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RCDs and the 18th Edition

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People talking about RCDs and the 18th Edition is, most of the time, a good thing, but occasionally it’s a dangerous thing, according to Electrium, home to brands Crabtree & Wylex.

Electrium has been hearing reports that one RCD company is encouraging its customers to standardise on one type of RCD. 

This type of advice doesn’t actually follow the requirements within the wiring regulations and it may prevent designers from carrying out proper electrical design assessments. 

Even worse, it could lead to the use of inappropriate devices that are not suitable for the intended purpose, a frightening thought.

So, what do the regulations actually say about types of RCD?

Types of RCD – Chapter 53 selection and erection of equipment

Regulation 531.3.3, for example, is new to the regs book, but the principle of this regulation is not new at all.

For example, regulation 132.8 (17th and 18th editions) requires protective devices to operate at values of current, voltage and time that are suitably related to the characteristic of the circuits and possibilities of danger.

Four types of RCD are mentioned in regulation 531.3.3 which also requires the appropriate RCD to be selected from those four options – Type AC, Type A, Type F, or Type B. 

Each RCD type has different operating characteristics to suit particular applications – including those where DC components and varying frequencies are present.

The requirements here are clearly stated. Designers and installers must select and specify the correct device for each circuit or item of equipment being protected, and that choice must be made through a proper technical assessment.

However, Regulation 531.3.3 is only part of the story:

  • 30mA RCDs are used for additional protection (415.1)
  • For general purposes Type AC RCDs may be used (531.3.3)
  • For EV chargers Type A or B are required (722.531.2)
  • In some medical locations Type A or B are required – depending on fault currents (710.411.3.2.1)
  • Installations in caravans require RCDs to break all live poles including the neutral (721.415.1)

There are many elements to consider in order to select the appropriate type of RCD, and so failing to factor all of the relevant circumstances and guidance from within BS7671 into a design assessment is very risky.

Putting this into perspective, the term RCD is used within the wiring regulations more than 250 times and the definition of RCD includes RCCB, RCBO, CBR and MRCD. Clearly, it’s essential to understand the full picture before making device selections.

Avoiding unwanted tripping – Chapter 53 selection and erection of equipment

Regulation 531.3.2 is another ‘new’ regulation that is intended to aid designers in making better design decisions. 

This supports the principles of Regulation 314.1 (17th and 18th editions) which requires every installation to be divided into the number of circuits required to avoid danger and hazards caused by unwanted tripping of RCDs – a single fault should not cause the loss of power to groups of circuits

Regulation 531.3.2 gives designers two options to use to avoid unwanted tripping of RCDs from earth leakage (PE current) during normal operation.

Option 1

Divide the installation into individual circuits, each using its own 30mA RCBO

Option 2

Design the installation so that the PE current cannot be more than 30% of the rated trip current i.e. no more than 9mA for a 30mA RCD. But how easy is that to achieve?

Individual RCBOs provide a solution that also complies with Regulation 314.1 to avoid danger and hazards caused by unwanted tripping of RCDs. 


A single fault should not cause the loss of power to groups of circuits. The safety of the people within buildings can only be ensured when power continuity is maintained to healthy circuits by the initial design.


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