Electrical Circuits and Protection Devices

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Questions and Answers

The fuse and circuit breaker are two types of automatic overload devices that are normally used in what?

  • Heating Systems
  • Plumbing Systems
  • Electrical Circuits (correct)
  • Ventilation Systems

Circuit breakers are classified into?

Ampere groupings and each group is classified by the largest ampere rating of its range.

The ampere rating of a circuit breaker is located where?

On the handle of the device.

Two organizations that both test and list devices are?

<p>The UL and Canadian Standards Association.</p> Signup and view all the answers

Standard interrupting capacity circuit breakers can be identified by what?

<p>Their black operating handles and black printed interrupting labels.</p> Signup and view all the answers

The FH-type circuit has an interrupting rating of?

<p>65,000A symmetrical to 240VAC.</p> Signup and view all the answers

Standard interrupting capacity circuit breakers are used where?

<p>In residential applications where the available fault current is seldom higher than 10,000A.</p> Signup and view all the answers

At currents below a threshold of about 1,000A, the current-limiting circuit breakers perform in a manner similar to what?

<p>Conventional thermomagnetic circuit breakers.</p> Signup and view all the answers

UL defines Class A devices as?

<p>One that will trip when a fault current to ground is 6mA or more.</p> Signup and view all the answers

Class A devices must not trip below?

<p>4mA.</p> Signup and view all the answers

A non-feed through type GFCI receptacle offers only protection at?

<p>The point of installation.</p> Signup and view all the answers

Because single element fuses have a high speed of response to short circuit currents, they are particularly suited for protection of breakers with?

<p>Low interrupting ratings.</p> Signup and view all the answers

What NEC Section requires that any cartridge fuses used for branch circuit or feeder protection must be plainly marked?

<p>NEC Section 240.6(C).</p> Signup and view all the answers

NEC Section 240.6(C) requires you to show what on the fuse barrel?

<p>Ampere rating, voltage rating, interrupting rating, current limiting (where applicable), the name or trademark of the manufacturer.</p> Signup and view all the answers

When sizing fuses for a given application....?

<p>A single-line schematic will suffice.</p> Signup and view all the answers

Coordination is the name given to?

<p>The time-current relationship among a number of overcurrent devices connected in series.</p> Signup and view all the answers

Harsh environments, general deterioration, accidental damage, damage from natural causes, excessive expansions, and overloading of the electrical system are all factors that contribute to?

<p>The occurrence of such overcurrents.</p> Signup and view all the answers

Reliable protective devices prevent or minimize costly damage to?

<p>Transformers, conductors, motors, equipment, and many other components that make up the complete electrical system.</p> Signup and view all the answers

A short circuit is probably the most common cause of?

<p>Electrical Problems.</p> Signup and view all the answers

Overloads are most often between?

<p>One to six times the normal current level.</p> Signup and view all the answers

Continuous (sustained) overloads can result from?

<p>Motor defects (such as worn motor bearings), overloaded equipment, or too many loads on a circuit.</p> Signup and view all the answers

What two types of automatic overload devices are normally used in electrical circuits?

<p>Fuse and circuit breakers.</p> Signup and view all the answers

What NEC sections contain the requirements for interrupting ratings and protection against fault current?

<p>NEC Sections 110.9 and 110.10.</p> Signup and view all the answers

A standard molded-case circuit breaker usually contains?

<p>A set of contacts, magnetic trip element, a thermal trip element, line and load terminals, bussing used to connect these individual parts, and an enclosed housing of insulated material.</p> Signup and view all the answers

On an overload, the circuit breaker opens itself, or trips. When tripped, the handle jumps to?

<p>The middle position.</p> Signup and view all the answers

Circuit breakers are classified into?

<p>Ampere groupings that match specific physical dimensions, commonly referred to as frame size.</p> Signup and view all the answers

Circuit breakers are classified by?

<p>The largest ampere rating of its range.</p> Signup and view all the answers

The lowest ampere ratings of circuit breakers frame sizes are?

