Electrical and mechanical fundamentals
Electrical and mechanical fundamentals
This domain tests basic electrical safety and machinery behavior that operators need before touching a breaker or starting a pump. Expect power calculations, protective device purpose, motor checks, and mechanical symptoms.
Core rules
- Electrical work requires lockout and tagout, verified isolation, and awareness of arc flash and stored energy. A breaker in the off position is not proof of a de-energized circuit until tested.
- For three-phase power, P equals the square root of 3 times V times I times power factor. Distinguish kW from kVA; the breaker and cable are sized for current, while the motor produces mechanical output.
- Mechanical equipment fails through lubrication loss, misalignment, cavitation, vibration, and valve packing problems. Check suction, discharge, coupling, and vibration after start.
- Do not bypass protective devices or interlocks to keep a unit running.
Worked example
A 480 V, three-phase motor draws 10 A at 0.8 power factor. Input power is the square root of 3 times 480 times 10 times 0.8, or about 6.65 kW. If the motor is 90 percent efficient, shaft power is about 5.99 kW. Use the input value for cable and breaker heating and the shaft value for driven equipment.
Traps
Using V times I for three-phase power; ignoring power factor; assuming a hot motor is bad without checking load and cooling; and starting a pump without confirming suction valve position.