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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 current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Carotid pulse - which is located in the neck next to the Adam's apple.
800 volts.
Grounded phase and ground fault devices were not operating.
The capacitors should be discharged.

2. What is the danger when working around capacitors?
An electrical shock hazard may exist.
100 volts.
Reverse its rotation.
Stator windings.

3. What value of voltage will be measured from phase-to-phase - in a wye-connected secondary - if 2 -300 volts is measured from phase-to-ground?
25 feet.
3 -979 Volts.
A grounding resistor.
480 volts.

4. To protect against over-current in a three-phase circuit over-current protection must be provided for...
They should be made by the same manufacturer.
200 volts AC.
At least two phases
Tied together

5. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Tied together
Capacitance.
25 feet.
75 turns.

6. 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.
Inductance.
5:1.
25 feet.
Clamp-on ammeter.

7. The size of a transformer is measured in...
Kilovolt ampere (KVA).
Short-circuited after power has been removed.
Check to make sure that the victim is not still in contact with the electricity.
A grounding resistor.

8. What position of a selector switch using a voltmeter would you select to check a 120-volt AC circuit to obtain the most accurate reading?
Zero amps.
200 volts AC.
Transformers.
Potential transformer.

9. Current that changes periodically in direction is...
A.C.
Inductance.
277 volts.
Alternating current.

10. Compared to the secondary of a loaded - step-down transformer - the primary has...
Cut around clothing that has adhered to the burned area.
Lower current and higher voltage.
Disconnect the frame-grounding conductor.
Reverse its rotation.

11. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
Nearly constant.
200 volts AC.
Disconnect the frame-grounding conductor.
The current will read zero.

12. A transformer with 3000 turns in the primary winding has a primary voltage of 4 -800 volts and a secondary voltage of 240 volts. How many turns are in the secondary winding?
277 volts.
A grounding resistor.
600 volts.
150 turns.

13. A burn causing a blister is classified as a...
Continuous.
75 turns.
No.6 AWG.
Second-degree burn.

14. Three-phase AC underground cables must be provided with overcurrent protection using a...
100 volts.
Second-degree burn.
At least two phases
Circuit Breaker.

15. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Because less current flowing at high voltage causes less line loss per kilowatt.
They should be made by the same manufacturer.
Nearly constant.
100 volts.

16. Compared to the secondary of a loaded - step-down transformer - the primary has...
H terminals.
Check to make sure that the victim is not still in contact with the electricity.
It provides good speed control.
Lower current and higher voltage

17. The standard frequency of alternating current used at a mine is...
Electro-magnetism for their operation.
They must be installed at least 7 feet above the ground.
60 Hertz
Reverse any two-phase leads.

18. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
At the source transformer.
480 volts.
Zero amps.
Grounded phase and ground fault devices were not operating.

19. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
600 volts.
Current changing constantly in magnitude and periodically in direction.
1 amp.
Continuous.

20. Not an acceptable method for the installations of surface transformers?
They must be installed at least 7 feet above the ground.
Reverse its rotation.
15 feet off the ground.
Check to make sure that the victim is not still in contact with the electricity.

21. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
Full-load - three-phase voltage.
600 volts.
5:1

22. 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?
Allow for metering voltage and Current.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
600 volts.
Transformer

23. Current transformers - or CT's - are used to...
Capacitance.
Derive a neutral for grounding purposes.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
200 volts AC.

24. The primary function of a power transformer is to provide...
Lower current and higher voltage
Full-load - three-phase voltage.
100 turns.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

25. A wound-rotor induction motor has all the following EXCEPT...
60 cycles per second (60 Hertz).
Because less current flowing at high voltage causes less line loss per kilowatt.
Stator windings.
Squirrel-cage rotor.

26. What is a capacitor?
A device used to store electrical energy.
Short-circuited after power has been removed.
480 volts.
The chest should rise or inflate.

27. Not true about AC power?
Zero amps.
Lower current and higher voltage.
Inductance.
It provides good speed control.

28. What type of 3-phase AC motor is used most often around a mine?
600 volts.
Squirrel-cage induction motor.
Carotid pulse - which is located in the neck next to the Adam's apple.
150 turns.

29. 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?
75 turns.
Check to make sure that the victim is not still in contact with the electricity.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
100 turns.

30. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
60 cycles per second (60 Hertz).
The current will read zero.
7 -630 volts.
A step-up transformer.

31. Two 10-hp induction motors are operating on 460 volts AC. One motor is connected by single- phase; the other is connected in three-phase. The three-phase will...
Draw less current.
Carotid pulse - which is located in the neck next to the Adam's apple.
An electromagnet.
Allow for metering voltage and Current.

32. Not a symptom of electrical shock?.
Hyperventilation.
12 to 15 times a minute.
The chest should rise or inflate.
1 amp.

33. What should the duty rating of a grounding resistor be?
2000 watts
Inductance.
Capacitance.
Continuous.

34. AC voltage is raised or lowered by means of...
Transformers.
Draw less current.
1 amp.
An electrical shock hazard may exist.

35. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
75 turns.
A device used to store electrical energy.
The chest should rise or inflate.
H terminals.

36. Find the turns ratio of a single-phase transformer with the following values: Primary power of 5KW - secondary current of 100 amps - and primary voltage of 250 volts.
5:1
X.
150 turns.
Squirrel-cage induction motor.

37. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
Two speeds
Grounded phase and ground fault devices were not operating.
Carotid pulse - which is located in the neck next to the Adam's apple.

38. All transformers step voltage up or down by...
Inductance.
Lower current and higher voltage.
Kilovolt ampere (KVA).
Four times less than that of the secondary.

39. At what rate should mouth-to-mouth resuscitation be applied?
277 volts.
At least two phases
12 to 15 times a minute.
Larger than that found in the primary winding.

40. The standard frequency of power supplied by the power company is...
480 volts.
Kilovolt ampere (KVA).
60 cycles per second (60 Hertz).
H terminals.

41. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Alternating current.
Because less current flowing at high voltage causes less line loss per kilowatt.
Derive a neutral for grounding purposes.
The motor rotation is reversed.

42. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Short-circuited after power has been removed.
75 turns.
X.
Electro-magnetism for their operation.

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

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44. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
An electrical shock hazard may exist.
2000 watts
The motor rotation is reversed.
5:1.

45. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
100 volts.
200 volts AC.
15 feet off the ground.
Two speeds

46. A transformer has 2000 turns in the primary and 500 turns in the secondary. The primary voltage is 3 -200 volts. What is the secondary voltage?
60 cycles per second (60 Hertz).
800 volts.
Alternating current.
X.

47. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Because less current flowing at high voltage causes less line loss per kilowatt.
Capacitance.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

48. The secondary windings on a 10:1 step-down transformer will have a wire size...
15 feet off the ground.
Larger than that found in the primary winding.
200 volts AC.
Capacitance.

49. How do you change the direction of a 3-phase AC motor?
60 cycles per second (60 Hertz).
Reverse any two-phase leads.
Short-circuited after power has been removed.
Lower current and higher voltage

50. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
7 -630 volts.
277 volts.
Inductance.
Squirrel-cage rotor.