<p>15A, 125A, 250A, 500A, and 1,200A.</p> Signup and view all the answers

The largest ampere ratings of circuit breaker frame sizes are?

<p>100A, 225A, 400A, 1,000A, and 2,000A frames.</p> Signup and view all the answers

Circuit breaker voltage ratings must be?

<p>Equal to or greater than the voltage of the electrical system on which they are used.</p> Signup and view all the answers

Circuit breakers have two types of current ratings. Those are?

<p>Continuous current rating and fault current interrupting capacity.</p> Signup and view all the answers

The ampere rating of a circuit breaker is located on?

<p>The handle of the device.</p> Signup and view all the answers

Generally, the circuit breaker current rating must be?

<p>Equal to or less than the load circuit conductor current carrying capacity (ampacity).</p> Signup and view all the answers

Most overcurrent protective devices are labeled with?

<p>Normal current rating and interrupting rating.</p> Signup and view all the answers

Interrupting rating of electrical equipment is divided into two parts:?

<p>Current at fault (short circuit) levels and current at operating levels.</p> Signup and view all the answers

Different methods to determine the short circuit requirements are?

<p>Approximate values, extensive computations, and the point by point method (most simple method).</p> Signup and view all the answers

NEMA tests are?

<p>Self-certification tests.</p> Signup and view all the answers

UL tests are?

<p>Certified by unbiased witnesses.</p> Signup and view all the answers

Currents between 7 to 15 times the rated current are handled as?

<p>Overcurrents by the thermal magnetic trip element.</p> Signup and view all the answers

Interrupting capacity of a circuit breaker is based on its?

<p>Rated voltage.</p> Signup and view all the answers

High interrupting capacity circuit breakers are used in places such as?

<p>Industrial plants.</p> Signup and view all the answers

Current-limiting circuit breakers operate extremely fast to provide?

<p>Downstream protection for other types of overcurrent devices.</p> Signup and view all the answers

One of the chief causes of circuit breaker failure is?

<p>High heat caused by loose connections at the load side of the breaker.</p> Signup and view all the answers

GFCIs are what Class of protective devices?

<p>Class A.</p> Signup and view all the answers

When installed properly, GFCIs continuously monitor the current in the?

<p>Grounded and ungrounded conductors.</p> Signup and view all the answers

If the GFCI senses a difference between the grounded and ungrounded conductors of more than 6mA, it will automatically trip the circuit and power is interrupted in?

<p>1/40 of a second or less.</p> Signup and view all the answers

Shunt trip circuit breakers have a built-in electric coil that causes?

<p>It to open the breaker contacts when the coil is energized.</p> Signup and view all the answers

Shunt trip circuit breakers are used for?

<p>Fire suppression circuits, pushbuttons, or alarm circuits.</p> Signup and view all the answers

Arc fault circuit breakers include electronics that monitor?

<p>Current flow in the breaker.</p> Signup and view all the answers

Arc fault circuit breakers detect?

<p>Patterns of surges or spikes typical of currents in a short, high resistance arcing circuit.</p> Signup and view all the answers

Switched neutral breakers disconnect?

<p>The neutral or ungrounded conductor simultaneously with all ungrounded conductors.</p> Signup and view all the answers

Switched Neutral breakers can be installed in?

<p>Fuel dispensing equipment.</p> Signup and view all the answers

Non-automatic breakers have?

<p>No devices to protect the circuit against overcurrent.</p> Signup and view all the answers

Non-automatic breakers can be used?

<p>As a means of manually disconnecting circuits by operating the handle.</p> Signup and view all the answers

A fuse is the simplest device for?

<p>Opening an electric circuit when excessive current flows due to an overload or such fault conditions as grounds or short circuits.</p> Signup and view all the answers

Most low voltage distribution fuses have?

<p>250V or 600V ratings (other ratings are 125V and 300V).</p> Signup and view all the answers

Voltage rating of a fuse must be at least?

