RCBOs (Residual Current Circuit Breakers with Overcurrent Protection)

RCBO “RCD + MCB” residual current circuit breakers with overload protection

RCBOs combine overload and short-circuit protection (MCB function) with residual current protection against earth leakage (RCD function) in a single device. They are used in residential and industrial distribution boards to protect people and the installation, according to the electrical design and applicable standards.

Why choose an RCBO

• Combined protection in one device: reduced wiring and panel space compared to separate MCB + RCD (depending on the architecture).
• Per-circuit RCD protection: dedicated residual protection per line helps reduce nuisance trips affecting the whole board.
• Faster fault identification: when it trips, the affected circuit is immediately clear.

Key technical elements

• MCB section: rated current (In) and trip curve (B/C/D, depending on model) to manage inrush and load types.
• RCD section: residual operating current IΔn (e.g., 30 mA for personal protection in many applications) and type based on expected leakage current waveforms.

How to select an RCBO (technical checklist)

Selection must match the network, loads, prospective short-circuit current and upstream protection scheme. Follow the design requirements and applicable standards.

• Rated current (In): aligned with conductor size and expected load.
• Trip curve (B/C/D): according to inrush currents (lighting, outlets, power supplies, motors).
• Residual trip current (IΔn): typically 30 mA for many final circuits; consider 10 mA (higher sensitivity) or 100/300 mA (installation/fire protection) where required by the design.
• RCD type:
  • Type AC: sinusoidal residual currents; often limited to fully compatible applications.
  • Type A: also covers pulsating DC components; suitable for many modern loads (SMPS, light power electronics).
  • Type F: for single-phase inverter loads and mixed residual currents (certain drives and advanced appliances).
  • Type B: for smooth DC residual currents and specific inverter/EV-related applications where required.
• Breaking capacity (Icn/Icu): suitable for the prospective fault current.
• Poles: 1P+N / 2P / 3P+N / 4P depending on the network and isolation needs.
• Mounting and accessories: DIN rail, module width, auxiliary contacts/trips depending on the system.

Compatibility and nuisance tripping

• Nuisance trips may be caused by cumulative leakage, EMC filters, electronic loads and installation quality. RCD type selection and circuit splitting are key.
• Upstream coordination: evaluate selectivity with upstream RCD/RCBO devices and overall protection coordination.
• Periodic testing: use the test button (T) as per manufacturer guidance and internal procedures.

FAQ

RCBO or separate MCB + RCD?
RCBOs are useful to protect individual circuits with dedicated residual protection and reduce board-wide trips. Separate devices may be preferred in certain panel architectures or for upstream selectivity strategies.

Type AC vs Type A: what is the difference?
Type A detects pulsating DC residual currents typical of many modern devices. Type AC is suitable only for fully compatible, more traditional loads.

When do I need Type F or Type B?
It depends on the load profile: certain single-phase inverter loads may require Type F; applications with smooth DC residual currents may require Type B according to the design and standards.