42 Questions Every Qualified Electrician Should Know

Electricians work in many fields, such as maintenance, operations, installation, and commissioning. Whatever the specialty, the fundamentals of electrical work remain the same. Do you still remember them?

Q&A Time

1. What are electric charge, current, voltage, resistance, a circuit, and electric power?

Electric charge — The quantity of charge carried by an object is called its electric charge. Its unit is the coulomb.

Current — The directed movement of electric charge through a conductor forms an electric current. Its unit is the ampere, abbreviated A. If one coulomb of charge passes through the cross-section of a conductor in one second, the current is one ampere.

Voltage — The potential difference between two points in a circuit is called voltage. Its unit is the volt.

Resistance — The opposition to the flow of current in a conductor is called resistance. Its unit is the ohm.

Circuit — The path through which current flows is called a circuit. A circuit consists of four basic parts: a voltage source, electrical devices, a switch, and connecting wires. The power source generates electrical energy; electrical devices include light bulbs and electric fans; and the connecting wires connect the power source, devices, and switch and must form a closed loop.

Electric power — This indicates how much work electricity does per unit time. It is abbreviated as power, and its unit is the watt. Consuming 1 kilowatt of electrical energy in 1 hour is called “1 unit of electricity.”

2. What is a safe voltage?

A voltage that causes no harm to any part of the human body when a person comes into contact with electricity is a safe voltage.

3. What are the dangers of an unbalanced three-phase load?

An unbalanced three-phase voltage produces a negative-sequence rotating magnetic field in the stator of an induction motor. The motor then operates under the influence of both positive- and negative-sequence magnetic fields. Because the positive-sequence rotating magnetic field is much stronger than the negative-sequence field, the motor rotates in the positive-sequence direction. However, the rotor’s negative-sequence impedance is very small, so the negative-sequence current is relatively large. The negative-sequence current and magnetic field produce a large braking torque, reducing the motor’s power.

4. What does “three-electricity” mean?

It means planned use of electricity, conservation of electricity, and safe use of electricity.

5. How does star-delta (Y-△) reduced-voltage starting work?

For a motor rated at 380V that operates with a delta connection, the stator windings can first be connected in a star during startup and then switched to a delta connection once the motor has started. This method is called star-delta reduced-voltage starting.

6. What is meant by “earth”?

When an electrical device develops a ground fault, the fault current flows through the grounding conductor and grounding electrode into the earth, spreading outward in a hemispherical pattern. Near the grounding electrode the hemisphere has a smaller surface area and higher resistance, so the voltage drop as the grounding current passes through it is greater and its potential is higher. Farther from the electrode, the hemisphere has a larger surface area and lower resistance, so the voltage drop is smaller and the potential is lower. At a spherical radius of approximately 20m centered on the grounding electrode, the potential approaches zero in practice; this is generally called earth or ground.

7. What is a grounding electrode?

A metal conductor, or group of metal conductors, in direct contact with the soil is called a grounding electrode or grounding-electrode group.

8. What are a grounding conductor and a grounding system?

A metal conductor connecting the part of electrical equipment that must be grounded to the grounding electrode is called a grounding conductor. The grounding electrode and grounding conductor together are called the grounding system.

9. What is protective neutral grounding?

It is the connection of the non-current-carrying metal parts of electrical equipment to the neutral conductor of the power grid.

10. What is the purpose of protective neutral grounding?

The grounded neutral point of a transformer or generator is called the neutral point, and the conductor connected to it is called the neutral conductor. Protective neutral grounding connects the metal enclosure of electrical equipment—which is normally not energized—to the neutral conductor. In supply systems below 1000V in which the neutral point is directly grounded, protective neutral grounding is generally used instead of protective earthing.

11. What are the key requirements for neutral grounding?

The following requirements must be met:

(1) In the same system, it is strictly prohibited to ground some electrical equipment while connecting other electrical equipment to neutral; this is extremely unsafe.

(2) Switches and fuses must not be installed on the neutral conductor of a three-phase, four-wire system.

(3) Pay attention to the quality of the neutral-conductor installation. To guard against a broken neutral conductor, install a sufficient number of repeated grounding points. The grounding resistance at repeated grounding points must not exceed 10 ohms.

(4) The neutral conductors of all electrical equipment must be connected in parallel to the neutral trunk or branch conductor.

(5) The cross-sectional area of the neutral conductor must be at least 1/2 that of the phase conductor.

12. What is a residual-current protective device?

When a single-phase ground-fault short circuit occurs in an electrical line or device, residual current is generated. A protective device that uses this residual current to disconnect the power supply to the faulty line or device is called a residual-current-operated protective device, commonly known as a leakage-current protector.

13. What are the three levels of protection commonly used for leakage-current protection?

Overcurrent, grounding, and short-circuit protection.

