Electricians in Shrewsbury

Why do we only install RCBO (not RCD) Distribution Boards?


An RCD (Residual Current Device) / RCBO (Residual Current circuit Breakers with Overcurrent protection) is a device that is designed to provide protection against electrocution by cutting off the flow of electricity automatically when it senses a ‘leakage’ of electric current from a circuit.

What is leakage current?

Earth leakage is electric current that finds its way to earth via an unintended path. There are two categories: unintentional earth leakage, which results from faulty insulation or equipment, and intentional earth leakage, which is a consequence of the way equipment is designed. It seems strange to “design in” earth leakage, but it’s sometimes unavoidable – IT equipment, for example, often produces some earth leakage, even when it’s working properly.

The most affected circuits are likely to be the ones that serve your kitchen appliances such as your fridge freezer, washing machine etc, or your entertainment systems such as computers, televisions, games consoles etc. The leakage current is likely to become higher the older your installation and your electric appliances are, a good reason to have a regular (at least every 10 years on a domestic property) electrical safety check up, known as an Electrical Installation Condition Report (EICR).​

Whatever the source of earth leakage, it must be prevented from causing electric shocks. This might happen if a faulty appliance such as an oven or kettle were to become live and you were to touch it – the leakage current would potentially travel through you to earth, resulting in an electric shock (see below to understand how little current it takes to create a potentially fatal electric shock).

An electric shock in such an instance is prevented by using either RCDs (Residual Current Device) or RCBOs (Residual Current Circuit breakers with Overcurrent protection). These measure the current in the line conductor and compare it with the current in the neutral conductor. If the difference exceeds the mA rating of the RCD or RCBO, 30mA (mA = milliamp  /  1 miiliamp = 0.001 amps) maximum in a domestic setting under current regulations, it will trip.

Why is a 30mA RCB0 / RCD so important?


To appreciate the importance of a working 30mA RCBO / RCD it is helpful to understand how much electrical energy it takes to kill a human being.


The smallest fuse used in a normal electric plug is 3 Amps; it takes less than one twentieth of that current to kill an adult in less than one tenth of a second.


40mA (a twenty-fifth of an amp) or more – flowing through the human body can cause irreversible damage to the normal cardiac cycle (‘ventricular fibrillation’) or death (‘electrocution’).

If disconnection takes place within 40ms fibrillation / electrocution is unlikely to occur.

Household RCDs (30mA maximum in a domestic setting under current regulations) are designed to disconnect the supply within 40ms at 150mA and within 300ms at 30mA tripping to protect the user.


With a fully functional 30mA RCD / RCBO installed, if the worst were to happen and you touched a live (faulty) appliance, you would most likely hear the “click” of the device disconnecting (“tripping”), but otherwise be completely unaware that anything was wrong. This being the case, if your RCD / RCBO does trip, you should always contact your electrician to investigate the reason for this.

Why RCBOs rather than RCDs?

The latest revision of BS 7671, The IET Wiring Regulations (18th Edition - January 2019) states in Regulation 411.3.4 “ the sum of the leakage currents shall not exceed 30% of the RCD residual current.”

All RCDs on a domestic installation must be a maximum of 30mA this means that the total leakage current of the circuits it covers must not exceed 9mA.

On a healthy electrical installation the leakage current to each circuit is usually around 1-3mA, well below the 9mA limit. However, on an older Single RCD Distribution Board (“fuseboard”) you may find a single RCD covering 6 or more circuits, or on a newer version, two RCDs (Dual RCD board), each covering 5 or more circuits. In such cases, the cumulative earth leakage (the total leakage of all the circuits protected by the RCD) may well exceed the 9mA total, even before the introduction of a faulty appliance, and does not, therefore, comply with the current wiring regulations which are designed to ensure maximum safety.

A competent electrician should, therefore, not be offering to install a dual RCD Distribution Board.

The solution to this is the installation of a full RCBO Distribution Board, a board in which each circuit is protected from earth leakage and overload / short circuit by its own 30mA device. In this case, the maximum permitted earth leakage to the individual circuit is now 9mA, a figure which is extremely unlikely to be exceeded except in the instant of a fault.

As well as complying with the IET Wiring Regulations, the RCBO Distribution Board has the added advantage that, if an earth leakage fault occurs, only the affected circuit will be disconnected. For example, if the heating element on your cooker becomes faulty (the single most common cause of call outs to “tripped” RCDs for us), only the RCBO supplying the cooker will trip. All other circuits will continue to operate normally. If this fault were to occur on a Dual RCD board, you would lose all the circuits protected by that device, likely to include at least half of your lights and sockets as well as the cooker, or, in the case of a single RCD board, all electrical power would be lost until the fault could be resolved.

This highlights another advantage of the RCBO Distribution Board. In the event of a fault, it is immediately evident to the electrician attending to resolve the issue which circuit the fault is on. Not having to inspect and test 5 or more circuits protected by the tripped RCD saves a considerable amount of time for the electrician, and money for you.

Not all RCBOs / RCDs are created equal

There are a variety of different types of RCBO / RCD available, and it is important that the right one is installed to meet both your individual requirements and those of the IET Wiring Regulations.

In the simplest terms, a competent electrician should not be installing Type AC RCBOs / RCDs in domestic properties as these can be rendered inoperable by leakage current from appliances with DC motors / inverters such as are to be found on most modern dishwashers / washing machines / tumble dryers etc. Type AC RCBOs / RCDs were standard for many years, and unfortunately some less competent / unscrupulous electricians continue to install them.

Type A RCBOs / RCDs will operate upon detection of both DC current leakage and AC current leakage, thus ensuring you and your family are fully protected. These should now be the standard for every domestic installation.

In some specific situations (although not common domestically), other types of RCBO / RCD may be required. If you require further information about this please do not hesitate to contact us.

Electricians in Shrewsbury