What Is The Difference Between RCD And MCB?

There are significant differences between Residual Current Devices (RCDs) and Miniature Circuit Breakers (MCB) in function, object of protection, working principle, application scenarios and structure composition. Specific differences are as follows:
1.Functional positioning
RCD: Focuses on leakage current protection by detecting the residual current in the circuit (i.e., the difference between the current of the live and neutral lines) to determine whether a leakage has occurred. When the remaining current exceeds a set value (e.g., 30mA), it quickly cuts the power supply to prevent electrocution or electrical fire.
Core Functions: personal safety protection, electrical fire prevention.
MCB: Focuses on overload and short-circuit protection by measuring the size and duration of the current to determine if a fault has occurred.
Overload Protection: When the current exceeds the rated value for a long time, the bimetal band bends under the heat, triggering a trip.
Short-circuit protection: When the current momentarily exceeds the rated value, the electromagnetic coil produces a strong magnetic field, causing the tripping mechanism to trip.
Core Functions: circuit and equipment protection.
2.Protected objects
RCD: Protects personal safety for high-risk situations such as wet spaces (such as kitchens and bathrooms), mobile devices and outdoor lighting.
For example, when the insulation fails, causing current to leak to the ground, the radar station immediately cuts off the power supply to prevent electrocution.
Protection of wiring and equipment for all circuit branches against fire or equipment damage caused by overload or short circuit.
For example, when multiple high-powered appliances are used at the same time, causing the line to overload, the MCB jumps and cuts the power supply.
3. Principles of work
Zero-sequence current transformer: both live and neutral lines pass through a circular iron core. Normally, the current is equal and the magnetic fields cancel each other out. When a leakage occurs, an unbalanced magnetic field is created, triggering the tripping mechanism.
Speed of action: Millisecond level (typically ≤0.3 seconds), fast power supply cut.
MCB: A bimetallic strip bend when heated, propelling the tripping mechanism (overload protection).
Electromagnetic Tripwire: The electromagnetic coil induces a short-circuit circuit current that creates a strong magnetic field that immediately attracts the armature and hits the tripwire (short-circuit protection).
Speed of action: Overload protection requires a certain amount of time (long delay) and short-circuit circuit protection requires a millisecond level (instantaneous).
4. Application Scenarios
RCD: Required installation scenarios: electrical circuits for sockets in bathrooms, kitchens, balconies; power supply circuits for portable equipment; outdoor lighting, water pumps and other electrical circuits that may come into contact with water.
Note: RCD cannot be used alone; it must be used in a series with MCB (or an integrated RCBO).
MCB: As the main switch and branch circuit switch, is the basic protection device of all circuits.
For example, in a home distribution box, the master switch is usually an MCB, and each branch circuit (such as lighting and sockets) also has an MCB.
V. Structural Composition
Core components: zero-sequence current transformer, tripping mechanism, test button (for periodic simulation of leakage current testing).
Wiring requirements: the live wire and neutral wire must be connected simultaneously, the inlet wire and the outlet wire must correspond strictly.
MCB: Core components: mobile contact, fixed contact, arc extinguishing chamber, thermal stroke unit, magnetic stroke unit, operating handle.
Wiring requirements: only live wire (single pole) or live wire and neutral wire (bipolar).