79 Key Professional Knowledge Areas in Electrical Engineering （71-79）

By clyde March 10th, 2025 14 views

71.Why does an AC contactor overheat when operated frequently?

When an AC contactor starts, the large air gap between the core and armature results in low electrical reactance, causing a high magnetizing current to flow through the coil. This current can be many times greater than the working current. Frequent starting causes the coil to repeatedly carry this high starting current, leading to overheating of the coil. In severe cases, this can result in the coil being burned out.

72.What are the regulations for control cables introduced into panels or cabinets?

① Cables introduced into panels or cabinets should be neatly arranged without crossing and must be securely fastened so that no stress is applied to the terminal boards.
② Armored or lead-sheathed cables should not enter the panel or cabinet directly; the steel tape should be tightly bound at the cut-off point.
③ For control cables used in transistor protection or control systems, if shielded cables are used, their shielding must be grounded. If non-shielded cables are used, one spare core must be grounded.
④ Rubber-insulated wires should be protected by additional insulating tubing.
⑤ Cable cores within panels or cabinets should be arranged horizontally or vertically without crossing, with appropriate length reserved for spare cores.

73.What are the rules for threading cables through conduit?

When laying cables and threading them through conduits, the following rules apply:
① Armored cables and lead-sheathed cables must not be run through the same conduit.
② A single conduit should only accommodate one power cable.
③ Power cables and control cables must not be run through the same conduit.
④ When threading bare lead-sheathed cables through conduits, the cable entry section should be protected with hemp or other soft materials, and excessive force during insertion should be avoided.

74.How are rigid busbars connected?

Rigid busbars are typically connected using either compression joints or welding. Compression joints use screws to clamp the busbars together, facilitating modification and disassembly. Welding uses electric or gas welding for connections, commonly used in applications where disassembly is not required. Soldering or binding is prohibited.

75.Under what circumstances should cables be installed in conduit? How is the conduit diameter selected?

Conduit protection for cables is required in the following situations:
① Where cables enter or exit buildings, tunnels, floors, or major walls.
② Where cables emerge above ground up to 2 meters and extend underground to 0.25 meters.
① Where cables cross or approach underground pipelines at distances that do not meet code requirements.
④ Where cables cross roads, tram tracks, or railways.
⑤ In industrial areas where mechanical damage or pedestrian access is possible.
When selecting conduit diameter, the inner diameter should be 50% larger than the outer diameter of the cable.

76.What are the phase sequence arrangements and color codes for busbars?

The phase sequence arrangement of busbars (as observed from the front of the equipment) follows these principles:

Left-to-right: Left = Phase A, Middle = Phase B, Right = Phase C.
Top-to-bottom: Top = Phase A, Middle = Phase B, Bottom = Phase C.
Far-to-near: Farthest = Phase A, Middle = Phase B, Nearest = Phase C.
Color coding:
Phase A: Yellow
Phase B: Green
Phase C: Red
Neutral (unearthed): Purple
Positive pole: Brown
Negative pole: Blue
Grounding wire: Black

77.Why are two blades used per pole in high-voltage disconnect switches?

According to electromagnetic principles, when two parallel conductors carry current in the same direction, they generate an electromagnetic force that pulls them closer together. The magnitude of this force depends on the distance between the conductors and the current flowing through them. During faults, large currents may flow through the blades, causing them to press tightly against the fixed contacts with significant pressure. This prevents the blades from vibrating loose and avoids the risk of accidents. Additionally, the electromagnetic force ensures tight contact between the moving blades (contacts) and fixed contacts, reducing contact resistance and preventing melting or welding of the contacts due to fault currents.

78.What type of electrical equipment should be installed in explosive hazard areas?
Electrical junction boxes and pull-boxes in explosive environments must comply with the following:
① In Q-1 and G-1 zones, except for intrinsically safe circuits, explosion-proof types must be used.
② In Q-2 zones, except for intrinsically safe circuits, any type of explosion-proof equipment can be used.
③ In Q-3 and G-2 zones, dustproof types can be used.
④ In Q-1 and Q-2 zones, except for intrinsically safe circuits, the class and group of junction boxes and pull-boxes must match or exceed the class and group of the explosive mixture in the area.
⑤ Intrinsically safe circuits in all explosive zones can use dustproof types.

79.What are the regulations for installing explosion-proof flexible conduit?

Explosion-proof flexible conduit should have no cracks, holes, mechanical damage, deformation, or other defects. Installation must comply with the following:
① In Q-1 and G-1 zones, explosion-proof types must be used. In Q-2 zones, explosion-proof safety types can be used. In other explosive zones, dustproof types can be used.
② In Q-1 and Q-2 zones, the class and group of the explosion-proof flexible conduit must match or exceed the class and group of the explosive mixture in the area.
③ Ambient temperature should not exceed ±40°C.
④ The bending radius should not be less than five times the conduit diameter.

79 Key Professional Knowledge Areas in Electrical Engineering （61-70）

Types and functional characteristics of transformers