14. What causes leakage current, and what are its consequences?

Causes: insulation damage (including aging, environmental factors, mechanical damage, and toxic gases).

Consequences: stray current is generated, losses occur, and line losses increase.

15. What causes leakage current in lighting circuits, and what are the methods and steps for checking it?

There are two paths for leakage current in a lighting circuit:

One is damage to the insulation resistance between the phase and neutral conductors, causing leakage between them.

The other is damage to the insulation between the phase conductor and earth, causing leakage between the phase conductor and earth.

The inspection can be carried out as follows:

(1) First determine whether there is leakage. Use a megohmmeter to measure the insulation resistance, or connect an ammeter at the main switch of the building under inspection, remove all bulbs, and turn on all light switches. If the ammeter needle moves, leakage is present.

(2) Determine whether the leakage is between the phase and neutral conductors, between the phase conductor and earth, or both. For example, when checking with an ammeter connected in the circuit, disconnect the neutral conductor and observe the current. If the ammeter reading does not change, there is leakage between the phase conductor and earth. If the ammeter reads zero, there is leakage between the phase and neutral conductors. If the reading decreases but does not reach zero, there is leakage between the phase and neutral conductors and between the phase conductor and earth.

(3) Determine the extent of the leakage. Remove the branch circuit breaker or open the knife switch. If the ammeter reading does not change, the leakage is on the main line; if the reading decreases but does not reach zero, there is leakage on both the main line and the branches.

(4) Find the leakage point by following the branch circuits, opening switches, and checking each section in turn; locate and eliminate the leakage point.

16. What are the methods of leakage-current protection?

Overall protection for the entire network, meaning that a protective device is installed at the low-voltage network power source;

final-stage protection for mobile electrical equipment, temporary electrical equipment, and household electrical installations;

multi-level protection for larger low-voltage networks.

All three methods automatically disconnect the power supply when the leakage-current protector operates.

17. What does electrical safety equipment include? What are its specific items?

Electrical safety equipment is classified by function into:

operating equipment and protective equipment.

Operating equipment includes voltage detectors, insulating rods, and insulating clamps.

Protective equipment includes insulating gloves, insulating boots, insulating mats, insulating platforms, barriers, and safety equipment for work at height.

18. What are the hazards of static electricity?

Explosions and fires, electric shock, and interference with production.

19. What methods are available to eliminate static-electricity hazards?

Grounding, leakage, neutralization, and process control.

20. What is a short circuit, and what can it cause?

A short circuit occurs when two power-supply conductors touch or come into direct contact without passing through a load.

A short circuit can overheat electrical equipment or even burn it out and cause a fire. The short-circuit current also produces a large electrodynamic force that can damage electrical equipment. A serious short-circuit accident can destabilize the system and waste electrical energy.

21. How are safe voltage values specified?

In China, safe voltage values are specified according to the particular environmental conditions:

65 volts in buildings without a high risk of electric shock;

36 volts in buildings with a high risk of electric shock;

12 volts in buildings with a particularly high risk of electric shock.

22. What are the safety requirements for grounding and neutral grounding?

(1) Electrical continuity;

(2) Reliable connections;

(3) Adequate mechanical strength;

(4) Adequate conductivity and thermal stability;

(5) Repeated grounding of the neutral conductor;

(6) Corrosion prevention;

(7) Protection against mechanical damage;

(8) Compliance with relevant requirements for underground installation distances;

(9) Grounding and neutral conductors must not be connected in series.

23. What does ensuring the normal operation of a transformer involve?

It includes checking whether the current, voltage, power factor, oil level, oil color, temperature readings, and contact condition are normal; checking equipment and lines for damage or heavy contamination; checking that auxiliary facilities such as doors, windows, and fences are intact; listening for normal operating sounds and any discharge noise; and checking for a burnt smell or other abnormal odors.

24. What is guarding?

Guarding is the use of protective devices to control unsafe factors—for example, using barriers, guards, covers, or enclosures to isolate live parts from the outside.

25. What are the main functions of transformer oil?

The main functions of transformer oil are cooling and insulation.

26. What is the function of an automatic air circuit breaker?

An automatic air circuit breaker, also called an automatic switch, is an important device for making and breaking circuits in a low-voltage distribution network. It provides time-delayed overcurrent protection, instantaneous short-circuit protection, and undervoltage protection.

27. What equipment is included among high-voltage electrical equipment?

High-voltage electrical equipment mainly includes high-voltage fuses, high-voltage disconnectors, high-voltage load switches, high-voltage circuit breakers, high-voltage switchgear, and power transformers.

