Electrical Systems and Equipment
Electrical Systems and Equipment
This domain tests distribution equipment, transformers, switchgear, motors, starters, variable-frequency drives, generators, UPS systems, protection coordination, and grounding. Expect questions on equipment function and correct application rather than deep design.
Core rules
- Overload protection protects the motor and conductors from sustained excess current; short-circuit and ground-fault protection clear faults. They are not interchangeable.
- Transformers, generators, and UPS units have distinct grounding, inrush, fault-current, and coordination requirements.
- Switchgear and panelboards require correct interrupting ratings, series ratings, labelling, and working clearance.
- VFDs require suitable cable, grounding, shielding, and motor insulation considerations; output is not a simple sine wave.
Worked example
A 480 V three-phase motor trips on overload after running for several minutes. Check the actual current against nameplate full-load current and the overload setting, then inspect mechanical load, voltage balance, cooling, and connections. Do not simply increase the overload to stop tripping: it may be protecting the motor. A separate branch short-circuit protective device handles fault current, while the overload relay handles sustained overcurrent.
Traps
Using the overload relay as fault protection; ignoring transformer inrush; applying standard cable to VFD outputs without checking requirements; assuming an upstream breaker’s rating equals the equipment interrupting rating.