Chapter 17 Review

Low-voltage power circuit breakers are listed under UL Standard ____.

1066

For motor branch circuits, the maximum setting of a conventional inverse-time circuit breaker must not exceed ____ percent of the full-load current of the motor.

250

Fused switches are available in ratings of up to ____ amperes in both 250 and 600 volts.

6000

Name and describe the class of fuses used for control circuit protection.

Class CC fuses are primarily used for control-circuit protection, for the protection of motor control circuits, ballasts, small transformers, and so on. They are UL listed as branch-circuit fuses. Class CC fuses are rated at 600 volts or less and have a 200,000-ampere interrupting rating in sizes from 1/10 ampere through 30 amperes. These fuses measure 1-1/2 in. x 13/32 in. (38.1 mm x 10.3 mm) and can be recognized by a button on one end of the fuse. This button is unique to Class CC fuses. When a fuseblock or fuseholder that has the matching Class CC rejection feature is installed, it is impossible to insert any other 1-1/2 in. x 13/32 in. (38.1 mm x 10.3 mm) fuse. Only a Class CC fuse will fit into these special fuseblocks and fuseholders. A Class CC fuse can be installed in a standard fuseholder.

Conventional plug fuses have a base referred to as a(n) ____ base. This base is of the same shape as the base on a light bulb.

Edison

Class ____ fuses formerly were called NEC® or Code fuses.

H

What is the purpose of peak let-through charts?

It is important to understand the use of peak let-through charts if one is to properly match the short-circuit current rating of electrical equipment with the let-through current values of the overcurrent protective devices (fuses or circuit breakers). Short-circuit current rating is defined in Article 100 of the NEC® as, "The prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria." The short-circuit current ratings of electrical equipment (such as panelboards, bus duct, switchboard bracing, controllers, and conductors) and the interrupting ratings of circuit breakers are given in the published standards of the Underwriters Laboratories, NEMA, and the Insulated Cable Engineering Association (ICEA). The short-circuit current (formerly "withstand") ratings may be based either on the peak current (Ip) or on the RMS current. Peak let-through charts give a good indication of the current-limiting effects of a current-limiting fuse or circuit breaker under "bolted fault" conditions. Current limitation is one of the principles used to obtain a series-rating listing for circuit breakers.

____ circuit breakers are the most common type in use today.

Molded-case

How do thermal-magnetic circuit breakers work?

On a continuous overload, the bimetallic element heats and bends until it unlatches the inner tripping mechanism of the breaker. Harmless momentary overloads do not cause the tripping of the bimetallic element. If the overload is heavy, or if a short circuit occurs, then the magnetic mechanism within the circuit breaker causes the breaker to interrupt the circuit instantly. The time required for the breaker to open the circuit completely depends on the magnitude of the current and the mechanical condition of the circuit breaker. This time may range from approximately one-half cycle to many minutes.

The Class ____ fuse is a nonrenewable cartridge type and has a minimum interrupting rating of 200,000 RMS symmetrical amperes.

R

____ is the ability of the equipment (the entire assembly) to withstand a fault current equal to or less than the rating for the length of time it takes the specified overcurrent device to open the circuit to the equipment.

Short-circuit rating

____ and the National Electrical Manufacturers Association establish standards for the ratings, types, classifications, and testing procedures for fuses and circuit breakers.

The Underwriters Laboratories Inc.

What are some factors that influence speed of response of an overcurrent device?

The time required for the fusible element of a fuse to open varies inversely with the magnitude of the current that flows through the fuse. As the current increases, the time required for the fuse to open decreases. The time-current characteristic of a fuse depends on its rating and type. A circuit breaker also has a time-current characteristic. For the circuit breaker, however, there is a point at which the opening time cannot be reduced further due to the inertia of the moving parts within the breaker. The time-current characteristic of a fuse or circuit breaker should be selected to match the connected load of the circuit to be protected. Time-current characteristic curves are available from the manufacturers of fuses and circuit breakers.

What are some prohibitions on the location of overcurrent devices?

be exposed to physical damage be installed in the vicinity of easily ignitable material be installed in clothes closets be installed in bathrooms. This requirement is for dwelling units and guest room of hotels and motels, not for commercial and industrial buildings be installed over the steps of a stairway

A dual-element, time-delay fuse has one or more fusible elements (links) connected ____.

in parallel

An interrupting rating is a(n) ____ rating, and measures the fuse's or circuit breaker's ability to safely open an electrical circuit under fault conditions, such as an overload, short circuit, or ground fault.

maximum

When you remove a fuse from a switch, and then take a resistance reading across the fuse using a(n) ____, a good fuse will show zero or a very minimal resistance.

ohmmeter

Series-rated equipment is also referred to as ____.

series-connected

In the case of a delta, 3-phase, corner-grounded "B" phase system, a device called a ____ can be installed in the "B" phase.

solid neutral

When compared to other types of fuses, the ____ fuse has the lowest energy let-through values.

straight current-limiting