Which statement correctly describes the typical uses of 30 mA, 100 mA, and RCBO protective devices?

• A. 30 mA RCD for personal protection; 100 mA for additional protection in certain circuits; RCBO combines residual current and overcurrent protection.
• B. 30 mA for overcurrent protection; 100 mA for personal protection; RCBO only handles surge protection.
• C. 30 mA for energy efficiency; 100 mA for lighting; RCBO for isolation.
• D. 30 mA reduces voltage; 100 mA increases current; RCBO reduces impedance.

Answer: (D)

Protective devices are all about fault detection and rapid disconnection to keep people and wiring safe. A residual current device rated at 30 mA is chosen for personal protection because it detects even small leakage currents that could pass through a person and trips quickly to interrupt the circuit, reducing the risk of electric shock. A 100 mA residual current device is used in some circuits as additional protection in situations where a slightly higher leakage threshold is acceptable or could avoid nuisance trips in specific installations. An RCBO combines residual current protection with overcurrent protection, so it can trip not only for earth leakage but also if a fault causes an overload or short circuit.

The statement about reducing voltage, increasing current, or reducing impedance doesn’t reflect how these devices work. They don’t regulate or change the circuit’s voltage or current; they monitor for fault conditions and disconnect the supply when those conditions are met to protect people and equipment.