Control, Protection and Switchgear
What this domain tests
Control, Protection and Switchgear tests fuses, MCBs, MCCBs, ELCBs/RCCBs, contactors, overload relays, relays and interlocking. The exam expects correct device selection and understanding of what each device does and does not protect.
Core rules
- A fuse and MCB protect against overload and short circuit; an RCCB protects against earth leakage and does not replace overload protection.
- A contactor switches load repeatedly; an overload relay senses sustained excess current; a relay can provide control or protection logic.
- MCB types B, C and D have different magnetic trip ranges; choose for inrush. A motor circuit needs coordination between starter, overload and short-circuit protection.
- Interlocks prevent unsafe simultaneous operation, such as forward and reverse contactors being energised together.
Worked example
A 7.5 kW, 400 V three-phase motor has pf 0.85 and efficiency 0.88. Line current I = 7500 / (√3 × 400 × 0.85 × 0.88) = 14.5 A. An overload relay set around 16 A allows normal starting but trips on sustained overload. The contactor must have an AC-3 rating at least equal to the motor’s rated current, and the short-circuit protection must be coordinated with it.
Traps that cost marks
Using an RCCB as the only protection, setting an overload relay to the starting current, choosing a contactor by thermal current only without AC-3 rating, and ignoring discrimination so the main breaker trips before the branch device. Never bypass an interlock to “make it work”.