Circuit Breakers and Fuses
What is normally the highest short circuit current in residential applications?
10,000 Amperes
In a feeder circuit with no motor load, a time-delay fuse must be sized for _____ percent of the continuous load.
125 - In a feeder circuit with no motor load, a time-delay fuse must be sized for 125 percent of the continuous load.
Circuit breakers ampere groupings
15A-100A 125A-225A 250A-400A 500A-1,000A 1,200A-2,000A
Type S plug fuses are available with capacities up to _____ amps.
30 - Type S cartridge fuses are available in 15 different amperage capacities ranging to 30 amps.
Every circuit breaker must have its interrupting rating shown on the circuit breaker unless it is rated for
5,000 Amperes
What is the highest voltage rating for circuit breakers used on DC systems that UL recognizes?
600 Volts
Molded Case Circuit Breaker
A Circuit breaker enclosed in an insulating housing
Choose the correct statement. A. A GFCI operates by detecting a voltage imbalance in excess of 10%. B. A GFCI must be installed at every outlet in a residence. C. A GFCI will open a circuit if a current imbalance of greater than 6 mA is detected
A GFCI will open a circuit if a current imbalance of greater than 6 mA is detected
What must be used in conjunction with Type S plug fuses when they are installed in a fuse holder?
A Type S Adapter
Circuit breaker
A device that is designed to open a circuit automatically at a certain overcurrent. It can also be used to manually open and close the circuit.
Cartridge fuse
A fuse enclosed in an insulating tube to confine the arc when the fuse blows.
Which of the following best describes a dual element fuse? A fuse having an overcurrent limit and a time delay before activation A fuse with two (2) attachment points A fuse with no barrels A fuse having two (2) ferrules on each end
A fuse having an overcurrent limit and a time delay before activation
Frame size
A method used to classify circuit breakers according to given current ranges.
Pole
A portion of a device associated exclusively with one electrically separated conducting path of the main circuit or device
Fuse
A protective device that opens a circuit when the fusible element is severed by heating due to a fault current or overcurrent passing through it.
Plug fuse
A type of fuse that is held in position by a screw thread contact instead of spring clips, as is the case with a cartridge fuse.
Overcurrent protection
De-energizing a circuit whenever the current exceeds a predetermined value; the usual devices are fuses, circuit breakers, or magnetic relays.
True or False. The terms overload current and short circuit mean the same thing.
False - An overload current is a current of short duration, generally up to six times normal current. A short circuit bypasses the normal circuit load.
True or False. A time-delay fuse reacts to normal overcurrents and high fault currents in the same way.
False - A time-delay fuse will open quickly on a high fault current, and it will delay opening on low overcurrents.
True or False. When non-time-delay fuses are used in a motor circuit with fuse protection only, the fuse should be sized for 150% of the full-load current.
False - In such an application, the fuse should be sized at close to 300% of the motor running full load current.
True or False. Feed-through GFCI receptacles are designed to interrupt current at one outlet and feed normal current through the circuit to the remaining outlets.
False - Feed-through GFCIs affect all the outlets in a circuit.
True or False. Single-element cartridge fuses are commonly used in motor control circuits.
False - Single-element cartridge fuses are subject to blowing due to nuisance or transient surge currents. Dual-element fuses are used in motor control circuits.
Which of the following best describes where the NEC allows the use of Edison Base fuses? For replacements in existing installations On circuits rated under 40 Amperes On circuits rated over 40 Amperes On new installations only
For replacements in existing installations
The voltage rating of a fuse in a circuit can be _____ than the circuit voltage.
Higher - The voltage rating of the fuse must be at least equal to the circuit voltage. It can be higher, but never lower.
Choose the correct statement. A. If a conductor is rated for 15 amps, the fuse should be no larger than 15 amps. B. The current rating of a fuse must be at least twice the normal continuous circuit current. C. The voltage rating of a fuse must be at least 80% of the circuit voltage.
If a conductor is rated for 15 amps, the fuse should be no larger than 15 amps.
Choose the correct statement. A. When replacing a fuse of circuit breaker, it is okay to use a substitute that has a higher current rating than the circuit components but you must never use one with a lower rating. B. Circuit breakers are built to withstand fault currents, so repeated overloads will not have a cumulative affect. C. It is important that all circuit components be compatible as far as voltage and current ratings are concerned.
It is important that all circuit components be compatible as far as voltage and current ratings are concerned.
When a common molded case circuit breaker trips, in what position will the operating handle be?
Middle - Halfway between the On and Off positions
Where is the Ampere Rating normally marked on Circuit Breakers?
On the Handle
On renewable cartridge fuses, which part of the fuse is normally renewed?
The Link
Which of the following is the most important consideration when working with over current protection devices? The brand name The age of the device The type of project The correct interrupting capacity
The correct interrupting capacity
Short circuit
The current that exists when an unintended path is created between any two components in a circuit.
Fault current
The current that exists when an unintended path is established between an ungrounded conductor and ground.
Fuse link
The fusible part of a cartridge fuse.
A 2,000A frame circuit breaker has a range of
The largest frame classification is 1,200A - 2,000A
Choose the correct statement. A. The current-limiting mechanism of a current limiting circuit breaker reacts instantaneously to a large fault current. The conventional mechanism opens when the fault current has been dissipated. B. When a large fault current occurs, the conventional trip mechanism of a current limiting circuit breaker opens first. C. The current-limiting mechanism of a current limiting circuit breaker reacts to a large fault current by diverting it directly to the circuit ground.
The current-limiting mechanism of a current limiting circuit breaker reacts instantaneously to a large fault current. The conventional mechanism opens when the fault current has been dissipated.
How to reset a tripped breaker
To reset, turn the handle to the Off position and then push it as far as it will go beyond this position (Reset); finally, turn it to the On position.
Circuit breakers have _____ type(s) of current ratings.
Two - Circuit breakers have two types of current ratings: continuous and fault current interrupting capacity.
The term AIC refers to the
amperage interrupting capacity - AIC stands for amperage interrupting capacity, which is the maximum short circuit current at which the breaker will safely interrupt the circuit.
NEC Section 240.85 covers circuit breaker
applications
In a system in which selective coordination is applied, the protective device _____ the fault source will open first.
closest to - The closer it is to the voltage source, the more circuits are affected when the protective device opens. Therefore, it is more efficient to design the circuit so that the protective device closest to the fault reacts faster than the upstream devices.
What are the two types of current ratings for Circuit breakers.
continuous current rating - The first and the one that is used most often is the continuous current rating. fault current interrupting capacity- The second is the fault current interrupting capacity.
Worn motor bearings are most likely to result in a(n) _____.
sustained overload
Choose the correct statement. A. Single-element cartridge fuses respond rapidly to short circuit currents. B. An advantage of modern cartridge fuses is that their links can be replaced. C. Time-delay plug fuses are not available.
Single-element cartridge fuses respond rapidly to short circuit currents.
