RCBO's - The Ultimate Guide


RCBO stands for 'Residual Current Breaker with Over-Current'. This type of consumer unit 'breaker' combines both the functionalities of an MCB and an RCD. Effectively an RCBO is a versatile, useful device that keeps yourself and your home safe. An RCBO protects individual circuits this gives you even more freedom when planning out your consumer unit install.

RCBO - Fusebox Shop complete guide


When to use an RCBO

In July 2008, the 17th edition wiring regulations were published, this stated that all circuits must be protected from earth leakage. This was put in place to significantly reduce the number of injuries caused by electric shock. In the majority of domestic homes, this is achieved by the use of an RCD which protects a bank of MCB circuits simultaneously.

Using a dual rcd consumer unit is a cost effective and still a popular solution, although over the last few years their popularity has been falling. Going this way does have a considerable drawback as an earth leakage fault on one of the circuits will knock out the power to all the other circuits on that RCD. Leaving you in the dark when it may only be a fault on your kitchen sockets!

The fault described above is better known as nuisance tripping. Let's use the same example to show why using RCBO's eliminates nuisance tripping. Your kitchen sockets have a fault, when using an RCD solution all circuits will cut off and these circuits being shut off unnecessarily could include your fridge/freezer.

On the other hand, if you have an RCBO consumer unit installed, the RCBO's combines the functionality of both an RCD and MCB. This means if you do have a fault only that individual circuit will shut off, and the other will remain working like normal.

How does a RCBO combine the two?

The MCB can only detect current overloads, and the RCD/RCCB can only detect current leakages. The RCBO can detect both, making it a great choice when installing a wiring system as it will protect the circuit and the resident from electrical accidents.

Different types of RCBO?

RCBOs are divided into various types based on the operating function.

Type A

Are used for residual pulsating DC of up to 6mA and alternating sinusoidal residual current

Type S

It is a sinusoidal residential current device that incorporates time delay. You can install it upstream from a Type AC RCB to offer selectivity. You cannot use this RCD for additional protection since it cannot operate under the needed time of 40mS.

Type AC

They are commonly installed in homes and meant to be used for alternating sinusoidal residual current to offer inductive, capacitive, or resistive equipment. These RCBOs operate instantaneously to detect imbalance and do not have a time delay.

Type F

They are used for frequency-controlled equipment and appliances, including dishwashers, washing machines, and air conditioning controllers with variable speed drives.

Type B

They are used for three and single-phase equipment such as escalators, lifts, photovoltaic systems, welding equipment, and inverters.

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Bidirectional RCBO's

Exploring Bidirectional Protective Devices

With the rise of alternative energy sources like solar photovoltaic (PV) and energy storage systems, bidirectional power flow has become a crucial consideration for certain protective devices. This guide delves into the selection and installation of protective devices for such sources, as outlined in the recently issued BEAMA bulletin on connecting unidirectional and bidirectional protective devices.

Understanding Unidirectional Protective Devices

Unidirectional protective devices are specifically designed to accommodate power flow in a single direction, typically from the supply to the load. These devices are marked to distinguish between line and load terminals, ensuring correct connection. Examples include single-module residual current breakers with over-current protection (RCBOs) and arc fault detection devices (AFDDs), commonly employing electronic circuits for residual current protection. It's essential to adhere to the manufacturer's connection instructions to ensure proper functionality.

Bidirectional Protective Devices Explained

In contrast, bidirectional protective devices lack markings indicating line and load terminals, making them suitable for power flow in either direction without risk of damage. For instance, traditional electromechanical residual current circuit-breakers (RCCBs) and electronic RCCBs are bidirectional, often not labeled for line or load connections.

Identification and Usage of Protective Devices

Protective devices like miniature circuit-breakers (MCBs), RCCBs, RCBOs, and AFDDs feature markings to differentiate between supply and load terminals, typically indicated by labels like "line" and "load" or directional arrows. However, it's crucial to consult the manufacturer to confirm bidirectional compatibility, especially for devices without explicit markings.


Implications for Existing Installations

When assessing existing installations, such as during electrical installation condition reports (EICRs), it's vital to consider device functionality in case of failure. For TT systems, RCD failure poses safety concerns and warrants immediate attention. Recommendations for improvement align with current standards, but judgment should consider individual circumstances.


In conclusion, bidirectional power flow necessitates careful selection of protective devices for generators and energy storage systems. While RCDs may not be obligatory for PV systems, their application should align with installation requirements and manufacturer guidelines. Selecting the appropriate RCD type and ensuring bidirectional compatibility are critical aspects to consider, emphasizing adherence to manufacturer instructions for optimal device performance.

RCBO Consumer Unit