Flashcards in Test Review Deck (46)
Why is speed a desirable quality to have in opening circuit breakers?
-facilitates extinguishing an arc
-minimizes damage caused by a fault
-reduces effects of a fault on lines and equipment not directly involved with the fault
What is an isolation transformer?
A transformer that is magnetically coupled, but electrically and mechanically isolated.
What are harmonics? When do they occur?
Spikes of frequency that occur at multiples of 60 Hz
What must be known about a transformer to determine the different values of voltage and current?
What part of the transformer is connected to the load and what part is connected to the power supply?
The secondary winding is connected to the load and the primary winding is connected to the power supply
True or False:
In a step down transformer, the power in does not equal the power out.
What is a transformer?
A magnetically operated machine that can change voltage, current, and impedance without changing frequency
How are transformers rated?
Name the three basic contacts of a PCB?
Butt, wedge, and bayonet
What is the meaning of electrically trip free regarding a PCB?
The PCB will not allow the closing coil to close under a fault condition.
What is the most common gas used in circuit breakers to quench an arc?
Sulfur hexafluoride (SF6)
What is continuous current?
The amount of current that a circuit breaker can carry continuously
What is interrupting time?
The time it takes a circuit breaker to open after the tripping coil is energized
How many transformers make an open delta connection?
What is the frequency of the second harmonic?
What is a triplen harmonic?
Odd multiple of the third harmonic
What is the primary function of a circuit breaker?
A transformer has a primary voltage of 240 V, a secondary voltage of 180 V, and the windings are connected subtractively. What voltage will appear across the entire connection?
240 V - 180 V = 60 V
What is another name for an arc?
Two transformers rated at 100 kVA each are connected in an open delta connection. What is the total output power supplied?
.866(100kVA + 100kVA) = 173.2 kVA
1.732(100 kVA) = 173.2 kVA
What is a knife switch?
The simplest form of a circuit breaker
How many seconds does it take for a circuit breaker to open?
1/20 of a second
A transformer has a primary voltage of 240 V and a secondary voltage of 80 V. What is the turn ratio?
Np/Ns = 240V/80V
Np/Ns = 3/1
How are circuit breakers rated?
-interrupting current capacity
-continuous current capacity
What are the three mediums that extinguish the arc in circuit breakers?
Air, oil, and SF6
A transformer has a turn ratio of 1:6 and a primary current of 18 A. What is the secondary current?
Np/Ns = Is/Ip
1/6 = Is/18
Is= 3 A
True or False:
Phase voltage and line voltage are 90 degrees out of phase on a three phase system.
They are 120 degrees out of phase
What two things are important about a wye connected three phase system?
Line current equals phase current
E(Line) = E(Phase) x 1.732
What two things are important about a delta connected three phase system?
Line voltage equals phase voltage
I(Line) = I(Phase) x 1.732