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AC Theory

Instructions:

Answer 50 questions in 15 minutes.
If you are not ready to take this test, you can study here.
Match each statement with the correct term.
Don't refresh. All questions and answers are randomly picked and ordered every time you load a test.

This is a study tool. The 3 wrong answers for each question are randomly chosen from answers to other questions. So, you might find at times the answers obvious, but you will see it re-enforces your understanding as you take the test each time.

1. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
The current will read zero.
15 feet off the ground.
They should be made by the same manufacturer.
The sum of the two capacitors.

2. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
Lower current and higher voltage
Alternating current.
Cut around clothing that has adhered to the burned area.

3. What should the duty rating of a grounding resistor be?
Cut around clothing that has adhered to the burned area.
Continuous.
Four times less than that of the secondary.
Tied together

4. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
The sum of the two capacitors.
At least two phases
Reverse any two-phase leads.
480 volts.

5. Not a symptom of electrical shock?.
Circuit Breaker.
Isolation transformer.
Inductance.
Hyperventilation.

6. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Short-circuited after power has been removed.
Stator windings.
They should be made by the same manufacturer.
277 volts.

7. Current in an AC circuit may be measured by...
Clamp-on ammeter.
Disconnect the frame-grounding conductor.
Cut around clothing that has adhered to the burned area.
No.6 AWG.

8. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Current that changes constantly in magnitude and periodically in direction.
No.6 AWG.
Full-load - three-phase voltage.
Larger than that found in the primary winding.

9. A transformer with 1 -500 turns in the primary winding has a primary voltage of 2 -400 volts and a secondary voltage of 120 volts. How many turns are in the secondary winding?
Hyperventilation.
75 turns.
Electro-magnetism for their operation.
Transformer

10. A control power transformer...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Clamp-on ammeter.
Tied together
Continuous.

11. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Stator windings.
H terminals.
277 volts.
Grounded phase and ground fault devices were not operating.

12. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Potential transformer.
600 volts.
A grounding resistor.
5:1.

13. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
600 volts.
Full-load - three-phase voltage.
Hyperventilation.
Four times less than that of the secondary.

14. What is the first step to be followed in aiding a victim of electrical shock?
Check to make sure that the victim is not still in contact with the electricity.
480 volts.
Second-degree burn.
12 to 15 times a minute.

15. AC voltage is raised or lowered by means of...
Transformers.
The capacitors should be discharged.
200 volts AC.
The current will read zero.

16. A wound-rotor induction motor has all the following EXCEPT...
Full-load - three-phase voltage.
Shunted and taped
Squirrel-cage rotor.
5:1.

17. The combined capacitance of two capacitors in parallel is...
Second-degree burn.
The sum of the two capacitors.
Disconnect the frame-grounding conductor.
A.C.

18. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
Grounded phase and ground fault devices were not operating.
Second-degree burn.
100 turns.

19. Not an acceptable method for the installations of surface transformers?
Lower current and higher voltage
5:1.
They must be installed at least 7 feet above the ground.
Derive a neutral for grounding purposes.

20. A solenoid coil is...
Derive a neutral for grounding purposes.
A step-up transformer.
An electromagnet.
Tied together

21. The most reliable method to use to determine whether or not the heart has stopped is by checking the...

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22. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
5:1.
H terminals.
The sum of the two capacitors.
Grounded phase and ground fault devices were not operating.

23. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Check to make sure that the victim is not still in contact with the electricity.
Circuit Breaker.
Grounded phase and ground fault devices were not operating.
X.

24. Compared to the secondary of a loaded - step-down transformer - the primary has...
Zero amps.
Lower current and higher voltage.
100 volts.
A step-up transformer.

25. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Inductance.
The motor rotation is reversed.
A step-up transformer.
150 turns.

26. Current that changes periodically in direction is...
Tied together
Alternating current.
The sum of the two capacitors.
The current will read zero.

