What does RCBO mean? Residual Current Devices Guide

An RCBO is usually found inside a consumer unit although can be installed in its own enclosure. The device protects against several electrical faults:

• EARTH FAULTS 
• SHORT CIRCUITS 
• OVERLOADS 

1) What is an RCBO and what does it do?

An RCBO is a type of Residual Current Device or RCD. An RCD is designed to detect electrical currents that stray away from electrical circuits to earth. They detect this 'Earth Leakage' and when the amount of leakage reaches the rating of the RCD it will trip the power off to the circuit or circuits which it protects. 

Many RCD devices are designed and installed to protect against electric shocks and electrocution. In these cases, the rating of these devices will be set to 30mA (milli-amps) or less to minimize the dangerous levels of current that humans and animals may be exposed to.

2) When should an RCBO be used?

An RCBO device should be considered where both overcurrent protection and earth leakage protection are required in a wiring system. The same level of protection can be achieved with an RCD device used in conjunction with an MCB device although this has some drawbacks. The RCD + MCB combination will be more expensive than an RCBO when protecting one circuit. Where an RCD protects multiple MCB devices (and therefore multiple electrical circuits) if there is a fault, all circuits will lose their electrical supply.

One of the ideal situations for an RCBO to be used is where an electrical shower needs to be installed. There are high currents used when an electrical shower is in operation and this puts stress on a standalone RCD that also protects other heavy circuits. Installing an RCBO will not only provide all the benefits of an MCB and RCD but will mean that if water gets into the electrical wiring or equipment on that circuit, the device will trip ONLY that one circuit off.

3) What other types of RCD devices are there?

​There are other types of residual current devices such as:

3.1) Fuse Spur RCDs

These devices sit in an electrical circuit away from the consumer unit and are designed to protect just a part of an electrical circuit. They are commonly used where bathroom wiring additions or alterations have taken place.

3.2) Portable RCDs

​These types of RCD are intended for use on individual pieces of equipment normally. You may find one on an electric lawnmower, a wet tile saw or other equipment intended for use outdoors. These devices are wired in line with the appliance flex or installed at the plug top end of the appliance and will only trip the power to the equipment that you are using.

3.3) Socket Outlet RCDs

Where it is undesirable to install an RCD across the whole electrical circuit, individual power sockets may have RCDs built in to protect against earth leakage currents at that point only. This is advantageous in circumstances where computer equipment shares the same electrical circuit for example. Computer equipment has natural earth leakage and where many computers will be used, a fixed RCD at the consumer unit can become overwhelmed due to the accumulation of earth leakage that many computers combined will produce. 

In the USA there are GFCI sockets which carry out a similar purpose, typically these devices are more sensitive and are set to trip at around 6-8mA as opposed to the UK varieties which trip at around 30mA.

3.4) Fixed RCDs

Fixed RCDs are installed at the origin of the electrical circuit or electrical installations. It is not sensible to have one RCD device protecting the entire electrical system as a single fault on a single circuit will cause complete power loss. Electrical systems should be divided so that a fault on one circuit does not affect the operation of other circuits. This is why an RCBO is a great addition to a consumer unit as they not only provide over-current protection but also protect against any earth leakage fault.

4) How does an RCD device work?

In a healthy electrical circuit with no faults, there should be an even flow of electricity around the circuit. This means that the Line Conductor (live wire) and Neutral Conductor (neutral wire) should have the exact same level of current flow. Where there are earth fault currents present, this means that there is current leakage in the circuit. This can leak from either the live or neutral and can happen for many reasons. 
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When electrical current flows through a wire there is an electromagnetic field produced. The live wire has a magnetic field and the neutral wire also has one. Where equal current flows in opposing directions, each magnetic field will be of the same strength but in opposing directions. These magnetic fields effectively cancel each other out where the currents are the same and the resultant magnetic field strength is 0.

Where there is an imbalance in current flow, a magnetic field is present. The strength of this magnetic field is proportional to the size of the current imbalance. The RCD device detects this imbalance using a sensor coil and when it reaches the maximum leakage current allowable according to the rating of the device, it will trip the power off to prevent risk of electrical shocks.

5) ​Different names for the same thing?

There are a few different names for RCD devices such as the RCBO. Generally speaking, an RCB (Residual Current Breaker), an RCCB(Residual Current Circuit Breaker) and an RCD(Residual Current Device) all perform the same job in the same manner. They monitor the current flow in electrical circuits and trip the power off when there is an imbalance between the neutral currents and line currents.

6) RCBO vs MCB, what is the difference?

An RCBO provides earth fault current protection, protection from overload and protection from overcurrent.

An MCB is a Miniature Circuit Breaker and will trip to protect against overload and overcurrent faults.

The primary functions of RCBOs are to perform BOTH the jobs of an MCB and an RCD device offering residual current protection and the benefits of an MCB in one single device. This dual functionality makes them a great choice for electrical safety and convenience in the event of a fault.

7) ​RCD vs RCBO, what is the difference?

An RCD (residual current device) covers the whole range of earth leakage circuit protection devices such as RCD, RCB, RCCB, RCBO. The RCBO is a member of the RCD family. A standalone RCD will generally not offer protection from overload or overcurrent and will only offer protection against earth leakage faults. The RCBO contains all the benefits of an RCD and an MCB device, this is the main difference.

8) How much do RCBOs cost?

The RCBO has come down in cost in real terms over the years and now they can cost as little as £15 - £40 depending upon the brand, rating, and type of RCBO that is being installed. HAGER make excellent quality products at a very reasonable price but are not as cheap as FuseBox at the time of writing. They are both great quality products but FuseBox are newer to the scene whereas Hager have been around for several decades.

Prices will vary between wholesalers and stores but all RCBOs will be manufactured to the minimum standards set out in BS61009.

9) How to test an RCBO or RCD device as a lay person

There are many electrical tests that an electrician can perform to check that the functions of a residual current device are still working correctly. These electrical devices are meant to trip out when subject to an earth leakage current in a specific time to prove safe operation and only an electrician can check this using specialised test equipment. Aside from these technical tests, a quick check should be performed every 6 months by pressing the test button on the device. If the device trips off when the button is pressed then it is operational. If the test button does NOT trip the device or requires several attempts to do so then the device should be replaced by an electrician.

10) How an RCBO or RCD is tested by an electrician

​Using an RCD tester or multifunction tester, the electrician injects a fault current into the device to present earth leakage conditions. Where RCD or RCBOs are installed to offer protection from electrical shocks, they should trip when subject to 30mA of fault current and are required to do so in less than 300 milli-seconds. The test should be performed on both sides of the supply cycle at 0 degrees and 180 degrees.

What causes an RCBO to fail?

​There are many reasons that an RCBO device will fail such as:

• Continued tripping from intermittent faults 
• Age
• Corrosion 
• Environmental factors
• Poor Installation
• Poor Maintenance

These devices should be checked regularly and if they fail to trip then they should be replaced.

11) ​Summary

Whilst there are many types of protective device, an RCBO is one of the best for modern consumer units. They offer protection from short circuit currents, DIY accidents, electrical fires (to an extent) and electrical shock. There are considerable differences in performance and costs between different types of circuit breakers and thought should be given to risk vs reward when installing cheaper alternatives to the RCBO. Protective devices in electrical installations are there to protect the cables, electrical equipment and persons using the installation. It is my opinion that only the best devices should be used to prevent electrical hazards and disconnect the power supply when faults occur.