Overcurrent Protection: Fuses and Circuit Breakers

1
Chapter 17

• Overcurrent ProtectionFuses and Circuit Breakers

2
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

3
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

4
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

5
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

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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
• 140F (60C) wire (14 AWG through 1 AWG) and
  167F (75C) for wires 1/0 AWG and larger
• May be equipped with ground-fault sensing and
  labels that indicate their intended use

7
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

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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
• Rating is usually based on the ampacity of
  circuit conductors

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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

10
Fuses and Circuit Breakers (cont'd.)

• When overcurrent device is rated above 800-ampere
• 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

11
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

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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

13
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

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Types of Fuses (cont'd.)

• 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 (cont'd.)

• 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

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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

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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
  short-circuit element plus an overload element
  connected in series with short-circuit element

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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

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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

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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

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Figure 17-18 Use of cable limiters in a service
entrance
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Delta, 3-Phase, Corner-Grounded B Phase System

• Fuses shall be installed in series with
  un-grounded 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

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Figure 17-20 Three-phase, 3-wire delta system
with grounded B phase
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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

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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

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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

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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

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Figure 17-28 Molded-case circuit breakers
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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

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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.

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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
  end-use equipment having lower interrupting
  rating

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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

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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
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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

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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

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Current-Limiting Breakers

• Current-limiting circuit breaker limits the
  let-through 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
  let-through current of the breaker

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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

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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

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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

• Check the nameplate carefullyand 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

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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