Electrical Level 3 Module 5 over current protection

110 overcurrent protection

Types of fault current include the following:

Overloads, short circuits, ground faults, arc faults

110 overload conditions

Continuous overloads can result from___

defective motors (motors with worn bearings) overload equipment, or too many loads on one circuit

120 short circuit conditions

NEC section 110.9 states that equipment intended to interrupt current at fault levels (fuses and circuit breakers) shall have an interrupting rating at nominal circuit voltage___ to the current that is available at the line terminals of the equipment

At least equal

130 Ground Faults

Unintentional conducting connection between any of the conductors of an electrical system and the conducting material that encloses the conductors or any conducting material, such as a metal enclosure, that is grounded or may become grounded

Ground fault

140 Arc faults

480V circuits, the arc current may be as low as___ of the available bolted fault current and seldom exceeds___

Low: 38% seldom exceeds 60%

211 voltage rating

The voltage rating of a fuse must be___

At least equal or greater than the circuit voltage

222 dual element time delay fuses

Unlike single element fuses, dual element time delays are often applied in circuits subject to temporary motor overloads and surge currents to provide both high performance short circuit and overload protection. ____

Oversizing to prevent nuisance openings is not necessary with this type of fuse.

230 fuse classes and applications

Safety is the primary consideration of the ___

Underwriters Laboratories, INC. (UL)

230 fuse classes and applications

(rejection) fuses are high performance 1/10A to 60A units, 250V and 600V, having a high degree of current limitation and a root-mean-square (RMS) symmetrical short circuit interrupting rating of up to 200,000A

UL Class R

230 fuse classes and applications

Rated to interrupt 200,000A:

Class J

235 fuses for motor overload and short

designed to withstand numerous trip and reset cycles without replacement

Overload relays

Normal condition,

Current bypsses the spring and latching mechanism

310 Circuit breaker classification

These groups are as follows

100A, 225A, 400A, 1000A, 2000A

320 Circuit breaker interrupting capacity ratings

UL tests now include

NEMA tests plus other ratings

411 over current devices for branch circuits

The branch circuit conductor and over current protective device must be sized for the actual non continuous load plus-

125% of the continuous load

412 over current devices for feeders

Over current protective devices for a feeder with no motor load must be -

125% of the continuous load plus 100% of the non continuous load

413 over current devices for services

Each ungrounded service entrance conductor must have an over current device in series with a rating___

not higher than the ampacity of the conductor

415 over current devices for motor circuit protection

This level of protection is commonly referred to as motor branch circuit protection. ___ are designed to provide this protection, as long as they are sized correctly.

Dual element fuses

420 applying short circuit calculations

Although several methods for determining short circuit currents are in use, the ___ of short circuit calculation is the often recommended method

point to point method

510 circuit breakers

___ protects the circuit from excessive heat damage generated by lower level currents such as those caused by an overload

The thermal portion

___ provides an instantaneous trip for currents of high levels such as those present in a short circuit.

The thermal portion

510 circuit breakers

When tripped most breaker handles move to a position _____

Halfway between on and off

520 fuses

A single element fuse is designed to act immediately upon over current, which may be either a ____ or _____

Short circuit or an overload