Are RCBOs Mandatory Under The 18th edition?

Are RCBOs Mandatory Under The 18th Edition RCD

Are RCBOs Mandatory Under The 18th Edition?

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It’s been a hot topic for debate recently, especially with all the changes and the introduction of Amendment 2 to the 18th edition of the wiring regulations. So once and for all, we’ll clear the confusion and answer the question: Are RCBOs Mandatory Under The 18th Edition?

Unfortunately the answer isn’t quite as clear as a simple ‘yes’ or ‘no’ (is it ever with the wiring regulations?!), however once we’ve laid out the particular requirements, it will become clear what is needed.

So let’s go, section by section and see what we need to do to be compliant for sure:

Division Of Circuits

This has long been a sticking point amongst electricians to be fair. When the previous 17th edition regulations introduced requirements to RCD protect pretty much every circuit, dual RCD boards were introduced as a cheap way of adding protection to every circuit.

Dual RCD board however, have never really met the requirements to provide adequate division between circuits to minimise inconvenience in the event of a fault. For example, if an earth fault on a socket circuit is connected to the same RCD as the lights of the property, they lose power too.

Are RCBOs Mandatory Under The 18th Edition RCD
Are RCBOs Mandatory Under The 18th Edition?

This can be particularly bothersome for domestic homeowners if a fault on a different circuit (lets say a garage circuit for example), blows the up front RCD and causes the power to the fridge/freezer socket to fail. What could have been a relatively minor issue then becomes a full blown emergency needing repair before the contents of the freezer defrost.

The only real way to comply with the requirement for the proper division of circuits, and ensure that one circuit tripping does not inadvertently affect another, is to use individual RCBOs for each circuit.

It should be noted that this particular ‘issue’ has been around as long as single/dual RCD boards have been, and is not something which has been brought about by the introduction of the 18th edition. Hoewever, a single fault should not cause a loss of power supply to a group of circuits, this is poor installation design.

The regulation in question is 314.1 which states “Every circuit should be divided, as necessary, to avoid hazards and minimize inconvenience in the event of a fault.”

Earth Leakage Currents Not To Be Above 30% Of Device Rating

This is a newer issue which was brought in with the 18th edition wiring regulations. This new regulation states that any earth fault protective device should not be subject to earth leakage current loadings of over 30% of their rated device current whilst in regular use.

Think of a dual RCD board where one single RCD can power many socket circuits (or even older single RCD main switch boards). With the advent of modern electronical equipment, computers and other items can have a significant amount of earth leakage whilst in normal usage.

Over the course of a large house with a couple of socket circuits, this can easily add up to over 9mA (30% of a standard 30mA RCD).

The 18th edition brought in a requirement with regulation 531.3.2 which effectively gives designers 2 choices:

  • Either install each circuit on its own 30mA RCBO
  • Or divide the installation up so that each RCD is not subject to more than 30% leakage current
Are RCBOs Mandatory Under The 18th Edition
Measuring Earth Leakage Currents Before A Board Change

Whilst it is still theoretically allowed to install dual RCD boards, accurately measuring the ‘in use’ leakage current of an installation is not something that many electricians do before a board change. Simply swapping the circuits over onto a dual RCD board and hoping is no longer an option.

This in theory basically makes RCBOs mandatory under the 18th edition unless you want to start measuring leakage currents and accepting the responsibility that comes with.

So: Are RCBOs Mandatory Under The 18th Edition?

So to answer the question ultimately, whilst RCBOS are not mandatory under the 18th edition in explicit terms, the regulations above give little options for professional electricians other than to just accept that RCBO equipped consumer units are the way forward for modern 18th edition installations.

Quite frankly, with the new lower prices of RCBO boards, it is madness to consider resorting to dual RCD boards simply to save a few £££.

Check the following resources which also help you decide whether you need to be fitting RCBOs on 18th edition installations:

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