Electrical Systems and Controls: Motors, Protection, and Safe Isolation
What this domain tests
Electrical questions test single- and three-phase fundamentals, motor starters and protection, control circuits, and safe work practices. Expect items on overloads versus short-circuit protection, contactor and relay logic, lockout, and interpreting nameplate data.
Core rules and concepts
- Ohm's law and power: V = IR; single-phase power = V x I x power factor; three-phase power = root-3 x V x I x power factor.
- Motor protection: overload protection protects the motor from sustained overcurrent and is set near full-load amps; short-circuit protection (fuses or breakers) clears faults. They are not interchangeable.
- Starters: direct-on-line, star-delta, soft start, and variable frequency drives. A VFD changes both voltage and frequency to control speed.
- Control circuits use contactors, relays, timers, and interlocks. A holding contact maintains a circuit after the start button is released; an interlock prevents unsafe simultaneous operation.
- Safe isolation: identify, isolate, lock out, prove dead with a tested instrument, and tag. Test the tester before and after use.
Worked example
A three-phase motor draws 30 A at 480 V with a power factor of 0.85. What is the input power, and where should the overload be set?
Reasoning: power = 1.732 x 480 x 30 x 0.85 = 21 200 W, about 21.2 kW. The overload is set to the motor nameplate full-load amps, typically 30 A or the next standard size, not to the calculated power. If the motor is service-factor 1.15, the overload may be set up to 1.15 x FLA only where permitted by the installation code; otherwise keep it at nameplate FLA.
Traps that cost marks
- Using the calculated current instead of the nameplate FLA for overload setting.
- Confusing overload protection (thermal, sustained) with short-circuit protection (magnetic, instantaneous).
- Forgetting the power factor in three-phase power calculations.
- Failing to prove dead with a tester proven on a known live source before and after.