Are you an electrical expert? If you can answer 15 or more of these 18 questions, you’re a “master”!
- What is a safety relay?
A safety relay is a device composed of multiple relays and circuits designed to complement each other’s potential faults. Its purpose is to achieve a complete, accurate, and low-error relay function through internal coordination. - What’s the difference between an electromagnetic relay and a solid-state relay?
The primary component in the control circuit of an electromagnetic relay is a coil, while in a solid-state relay, it’s an optocoupler—this is their key distinction. Electromagnetic relay coils have a nominal resistance, which serves as a reference during testing. Typically, the measured resistance should closely match the nominal value; too low indicates a short circuit, while too high suggests an open circuit. - How does a surge protector work?
A surge protector, essentially a lightning protection device, is a primary method for safeguarding weak electrical equipment from lightning. “Surge” refers to transient overvoltages or overcurrents in a circuit caused by lightning strikes, voltage fluctuations in the grid, electrostatic discharge, electromagnetic interference, or excessive potential differences. - How does a solenoid valve work?
A solenoid valve is an actuator that controls fluid media in pipelines. It consists of an electromagnetic coil that generates magnetic force when energized, overcoming the pressure or tension of a spring to drive a ferromagnetic component. This moves a piston within the valve body, changing its position to open or close the valve.
Three-wire staggered connection method:
The so-called staggered connection ensures that each joint is not at the same point, spaced apart as much as possible. First, cut two-thirds off one wire and connect it to another wire, securing it properly. Then, cut one-third off a second wire and connect it elsewhere, ensuring the joints are staggered across the wires. Finally, cut two-thirds off the third wire in the opposite direction and secure it. After completion, each joint should be about 2 cm apart, reducing the risk of overheating or short-circuiting. - What organizational measures ensure safety when working on high-voltage equipment?
① Work permit system; ② Work authorization system; ③ Work supervision system; ④ Work interruption and transfer system; ⑤ Work completion system. - What requirements must be met to de-energize equipment for maintenance?
① All power sources to the equipment must be completely disconnected; ② Working on equipment disconnected only by a switch is prohibited; ③ Isolation switches must be opened to ensure at least one clear disconnection point in all directions; ④ Transformers and voltage transformers related to the de-energized equipment must be disconnected on both high-voltage sides to prevent backfeeding. - What should you do if an instrument starts smoking?
Instrument smoking is typically caused by overload, reduced insulation, excessive voltage, degraded resistance, or loose current connections leading to poor contact or open circuits. Upon discovery, quickly short-circuit the meter and its circuit, disconnect the voltage circuit, and take care not to short the voltage coil or open the current circuit during operation. This prevents protection mismatches or human errors like accidental contact. - What are the effects of excessively high or low DC bus voltage?
When DC bus voltage is too high, continuously energized components like relays, indicators, or meters can overheat and fail. When too low, protective devices may malfunction or fail to operate. Generally, the allowable voltage variation range is ±10%. - Can AC and DC circuits share the same cable?
In simple terms, AC and DC circuits should not share a cable. The main reasons are: AC and DC are independent systems, and sharing a cable causes mutual interference, reducing insulation resistance for DC. Additionally, DC systems are insulated, while AC systems are grounded, increasing the risk of short circuits. Thus, they cannot share a cable. - Why is short-circuiting prohibited on the secondary side of a PT (potential transformer) during operation?
The secondary load of a PT is akin to a transformer in a no-load state. With its small impedance and limited capacity, a short circuit on the secondary side—without a fuse blowing—can easily burn out the PT. - When does a motor require reduced-voltage starting?
Reduced-voltage starting is needed when: the motor power exceeds 7.5 kW; the motor power is greater than 20% of the transformer capacity; frequent starts cause a voltage drop exceeding 10%; or occasional starts cause a voltage drop exceeding 15%. - What should you do if a motor’s bearing temperature is too high during operation?
Causes may include unsuitable grease, too much or too little grease in the bearing housing, debris in the grease, bearing misalignment (inner or outer race), excessive motor vibration, incorrect bearing type, or misaligned coupling. - What’s the relationship between phase/line voltage and current for three-phase power with delta- and star-connected loads?
- Delta connection: Line voltage equals phase voltage; line current is √3 times the phase current and lags the corresponding phase current by 30°.
- Star connection: Line voltage is √3 times the phase voltage and leads the corresponding phase voltage by 30°; line current equals phase current.
- What are the grounding requirements for a 10 kV substation or distribution station?
Grounding is required for metal enclosures of transformers, switchgear, and mutual inductors (PTs, CTs), as well as distribution cabinets, control panels, metal frames, lightning protection devices, cable heads, and metal barriers. Specific requirements include:
① Indoor angle steel foundations and supports must be connected with flat steel (minimum 25×4 mm²) as a grounding bus, then extended outdoors to connect with the external grounding system;
② Grounding electrodes should be placed at least 3 meters from the substation walls, with a length of 2.5 meters and a spacing of 5 meters between electrodes;
③ A closed-loop grounding grid is preferred; if resistance requirements aren’t met, additional external grounding electrodes can be added;
④ The total grounding resistance of the grid must not exceed 4 ohms. - Why is an open circuit prohibited on the secondary side of a CT (current transformer) during operation?
CTs are often used in high-current conditions. If the secondary circuit opens, the CT core becomes severely saturated, with flux density exceeding 1,500 gauss. Since the secondary coil has far more turns than the primary, an extremely high voltage—many times the normal value—is induced across the secondary terminals. This poses significant danger to all secondary circuit equipment and personnel. Additionally, an open secondary can overheat the core due to saturation, potentially causing burnout. - What is overcurrent protection, and how does it work?
When a short circuit occurs, a key characteristic is a sharp increase in line current. Overcurrent protection is a device that reacts to this rise and trips when the current exceeds a predetermined value. The tripping current is set based on maximum load current, and coordination is achieved through stepped time delays. - What causes shaft voltage in a generator?
① An unbalanced stator magnetic field induces voltage on the generator shaft, often due to localized high magnetic reluctance in the stator core (e.g., rust) or uneven air gaps between the stator and rotor.
② In steam turbine generators, poor shaft seals, high-speed steam leakage along the shaft, or high-velocity steam injection in the cylinder can cause the shaft to carry static charges.
These 18 foundational electrical questions separate the novices from the experts: knowing half makes you a beginner, 15 or more makes you a “master,” and knowing all lets you “rule the roost” at the company! Veteran electricians, where do you stand?