27. The property of an electric circuit that opposes any change in current is called...
Inductance.
Circuit Breaker.
Continuous.
Cut around clothing that has adhered to the burned area.

28. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Four times less than that of the secondary.
Cut around clothing that has adhered to the burned area.
25 feet.
Allow for metering voltage and Current.

29. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Hyperventilation.
Derive a neutral for grounding purposes.
Short-circuited after power has been removed.
No.6 AWG.

30. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Shunted and taped
12 to 15 times a minute.
Two speeds
An electrical shock hazard may exist.

31. When not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
They must be installed at least 7 feet above the ground.
Provides single-phase power to smaller loads such as lights - heaters - etc.
100 turns.

32. A transformer has a primary voltage of 2 -800 volts and a secondary reading of 140 volts. If there are 2000 turns on the primary - how many turns are on the secondary?
Check to make sure that the victim is not still in contact with the electricity.
75 turns.
1 amp.
100 turns.

33. The primary function of a power transformer is to provide...
Second-degree burn.
Derive a neutral for grounding purposes.
It provides good speed control.
Full-load - three-phase voltage.

34. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
15 feet off the ground.
Disconnect the frame-grounding conductor.
Squirrel-cage induction motor.
H terminals.

35. Where should grounding resistors be located?
At the source transformer.
5:1.
Transformers.
60 Hertz

36. The secondary windings on a 10:1 step-down transformer will have a wire size...
Larger than that found in the primary winding.
Current that changes constantly in magnitude and periodically in direction.
They must be installed at least 7 feet above the ground.
Clamp-on ammeter.

37. The standard frequency of power supplied by the power company is...
X.
60 Hertz
60 cycles per second (60 Hertz).
Full-load - three-phase voltage.

38. What is a capacitor?
Disconnect the frame-grounding conductor.
Short-circuited after power has been removed.
A device used to store electrical energy.
Full-load - three-phase voltage.

39. The standard frequency of alternating current used at a mine is...
Circuit Breaker.
Alternating current.
60 Hertz
5:1.

40. The size of a transformer is measured in...
Kilovolt ampere (KVA).
Larger than that found in the primary winding.
Alternating current.
Two speeds

41. Find the turns ratio of a single-phase transformer with the following values: Primary power of 10 KW - secondary current of 200 amps - and primary voltage of 250 volts.
5:1.
An electrical shock hazard may exist.
Full-load - three-phase voltage.
Squirrel-cage rotor.

42. Current transformers - or CT's - are used to...
75 turns.
Short-circuited after power has been removed.
Draw less current.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

43. 460 volts AC is required to operate two motors - one connected in single-phase and the other connected in three-phase. The three-phase motor will...
The current will read zero.
Current changing constantly in magnitude and periodically in direction.
Draw less current.
100 turns.

44. High voltage is used to transmit power over long distances...
H terminals.
600 volts.
Because less current flowing at high voltage causes less line loss per kilowatt.
Allow for metering voltage and Current.

45. A transformer has 1 -600 turns in the primary and 400 turns in the secondary. The primary voltage is 2 -400 volts. What is the secondary voltage?
600 volts.
Squirrel-cage rotor.
Squirrel-cage induction motor.
15 feet off the ground.

46. The transformer used only to measure voltage is a...
It provides good speed control.
Cut around clothing that has adhered to the burned area.
Derive a neutral for grounding purposes.
Potential transformer.

47. Under fault conditions - the grounding resistor in a high voltage circuit limits the voltage drop in the grounding circuit external to the resistor to not more than...
Draw less current.
100 volts.
Isolation transformer.
25 feet.

48. Not true about AC power?
It provides good speed control.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
A grounding resistor.
Hyperventilation.

49. A burn causing a blister is classified as a...
Reverse its rotation.
Draw less current.
Disconnect the frame-grounding conductor.
Second-degree burn.

50. How do you change the direction of a 3-phase AC motor?
Electro-magnetism for their operation.
Lower current and higher voltage.
Stator windings.
Reverse any two-phase leads.