Questions and Answers
The two most commonly used overcurrent protection devices are ___
fuses and circuit breakers
___ and ____ establish standards for rating, types, classes, and testing procedures for fuses and circuit breakers.
UL, NEMA
Fused switches are available in ____ and ___.
250v, 600v
Fused switches are for use with ___ conductors unless marked to indicate that the terminals are suitable for use with ____ conductors.
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Fused switches are rated based on 60 degrees C _____, and 75 degrees C ____ unless marked otherwise.
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Article ___ covers accessibility for every occupant to the overcurrent devices protecting the conductors supplying the occupancy.
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Article ____ covers switches containing fuses and circuit breakers being readily accessible and installed so that the center of the grip of the operating handle is not more than 6'7" above the floor.
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____ is the voltage that a circuit can be operated without any damage. It shall be greater than the voltage of the circuit in which the fuse or CB is to be used.
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____ is the amount of current that the device can carry continuously without interrupting the circuit.
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___ is the highest current at rated voltage that a device is intended to interrupt under standard test conditions.
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___ is the ability to withstand fault current to or less than the short circuit rating for the length of time it takes the specified overcurrent device to open the circuit.
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___ is the time required by the fusible element of a fuse to open which varies inversely with a magnitude of the current that flows through the fuse.
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___ is the process of imposing the maximum current limit to the electrical equipment for the purpose of protecting the circuit.
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___ is the maximum current that flows in protective devices during an overcurrent condition.
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What does I(2)t represent?
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___ provides a time delay in low-overload range to eliminate unnecessary opening of the circuit because of harmless overloads.
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____ motors have higher starting current characteristics than older style motors (Design B motors).
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Make sure to check ___ curves of fuses and circuit breakers to ensure that they will handle the momentary motor starting inrush currents.
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Article ___ covers maximum size permitted for duel element fuses is based on a maximum of 175% of FLC of the motor.
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Dual-element time delay fuses, current limiting fuses can handle current ___ times their ampere rating for at least ___ seconds.
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___ have extremely fast response in low overload and short circuit ranges.
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Article 240, part VI states that all fuses must be marked to show: ______
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Most low-cost, common, standard, nonrenewable, one-time fuses are ___ fuses.
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____ are grouped into three categories, _1, _5, _9, and have varying degrees of current-limiting ability, depending on the _ rating.
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Class __ are current limiting fuses with a minimum interrupting rating of 200,000 rms symmetrical amperes.
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______ fuses are considered to be dual-element time delay fuses, are physically smaller than Class H fuses, and have a minimum interrupting rating of 200,000 rms symmetrical amperes.
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____ is a nonrenewable cartridge type and has a minimum interrupting rating of 200,000 rms symmetrical amperes.
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___ is physically smaller than ordinary fuses and is available in both fast-acting and dual-element time delay types.
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___ are primarily used for control-circuit protection, for the protection of motor control circuits, ballasts, small transformers, and so on.
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Article ____ spells out the requirements for plug fuses.
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____ have three types: 1. Standard fuse link type 2. Loaded link type 3. Dual-element time-delay type.
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___ act clearly states that electrical equipment must not be worked on when it is energized.
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On ___ extreme caution must be exercised when checking fuses.
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Using a ___, the first step is to be sure to set the scale to the correct voltage setting.
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Reading from the ___ side of a good fuse should show zero voltage or else an extremely small voltage across the fuse.
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Remove the fuse from the switch and then take a ___ reading across the fuse using an ohmmeter.
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An ____ fuse will generally show a high resistance reading or an open circuit indication.
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___ are short circuit devices that can isolate a faulted cable rather than having the fault open the entire phase.
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____ is required to be installed in series with ungrounded conductors for overcurrent protection.
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The connection of one phase conductor to ground in a delta-connection, 3-phase system is sometimes called a ____.
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A ___ is made of copper bar or brass tubing that has exactly the same physical dimensions as a fuse for a given ampere rating and voltage rating.
