABLE LIMITERS

�Used with parallel cables for service entrances

and feeders

� Isolate a faulted cable

� Selected on the basis of conductor size

� Available for cable-to-cable or cable-to-bus

installation

Three-phase, 3-wire delta system with

grounded “B” phase

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

CIRCUIT BREAKERS

� 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 end-

use equipment having lower interrupting rating

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SERIES-RATED APPLICATIONS

� 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

SERIES-RATED APPLICATIONS

SERIES-RATED APPLICATIONS

�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

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

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HEATING, AIR-CONDITIONING, AND

REFRIGERATION OVERCURRENT PROTECTION

� Nameplate on HVAC equipment might

indicate “maximum size fuse,” “maximum

size fuse or circuit breaker,” or “maximum

size fuse or HACR circuit breaker”

SHORT-CIRCUIT CALCULATIONS

3-Phase Fault

Current

Calculation

SHORT-CIRCUIT CALCULATIONS SHORT-CIRCUIT CALCULATIONS

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Electrical system for a Commercial Building

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SHORT-CIRCUIT CURRENT

� Arcing fault multipliers (approximate)

Hazard Risk

Category 0

Hazard Risk

Category 1

Hazard Risk

Category 2

Hazard Risk

Category 3

Hazard Risk

Category 4

PPE:

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Hazard Risk

Category 0

Hazard Risk

Category 1

Hazard Risk

Category 2

Hazard Risk

Category 3

Hazard Risk

Category 4

COORDINATION OF OCPD

� Selective coordination: localization of

overcurrent condition to restrict outages to

the circuit or equipment affected,

accomplished by choice of overcurrent

protective devices and their ratings or

settings

Nonselective system verification

Selective system verification

Nonselective system verification

CURRENT-LIMITING FUSES

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CURRENT-LIMITING FUSES CURRENT-LIMITING FUSES

CURRENT-LIMITING FUSES