<p>Equal to the circuit voltage. It can be higher but never lower.</p> Signup and view all the answers

Generally, the ampere rating of a fuse and switch combination should be selected at?

<p>125% of the load current.</p> Signup and view all the answers

The interrupting rating of most branch circuit, molded-case circuit breakers in service entrance boxes is?

<p>10,000A.</p> Signup and view all the answers

Current-limiting fuses have an interrupting capacity of 200,000A and are commonly used to protect?

<p>Lower rated circuit breakers.</p> Signup and view all the answers

Edison base fuses are permitted only as?

<p>Replacements in existing installations.</p> Signup and view all the answers

Type S fuses were developed to?

<p>Reduce the possibility of over-fusing a circuit.</p> Signup and view all the answers

All type S fuses are?

<p>Time delay fuses.</p> Signup and view all the answers

Plug fuses are normally permitted to be used in circuits of no more than?

<p>125V between phases.</p> Signup and view all the answers

A plug fuse is a?

<p>Nonrenewable fuse.</p> Signup and view all the answers

In most industrial and commercial applications, cartridge fuses are used because?

<p>They have a wider range of types, sizes, and ratings than plug fuses.</p> Signup and view all the answers

Single element fuses have?

<p>A very high response rate to overcurrents.</p> Signup and view all the answers

Most fuses are used for protection of?

<p>Branch feeders on systems operating at 600V or below.</p> Signup and view all the answers

Flashcards

Overload Protection Devices

Automatic devices designed to protect electrical circuits from overloads and short circuits.

Fuse

A type of overload protection device that melts and breaks the circuit when excessive current flows through it.

Circuit Breaker

A type of overload protection device that uses a mechanism to interrupt the flow of electricity when an overload occurs.

Ampere Rating

The maximum current a circuit breaker can safely handle without tripping.

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Interrupting Capacity

The ability of a circuit breaker to safely interrupt a fault current.

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Standard Interrupting Capacity Breaker

A specific type of circuit breaker designed for residential use, typically with a lower interrupting capacity.

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FH-Type Circuit Breaker

A type of circuit breaker with a specific interrupting rating of 65,000A symmetrical at 240VAC.

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Ground Fault Circuit Interrupter (GFCI)

A type of overload protection device that is designed to trip when a ground fault occurs, protecting against electrical shock.

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Non-Feed Through GFCI

A type of GFCI that only provides protection at the specific location where it is installed.

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Class A GFCI

A classification of GFCI devices, defined by UL, that require a specific ground fault current to trip.

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Single Element Fuse

A type of fuse that reacts quickly to short circuits, ideal for protecting circuits with low interrupting ratings.

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NEC Section 240.6(C)

A regulation in the National Electrical Code requiring that cartridge fuses used for branch circuit protection must be clearly marked with specific characteristics.

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Coordination

The time-current relationship between multiple overcurrent devices in a system.

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Overcurrents

Conditions that cause an excessive flow of current in an electrical circuit, potentially leading to damage.

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Short Circuit

The interruption of the normal flow of current in a circuit due to a low-resistance path, typically caused by a fault.

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Overload

A condition where the current flow in a circuit exceeds its normal operating level, often caused by excessive load.

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Interrupting Rating

The maximum fault current a circuit breaker can safely interrupt.

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Molded-Case Circuit Breaker

A type of circuit breaker commonly used in residential and commercial applications, consisting of components enclosed in an insulated casing.

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Continuous Current Rating

The current rating that a circuit breaker can continuously handle without tripping.

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Shunt Trip Circuit Breaker

A type of circuit breaker with an electrical coil that allows it to be tripped remotely by a signal from an external device.

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Arc Fault Circuit Breaker (AFCI)

A specialized type of circuit breaker designed to detect and interrupt arcing faults, which can occur due to damaged cords or other electrical issues.

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Switched Neutral Breaker

A circuit breaker that allows the neutral conductor to be switched off simultaneously with the live conductors, useful in some industrial applications.

