Test Review Flashcards
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?
Turn ratio
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.
False
What is a transformer?
A magnetically operated machine that can change voltage, current, and impedance without changing frequency
How are transformers rated?
kVA
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?
Two
What is the frequency of the second harmonic?
120 Hz
What is a triplen harmonic?
Odd multiple of the third harmonic
What is the primary function of a circuit breaker?
Protection
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?
Plasma
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
Or
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
3:1
How are circuit breakers rated?
- interrupting current capacity
- continuous current capacity
- voltage class
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.
False
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
True or False:
Voltage, current, frequency, and impedance are things that can be changed by a transformer.
False
Frequency can’t be changed
In a transformer with a power output of 750 VA and a primary voltage of 120 V, what is the primary current?
P = I x E
750 VA = I x 120V
I = 6.25 A
What is the formula to find the phase voltage if given line voltage in a three phase wye connected system?
E(phase) = E(line) / 1.732
An open delta three phase transformer system has one transformer center tapped to provide a neutral for single phase voltages. If the voltage from the line to center tap is 277 V, what is the high leg?
Show the math.
277 V x 1.732 = 479.76 V
What kind of transformer uses one winding for both the primary and secondary windings?
An autotransformer
What is mutual inductance?
When a coil induces voltage into another coil
A transformer has a turn ratio of 2.5:1 and a primary voltage of 220 V. What is the secondary voltage?
Np/Ns = Ep/Es 2.5/1 = 220/Es Es(2.5) = 220(1) Es = 220/2.5 Es = 88 V
A transformer has a primary voltage of 240 V, apparent power of 360 VA, and a secondary voltage of 48 V. What is the secondary current?
P = Is x Es
360 VA = Is x 48 V
Is = 7.5 A
All values of a transformer are proportional to what?
Turn ratio of the windings
What is self-induction?
When a coil induces voltage onto itself
What is the formula to find primary current in a transformer?
Np/Ns = Is/Ip = Ep/Es
Name three things that dielectric oil does in a transformer.
Insulates, cools, and moisture prevention
What is the disadvantage of SF6 in circuit breakers?
It’s corrosive
What is the disadvantage of oil circuit breakers?
They are explosive
How can a breaker be tripped free?
Electrically, mechanically, and pneumatically
Why are heaters used with SF6 breakers?
It liquifies readily
What 8 things are on a transformer nameplate?
- Name of manufacturer
- Rated kilovolt-ampere
- Frequency
- Primary and secondary voltage
- Impedance of transformers rated 25kVA and larger
- Required clearances of transformers with ventilated openings
- Amount and kind of insulating liquid where used
- The temperature class for the insulating system of dry-type transformers
