Electrical Circuits and Protection Devices

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?

The UL and Canadian Standards Association. Signup and view all the answers

Standard interrupting capacity circuit breakers can be identified by what?

Their black operating handles and black printed interrupting labels. Signup and view all the answers

The FH-type circuit has an interrupting rating of?

65,000A symmetrical to 240VAC. Signup and view all the answers

Standard interrupting capacity circuit breakers are used where?

In residential applications where the available fault current is seldom higher than 10,000A. 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?

Conventional thermomagnetic circuit breakers. Signup and view all the answers

UL defines Class A devices as?

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

Class A devices must not trip below?

4mA. Signup and view all the answers

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

The point of installation. 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?

Low interrupting ratings. 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?

NEC Section 240.6(C). Signup and view all the answers

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

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

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

A single-line schematic will suffice. Signup and view all the answers

Coordination is the name given to?

The time-current relationship among a number of overcurrent devices connected in series. 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?

The occurrence of such overcurrents. Signup and view all the answers

Reliable protective devices prevent or minimize costly damage to?

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

A short circuit is probably the most common cause of?

Electrical Problems. Signup and view all the answers

Overloads are most often between?

One to six times the normal current level. Signup and view all the answers

Continuous (sustained) overloads can result from?

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

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

Fuse and circuit breakers. Signup and view all the answers

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

NEC Sections 110.9 and 110.10. Signup and view all the answers

A standard molded-case circuit breaker usually contains?

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. Signup and view all the answers

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

The middle position. Signup and view all the answers

Circuit breakers are classified into?

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

Circuit breakers are classified by?

The largest ampere rating of its range. Signup and view all the answers

The lowest ampere ratings of circuit breakers frame sizes are?

15A, 125A, 250A, 500A, and 1,200A. Signup and view all the answers

The largest ampere ratings of circuit breaker frame sizes are?

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

Circuit breaker voltage ratings must be?

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

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

Continuous current rating and fault current interrupting capacity. Signup and view all the answers

The ampere rating of a circuit breaker is located on?

The handle of the device. Signup and view all the answers

Generally, the circuit breaker current rating must be?

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

Most overcurrent protective devices are labeled with?

Normal current rating and interrupting rating. Signup and view all the answers

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

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

Different methods to determine the short circuit requirements are?

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

NEMA tests are?

Self-certification tests. Signup and view all the answers

UL tests are?

Certified by unbiased witnesses. Signup and view all the answers

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

Overcurrents by the thermal magnetic trip element. Signup and view all the answers

Interrupting capacity of a circuit breaker is based on its?

Rated voltage. Signup and view all the answers

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

Industrial plants. Signup and view all the answers

Current-limiting circuit breakers operate extremely fast to provide?

Downstream protection for other types of overcurrent devices. Signup and view all the answers

One of the chief causes of circuit breaker failure is?

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

GFCIs are what Class of protective devices?

Class A. Signup and view all the answers

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

Grounded and ungrounded conductors. 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?

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

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

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

Shunt trip circuit breakers are used for?

Fire suppression circuits, pushbuttons, or alarm circuits. Signup and view all the answers

Arc fault circuit breakers include electronics that monitor?

Current flow in the breaker. Signup and view all the answers

Arc fault circuit breakers detect?

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

Switched neutral breakers disconnect?

The neutral or ungrounded conductor simultaneously with all ungrounded conductors. Signup and view all the answers

Switched Neutral breakers can be installed in?

Fuel dispensing equipment. Signup and view all the answers

Non-automatic breakers have?

No devices to protect the circuit against overcurrent. Signup and view all the answers

Non-automatic breakers can be used?

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

A fuse is the simplest device for?

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

Most low voltage distribution fuses have?

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

Voltage rating of a fuse must be at least?

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

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

125% of the load current. Signup and view all the answers

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

10,000A. Signup and view all the answers

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

Lower rated circuit breakers. Signup and view all the answers

Edison base fuses are permitted only as?

Replacements in existing installations. Signup and view all the answers

Type S fuses were developed to?

Reduce the possibility of over-fusing a circuit. Signup and view all the answers

All type S fuses are?

Time delay fuses. Signup and view all the answers

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

125V between phases. Signup and view all the answers

A plug fuse is a?

Nonrenewable fuse. Signup and view all the answers

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

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

Single element fuses have?

A very high response rate to overcurrents. Signup and view all the answers

Most fuses are used for protection of?

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

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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Description

This quiz focuses on electrical circuits and the role of protection devices such as fuses and circuit breakers. It covers identification, ratings, classifications, and typical applications in residential settings. Test your knowledge on the critical aspects of overload protection and safety standards in electrical engineering.