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Fuse

A simple, non-renewable overload protection device that melts and breaks the circuit when excessive current flows.

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Current-Limiting Fuse

A type of fuse that can handle extremely high current levels, often used for protecting low-rated circuit breakers.

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Edison Base Fuse

An older type of fuse used in older installations, typically used for 125V applications.

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Type S Fuse

A type of fuse designed to reduce the risk of over-fusing, always time-delay types.

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Plug Fuse

A type of fuse designed for use in plug-in outlets, limited to 125V applications.

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Cartridge Fuse

A type of fuse often preferred for industrial applications, offering a wide range of sizes and current ratings.

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Branch Feeder Protection

The primary purpose of most fuses, which is to protect branch feeders operating at voltages of 600V or below.

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Study Notes

Electrical Circuits and Protection Devices

  • Fuses and circuit breakers serve as automatic overload devices in electrical circuits.
  • Circuit breakers are categorized by ampere groupings, based on their maximum ampere ratings.

Circuit Breaker Identification and Ratings

  • Ampere ratings of circuit breakers are indicated on their handles.
  • Standard interrupting capacity circuit breakers feature black operating handles and labels.
  • The FH-type circuit has a specific interrupting rating of 65,000A symmetrical at 240VAC.
  • Typical use for standard interrupting capacity circuit breakers is in residential settings, usually where fault currents are below 10,000A.

Automatic Overload Device Classifications

  • Class A devices, as defined by UL, trip at a ground fault current of 6mA or more but must not trip below 4mA.
  • Non-feed through GFCI receptacles provide protection only at the installation point.

Fuse and Ground Fault Circuit Interruption

  • Single element fuses react quickly to short circuit currents, making them optimal for protecting low interrupting rating breakers.
  • NEC Section 240.6(C) mandates that cartridge fuses used for branch circuit protection be clearly marked with critical ratings including ampere and voltage.

Overcurrent and Fault Protection

  • Coordination refers to the time-current relationship between multiple overcurrent devices in a system.
  • Factors contributing to overcurrents include environmental conditions, equipment damage, and system overloading.
  • Effective protective devices safeguard valuable components like transformers and motors from costly damage.

Short Circuits and Overloads

  • Short circuits are common culprits of electrical issues, while overloads typically occur at one to six times the normal current.
  • Continuous overloads can arise from defective motors or excessive loads on a circuit.

Interrupting Ratings and Capacity

  • Interrupting ratings and fault current protection requirements are outlined in NEC Sections 110.9 and 110.10.
  • Standard molded-case circuit breakers include components such as contacts and trip elements in insulated housing.

Ratings and Classifications of Circuit Breakers

  • Circuit breakers must have ratings equal to or exceeding the system voltage they are connected to.
  • Two primary ratings are continuous current and fault current interrupting capacity.

GFCI and Shunt Trip Devices

  • GFCIs belong to Class A protective devices, monitoring current in grounded and ungrounded conductors.
  • Shunt trip circuit breakers, featuring an electric coil, automatically open during an alarm or trigger from external devices.

Specialized Circuit Breakers

  • Arc fault circuit breakers are designed to detect patterns indicating arcing, such as damaged cords.
  • Switched neutral breakers cut power to the neutral conductor at the same time as the live conductors and can be used in fuel dispensing.

Fuse Characteristics and Applications

  • Fuses serve as simple overload protection devices and must match or exceed the circuit's voltage rating.
  • Current-limiting fuses can handle capacities up to 200,000A, commonly used for protecting lower-rated circuit breakers.
  • Edison base fuses are only allowed as replacements in existing installations, while Type S fuses reduce over-fusing risks and are always time-delay types.

Fuses vs. Circuit Breakers in Applications

  • Plug fuses are limited to 125V use and provide nonrenewable protection.
  • Cartridge fuses are preferred in industrial settings due to their versatility in size and rating.
  • Most fuses are primarily used for the protection of branch feeders operating at voltages of 600V or below.

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