Transcript Overcurrent Protection: Fuses and Circuit Breakers

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