RCD vs RCBO: Key Differences and Selection Guide
Differences between an RCD and an RCBO: operating principles, wiring diagrams, rated residual currents (10 mA and 30 mA), types AC, A, F, and B, and application guidelines.
Differences in functionality. An RCD monitors only earth leakage current and disconnects the circuit in the event of electric shock or insulation failure, but it does not provide protection against overloads or short circuits – therefore, an MCB must always be installed upstream. An RCBO combines an MCB and an RCD in a single unit, providing simultaneous protection against overloads, short circuits, and leakage current.
Which to choose: an "RCD + MCB" combination is suitable when one RCD protects multiple circuits, when it is important to immediately identify the cause of a trip in distribution boards with many circuits, or for general installation. An RCBO is preferred when space in the consumer unit is limited or for specific critical circuits (e.g., washing machines, water heaters). Rated residual current settings: 30 mA for socket-outlet circuits, 10 mA for wet rooms, and 100–300 mA for fire protection RCDs at the inlet.
What is an RCD?
An RCD (residual current device) is a device that monitors the current balance between the phase and neutral conductors. In the event of an earth leakage current (for example, if a person touches a live wire or if insulation is damaged), the RCD disconnects the circuit within a time not exceeding 0.03–0.3 s (selective versions may take up to 0.5 s).
Important: An RCD does not protect against overloads or short circuits. Therefore, an MCB must always be installed upstream of the RCD. The rated current of the RCD should be selected to be no lower than the rating of the upstream MCB, or ideally one step higher: for example, after a 25 A MCB, an RCD rated at 25 A or 40 A should be used.
Based on their operating principle, RCDs are divided into electromechanical and electronic types. Electromechanical types operate using the energy from the leakage current itself and remain functional even if the neutral conductor is disconnected; electronic types require mains power to operate, meaning they may fail to provide protection if the neutral is lost. Electromechanical types are generally considered more reliable, while electronic types are typically more cost-effective.
What is an RCBO?
An RCBO (residual current breaker with overcurrent protection) is a combined device that integrates the functions of an MCB and an RCD into a single housing. It provides simultaneous protection against overloads, short circuits, and leakage current.
Comparison of characteristics
| Parameter | RCD + MCB | RCBO |
|---|---|---|
| Overload protection | MCB | Integrated |
| Short-circuit protection | MCB | Integrated |
| Leakage protection | RCD | Integrated |
| Space in consumer unit | 3 modules (2P RCD + 1P MCB) | 2 modules; compact 1-module series available |
| Cost | Lower in total | Higher |
| Fault diagnostics | Easy to see which device tripped | Cause is not always immediately clear |
| Replacement upon failure | Only one device needs replacing | The entire unit must be replaced |
Residual current: 10 mA or 30 mA
- 10 mA – for wet rooms (bathrooms, saunas) and dedicated circuits for washing machines or dishwashers
- 30 mA – standard rating for socket-outlet circuits and general protection according to HD 60364
- 100–300 mA – fire protection RCD installed at the inlet to protect against fire caused by leakage currents
RCD Type: AC or A
- Type AC – responds to sinusoidal AC residual currents. The most common and cost-effective type.
- Type A – responds to both sinusoidal AC and pulsating DC residual currents. Recommended for circuits containing washing machines, variable speed drives, and switch-mode power supplies.
In addition to AC and A, there is Type F – designed for single-phase circuits with frequency converters (e.g., inverter-driven washing machines, air conditioners), and Type B – which responds to smooth DC residual currents, required for EV charging stations, photovoltaic systems, and variable speed drives. Regarding availability, the market is straightforward: Type AC and Type A are widely available in retail, Type F is rarely found in mass-market ranges, and Type B is a premium component typically available via special order. There are also selective versions marked with an 'S' (time-delay); these are installed upstream to ensure that in the event of a fault, the downstream branch device trips before the main inlet device.
When to use which
RCD + MCB:
- When one RCD protects multiple circuits (cost-effective)
- When diagnostics are important: the indicator flag on the MCB or RCD shows exactly which device has tripped
- In distribution boards with a large number of circuits
RCBO:
- When space in the distribution board is limited
- For dedicated critical circuits (washing machines, water heaters)
- During renovations when adding protection without replacing the entire board
Regulatory Requirements
The use of RCDs in residential buildings is regulated by the HD 60364 series. For socket-outlet circuits, HD 60364-4-41 requires an RCD with a rated residual current not exceeding 30 mA. This requirement is mandatory for outdoor socket-outlets and for circuits in bathrooms or other areas with increased risk. For the main inlet, the installation of a fire-protection RCD (100–300 mA) is recommended.