Basic Electrical and Instrumentation Knowledge for Maintenance
What this domain tests
Basic Electrical and Instrumentation Knowledge for Maintenance covers the electrical facts a mechanical technician needs: voltage, current, resistance, power, AC/DC, three-phase motors, starters, protection, earthing, and the sensors and instruments used to monitor machines.
Core rules
- Ohm's law: V = IR; power P = VI = I²R = V²/R. For three-phase motors, approximate line current I = P/(√3 × V × pf × efficiency).
- A clamp meter must clamp one conductor only; clamping a two-core cable reads near zero because currents cancel.
- Earthing gives a low-resistance path for fault current and stabilises voltage. Never work on a live circuit; isolate, lock, test and prove dead.
- Instruments: RTD (Pt100) for accurate temperature, thermocouple for higher range and faster response, pressure gauge or transmitter for pressure, limit switch and proximity sensor for position, vibration sensor for condition monitoring.
- A 4–20 mA loop uses 4 mA as live zero, so a broken wire reads below 4 mA and is distinguishable from a true zero.
Worked example
A 230 V single-phase heater draws 5 A. Its resistance is R = V/I = 230/5 = 46 Ω, and power is P = 230 × 5 = 1150 W. If a clamp meter reads 0 A on the supply cable but the heater is warm, suspect the meter is clamped around both conductors or is on the wrong range - not necessarily a dead circuit.
Traps
Measuring resistance on a live circuit; using a clamp meter on a multicore cable; assuming a motor is safe because the local switch is off; confusing RTD and thermocouple wiring; ignoring instrument calibration.