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The electrician must be able to use the following types of fuse data: ___ and ___ charts.
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If one is to ____ match the short-circuit current rating of electrical equipment with the let-through current values of the overcurrent protective devices.
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There are three types of Circuit breakers: ____ , Power, Insulated-case.
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Study Notes
Overcurrent Protection Devices
- Commonly used overcurrent protection devices include fuses and circuit breakers.
- Standards for rating, types, classes, and testing procedures are established by UL (Underwriters Laboratories) and NEMA (National Electrical Manufacturers Association).
Fused Switches
- Available voltage ratings for fused switches are 250V and 600V.
- Designed primarily for use with copper conductors; aluminum conductors require proper marking for terminal suitability.
- Rated based on temperature at 60 degrees Celsius for 14-1 AWG and 75 degrees Celsius for 1/0 AWG and larger.
Accessibility and Installation
- Article 240.24(B) mandates accessibility to overcurrent devices for occupants.
- Article 240.24(A) requires that switches with fuses or circuit breakers are readily accessible, with handles not exceeding 6'7" from the floor.
Electrical Ratings and Characteristics
- Voltage rating indicates the maximum voltage a circuit can handle without damage, exceeding the circuit’s operating voltage.
- Continuous current rating refers to the maximum current a device can carry without interrupting the circuit.
- Interrupting rating is the maximum current a device can interrupt at rated voltage under standard test conditions.
- Short circuit current rating indicates the device's ability to withstand fault currents until the overcurrent protection opens the circuit.
Current and Energy Concepts
- Speed of response is inversely related to the current's magnitude flowing through a fuse.
- Current limitation refers to the maximum current imposed for protecting the circuit.
- Peak let-through is the maximum current during an overcurrent condition.
Fuse Types and Features
- Dual-element time delay fuses prevent unnecessary tripping during harmless overloads and react quickly to short circuits.
- Energy-efficient motors feature higher starting currents, which can cause nuisance tripping of fuses.
- Time-current curves should be checked to ensure fuses and circuit breakers manage inrush currents effectively.
- Article 430.52 states dual-element fuses may not exceed 175% of a motor's full-load current (FLC).
Fuse Classes and Specifications
- Class H fuses are common, low-cost, and nonrenewable.
- Class K fuses are categorized into varieties (K1, K5, K9) based on current-limiting ability.
- Class J fuses, dual-element time delay types, have a minimum interrupting rating of 200,000 rms symmetrical amperes.
- Class R is a nonrenewable, cartridge type with the same interrupting rating as Class J fuses.
- Cube fuses are more compact, with both fast-acting and time delay types, requiring specific opening times under overload conditions.
- Class CC fuses protect control circuits.
Safety and Testing Procedures
- OSHA regulations prohibit working on energized electrical equipment.
- Extra caution is necessary on live circuits to avoid hazards during testing.
- When testing with a voltmeter, ensure proper voltage settings before measuring.
- An open (blown) fuse will show high resistance or an open circuit indication when tested.
Additional Equipment Insights
- Cable limiters isolate faults in cables, preventing total circuit interruption, available for copper or aluminum conductors.
- Overcurrent devices are essential for series installation with ungrounded conductors.
- Corner ground refers to a specific ground connection in a delta-connection, three-phase system.
- Solid neutrals are used to match the physical dimensions of fuses for corresponding ampere and voltage ratings.
Technical Knowledge and Application
- Electricians must be proficient in interpreting time-current characteristic curves and peak let-through charts for fuse data.
- It's critical to align the short-circuit current rating of electrical equipment with the let-through current from protective devices.
- Circuit breakers are categorized into molded case, power, and insulated-case types, each suited for specific applications and protection levels.
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Description
Test your knowledge on overcurrent protection devices including fuses and circuit breakers. This quiz covers their standards, ratings, and installation requirements based on established guidelines. Understand the critical aspects of accessibility and electrical ratings in overcurrent protection.