28. What is the operating principle of an induction motor?

An induction motor operates using a magnetic field. When three-phase AC is supplied to the stator windings, the three-phase currents in the stator windings continuously change, and the magnetic field they produce continuously rotates in space. As the magnetic field rotates, the rotor conductors cut the magnetic flux lines. Since the two ends of all rotor conductors are connected together by two copper rings, they form a closed circuit. This produces rotor current. The current experiences electromagnetic force in the rotating magnetic field and produces electromagnetic torque on the shaft, causing the rotor to turn in the direction of the rotating magnetic field.

29. Why is it important for the phase conductor to go to the switch?

When installing lighting circuits, the switch must control the phase conductor. This ensures that when the switch is off, the lamp holder is not energized, preventing electric shock when replacing a bulb or repairing the lamp holder.

30. What causes a motor to overheat severely?

(1) The power-grid voltage is too low or too high, the driven machinery is jammed, or lubrication is poor;

(2) Motor ventilation is poor or the fan cover is clogged with lint and debris;

(3) A rotor bar is broken;

(4) Forward/reverse operation is too frequent or the motor is started too many times;

(5) The rotor rubs against the stator;

(6) There is a localized short circuit in the stator winding or a localized ground fault in the stator winding.

31. What insulating tools are commonly used?

Insulating rods, insulating clamps, rubber insulating gloves and boots, rubber mats, and insulating platforms.

32. What is the characteristic of a diode?

A diode conducts in one direction only.

33. What are common lightning-protection devices?

Lightning rods, overhead ground wires, lightning-protection meshes, lightning-protection strips, and surge arresters.

34. What are the insulation classes of motors, and what are their respective temperatures?

The insulation classes are Class A, Class E, Class B, Class F, and Class H.

Class A: maximum permissible temperature 105℃, temperature rise 60℃

Class E: maximum permissible temperature 120℃, temperature rise 75℃

Class B: maximum permissible temperature 130℃, temperature rise 80℃

Class F: maximum permissible temperature 155℃, temperature rise 100℃

Class H: maximum permissible temperature 180℃, temperature rise 125℃

35. What are the braking methods for a three-phase induction motor?

(1) Mechanical braking; (2) plugging (reverse-current braking); (3) dynamic braking.

36. What are the requirements for lubricating motor bearings?

For a motor running at 3000 revolutions/minute, fill approximately 2/3, but slightly less;

for a motor running at 1500 revolutions/minute, fill approximately 2/3;

for motors running above 1500 revolutions/minute, generally use calcium-sodium-base grease, a high-speed grease;

for low-speed motors running below 1000 revolutions/minute, generally use calcium-sodium-base grease.

37. What is the difference between DC and AC?

Direct current, abbreviated DC, is current whose direction does not change over time. In a circuit with constant resistance, a constant-voltage source produces current whose magnitude and direction are both constant. Alternating current, abbreviated AC, is current whose magnitude and direction change periodically over time.

38. What is the load factor, and how can it be improved?

The load factor is the percentage ratio of the average active load to the maximum active load over a given period. It measures the difference between the average load and the maximum load. To improve the load factor, the main measures are to reduce peak load and increase average load.

39. What causes abnormal noise during motor operation?

Electrical causes:

(1) Winding ground fault or phase-to-phase short circuit;

(2) Inter-turn short circuit in a winding;

(3) Incorrect polarity of a winding or some of its coils;

(4) Single-phasing operation.

Mechanical causes:

(1) The fan blades touch the end cover;

(2) The bearing is badly worn or a rolling element is damaged;

(3) The bearing inner ring is not securely fitted to the shaft;

(4) Rotor-stator rubbing.

40. What should be done if a motor cannot be started by direct starting?

(1) Use a star-delta starter;

(2) Use an autotransformer starter;

(3) Use resistance or reactance reduced-voltage starting.

41. What is transformer utilization?

It is the ratio of the actual output power of an operating transformer to its rated output power.

42. How can the power factor be improved?

Use natural adjustment:

(1) Minimize the installed capacity of idle transformers and motors;

(2) Increase equipment utilization and reduce no-load operation;

(3) Change delta-connected motors to star connection. For manual adjustment, installing capacitors is the most economical and effective method.

Jiangsu Xinya High Voltage Test Equipment Co., Ltd. is a high-tech enterprise specializing in the R&D, production, and sales of high-voltage test equipment. It has been recognized as a “National High-Tech Enterprise,” is a member of the Testing Committee of the Chinese Society for Electrical Engineering, and was one of the earliest manufacturing enterprises to participate in drafting the national electric power industry standard “Partial Discharge Detector” (DL/T846.4 - 2004). The company holds 2 authorized invention patents, 4 software copyrights, and 23 utility model patents; has 7 provincial high-tech products; has obtained ISO9001:2015 quality management, ISO14001:2015 environmental management, and ISO45001:2018 occupational health management system certifications; and was approved for the 2017 “Yangzhou Lvyang Jinfeng” talent program.