Lesson 2: Overcurrent Protective Device Categories

Under short-circuit conditions, which of the following represents an OCPD's level of thermal effect protection for circuit components?

A. I2t

The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s) best defines a ? .

A. branch circuit Nec . 100

UL classes of fuses include branch-circuit fuses such as ? fuses.

B. Class L, J, and RK1

A device capable of providing protection for service, feeder, and branch circuits, and equipment over the full range of overcurrents between its rated current and its interrupting rating is a(n) ?

B. branch-circuit overcurrent protective device Nec. 100

The term branch circuit has different meanings as used in the NEC. Which of the following statement(s) is/are correct?

C. Both a. and b. are correct. A branch circuit is a type of circuit in an electrical distribution system connected from the final OCPD to the outlet(s) and is not limited to the type of wiring method used. A branch circuit is a category of overcurrent protective device used for services, feeders, branch circuits, and equipment providing a full range of protection against short circuits, ground faults, and overloads.

Which are branch-circuit rated circuit breakers? I. Instantaneous (only) circuit breaker II. Insulated case circuit breaker III. Low-voltage power circuit breaker IV. Molded case circuit breaker V. Supplementary circuit breaker

D. II., III., and IV.

An OCPD category used to protect luminaires, appliances, and other equipment, but not permitted to be used as a substitute for required branch-circuit OCPDs or in place of the required branch-circuit protection, best describes ? .

D. supplementary overcurrent protection

Under fault conditions, the greatest damage done to components by a fault current often occurs in the first half-cycle. What is/are a common adverse effect(s) that result when electrical equipment is not properly protected from fault current?

E. a., b., and c.

A fuse or circuit breaker that is permitted for branch-circuit overcurrent protection is required to be marked "branch-circuit fuse," "branch-circuit breaker," or "branch-circuit overcurrent protective device."

False

Application-limited OCPDs are always suitable for use in place of branch-circuit OCPDs.

False

Creepage spacing is the shortest distance through the air for parts designed to be at different voltage potentials, such as phase A and phase B or one phase-to-ground. Clearance is the shortest distance over a surface for parts designed to be at different voltage potential.

False

Supplementary OCPDs are described as OCPDs permitted for only specific branch-circuit applications, under limited conditions, per the specific reference in the NEC.

False

Typically, selecting a branch-circuit OCPD for an application by its voltage rating and ampere rating satisfies all the NEC requirements for the level of protection and safety required.

False

Where supplementary overcurrent protection is used for luminaires, appliances, and other equipment or for internal circuits and components of equipment, it shall be permitted to serve as a substitute for required branch-circuit overcurrent devices or in place of the required branch-circuit protection.

False Nec. 240.10

A branch-circuit overcurrent protective device that is current-limiting will be marked "current-limiting."

True

Branch-circuit fuses or circuit breakers 600 volts or less that are marked "current-limiting" have met specific current-limiting performance requirements per their respective product standards.

True

Overcurrent protective devices have product standards that establish requirements for certain minimum product performance criteria and physical specifications. UL 248 is a fuse product standard and UL 489 is a molded case circuit breaker product standard.

True

Per NEC 240.21, overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply, except as specified in 240.21(A) through (H).

True

The maximum magnetic forces exerted on equipment and circuit components during a short-circuit condition is proportional to the peak current squared. If a current-limiting OCPD can cut the peak current to one third of the available peak current, then the magnetic or mechanical stress will be cut to one ninth (1/(32)) of that available.

True

When the fault current is in the OCPD's current-limiting range, a current-limiting OCPD prevents the fault current from building up to the full available fault current value in the first half cycle of fault current. This action reduces damage to equipment and can help protect workers from devastating arc-flash and arc-blast incidents.

True