Protection Devices Flashcards
Do general distribution circuits require protection against short circuit and overloads?
yes
Do short circuits normally have lower or higher values than overloads?
higher
Does a short circuit path have higher impedance than normal?
no
lower
What do overcurrent devices protect against?
protect circuits and equipment from excessive temperatures by currents than are higher than normal
What are two protective devices?
overcurrent and overload
What are the two main types of fuses?
time delay, non-time delay
The instantaneous trip mechanism in a circuit breaker, which provides for short circuit protection, is provided by a _____ _____ element
electromagnetic responsive
What are the two current ratings on an overcurrent device?
continuous, interrupting
What are the two main types of circuit breakers?
instantaneous and time limit
On what type of circuit would you find an overload device?
motor
What are the two types of protection offered by fuses?
non-time delay, time delay
What are the two ampere ratings that overcurrent devices have?
continuous rating, interrupting rating
What are the two mechanisms of a thermal magnetic circuit breaker?
a bimetal strip, short circuit sensing element or solenoid
Which mechanism of thermal magnetic circuit beakers causes the breaker to trip during a prolonged overload condition?
half, quarter
A non-time delay fuse can clear a short circuit condition in less than ____ of a cycle and higher short circuit current in less than _____ of a cycle
the bimetal strip
Describe the sequence of events that occur to a time delay fuse responding to a short circuit condition
the fuse link vaporizes and the surrounding arc quenching filler extinguishes the arcing current
What is the best fuse selection for sensitive equipment
an ultra acting fuse
Why is it important to know the interrupting rating of a fuse or breaker?
it must be able to turn off the circuit without blowing up
List the four factors that affect the maximum available short circuit current
kVA of transformer
Percent IZ
Circuit Voltage
Impedance of short circuit path
Is a circuit breaker a means of disconnecting in a motor circuit?
yes
What must a disconnect in a motor circuit be able to disconnect?
all ungrounded conductors
Is a magnetic motor starter a motor disconnect?
no
List the two basic control requirements for a motor branch circuit
disconnecting means, motor control
Can a non-time delay fuse provide both short circuit and overload protection?
no
A motor attempts to drive _____ load that is connected to it
any
Motor starting current is usually _____ than its full load current
several times higher
True or False?
Most motors are not damaged by a brief overload
true
Do breakers and non-time delay fuses protect against overcurrent protection?
yes
True or False?
A factory installed bimetal disk in a thermally protected motor protects the motor against short-circuit currents
false
The heater of an overload device is connected directly into the motor control circuit.
True or false?
true
Can a motor controller have a lower horsepower rating than the motor it controls?
no
