Electrical Engineer Interview Questions

Power factor (cos φ) represents the phase angle between voltage and current waveforms. Unity power factor (1.0) means voltage and current are perfectly in phase; lower power factors indicate reactive components (inductance from motors and transformers) that consume reactive power without doing useful work. Poor power factor (e.g., 0.7) requires larger cables to supply the same real power, increases transformer losses, and incurs penalties from utilities (reactive power charges). Industrial facilities improve power factor by installing capacitor banks that neutralise inductive reactance, bringing cos φ closer to 0.95. The practical impact: a 100 kW motor at 0.7 power factor requires the same cable capacity as a 140 kW motor at unity power factor. Utilities mandate minimum 0.95 power factor to reduce distribution losses and improve grid efficiency. As an electrical engineer, designing power factor correction is a key part of efficient industrial electrical system design.

Protection coordination ensures that if a fault occurs on a feeder, only the relay at the fault point trips, isolating that line while the rest of the network stays live. This requires careful setting of protective relays (overcurrent, earth fault, distance relays) with time-current characteristics that are selective—relays closer to the fault trip faster than upstream relays. Use relay coordination software or manual plotting of time-current curves to verify that settings are staggered appropriately (typically 0.2-0.3 seconds between relay operations). Consider different fault scenarios (three-phase faults, single-phase earth faults) as relay settings differ. During commissioning, test relay response with secondary injection (injecting scaled signals) to verify settings before full-voltage testing. Coordination also depends on system impedance and fault level, which may change if network topology changes (new generation, new loads). Ongoing monitoring of relay performance through fault record data helps optimise settings over time.

Wind and solar generation are variable and weather-dependent—they don't provide consistent baseload power like conventional power stations. High renewable penetration creates several grid challenges: (1) Reduced inertia—traditional synchronous generators provide stabilising inertia through spinning mass; inverter-based renewables don't, making grids more susceptible to frequency collapse if large generators trip suddenly. (2) Voltage support and reactive power—grids need sufficient reactive power reserves to maintain voltage stability; renewables provide limited reactive support unless equipped with advanced power electronics. (3) Forecast uncertainty—grid operators must maintain spinning reserve to cover renewable variability (clouds over solar farms, wind drops). (4) Curtailment losses—in periods of high renewable generation and low demand, grids must reduce renewables output, wasting energy and reducing returns on investment. Solutions include energy storage systems (batteries, pumped hydro) to buffer variability, demand-side response (flexible industrial loads, EV charging schedules) to match consumption to generation, and grid reinforcement to accommodate power flows from distributed renewables. The 2020s will be defined by engineers solving these integration challenges.

Power system stability is the ability of the grid to maintain steady operation after disturbances (large generator trip, sudden load change, faults). Synchronous machines (traditional power stations with spinning generators) provide several forms of stability: (1) Frequency support through inertia—when demand suddenly exceeds supply, spinning generators naturally slow down, releasing kinetic energy to prevent frequency collapse. (2) Voltage support through reactive power—generators automatically supply reactive power to maintain voltage when demand increases. (3) Damping oscillations—governors and exciters on synchronous machines actively damp power oscillations that would otherwise destabilise the grid. Inverter-based renewables (solar, wind) lack inherent inertia and reactive support unless equipped with advanced controls. High renewable penetration reduces the synchronous machine base, reducing the grid's natural stability margins. Solutions include synthetic inertia (fast-acting power electronics that mimic synchronous inertia), fast-frequency support from batteries, and grid-forming inverters that actively maintain voltage. Managing this transition from synchronous to inverter-dominated grids is one of the defining engineering challenges of the 2020s.

Arc flash is the sudden release of energy when an electric current arcs across a gap in an electrical circuit, creating a plasma column with temperatures exceeding 4000°C—hotter than the surface of the sun. The energy release causes severe burns, eye damage (arc flash blindness), and blast pressure that throws workers across rooms. Arc flash hazard is highest in systems with high available short-circuit current and long fault-clearing times. Electrical engineers assess arc flash risk using IEEE 1584 methods: calculate available short-circuit current at each location, determine protective relay clearing time, and estimate incident energy (calories per square centimetre). Results determine required personal protective equipment (PPE) category. Mitigation strategies include reducing short-circuit current (higher impedance transformers), faster fault clearing (lower relay settings, current-limiting fuses), or geographic barriers (zone boundaries, locked switchgear). Modern switchgear includes arc-flash mitigation features (quick-tripping, reduced clearance times). Arc flash assessment is mandatory in industrial electrical design; failure to properly assess and communicate arc flash hazards is a serious safety and legal liability.

Cable sizing involves selecting a conductor cross-section that safely carries the load current whilst keeping voltage drop within acceptable limits (typically 3-5% for distribution, 1-2% for final circuits). The process requires: (1) Calculate full-load current (I = P / (√3 × V × cos φ) for three-phase). (2) Select cable with ampacity (current-carrying capacity) rating that accommodates the load, considering installation method (in conduit, buried, free air) and ambient temperature. Standards like BS 7909 provide derating factors for various conditions. (3) Calculate voltage drop (V_drop = I × R, where R depends on conductor resistance and length) and verify it meets code limits. (4) Check short-circuit withstand—cables must survive the thermal and mechanical stress of short-circuit current without melting. (5) Consider future expansion—slightly oversizing cables can accommodate load growth. (6) Cost-benefit analysis—larger cables reduce loss and voltage drop but cost more; balance is project-specific. Tools like ETAP automate cable sizing, but understanding the fundamentals is essential for sound design decisions and troubleshooting inadequate cable designs.