The technical team at Wylex takes a look at the recent changes to the wiring regulations and what this means for consumer units and domestic residential installations.
There are several new regulations in the 18thEdition that are relevant to electrical installations within domestic household premises that designers and installers need to know about. For example it’s now a requirement for all lighting circuits within domestic household premises to be protected by 30milliamp residual current devices. There are no exceptions to this in regulation 411.3.4.
It’s also a requirement of regulation 314.1 to reduce the possibility of unwanted tripping of RCDs from PE currents, and to take account of hazards that arise from the failure of a single circuit. In particular, unwanted tripping of RCDs can cause dangers and hazards if residents are plunged into darkness when the lighting circuits are not actually faulty.
Avoiding unwanted tripping of RCDs from PE currents
Regulation 531.3.2 supports the principles of Regulation 314.1 (which is not new). This requires every installation to be divided into the number of circuits required to avoid danger and minimise inconvenience in the event of a fault. It is intended to make sure that a single fault on one circuit should not cause the loss of power to other circuits or introduce hazards, dangers or risks to occupants.
531.3.2 is new to the regulations, and requires designers to select RCDs so as to limit the risk of unwanted tripping. It gives designers two options to use to avoid unwanted tripping of RCDs from PE current (non-fault current from equipment that is used in the installation) flowing during its normal operation:
Subdivide the installation into individual circuits. For each circuit using its own RCD, in domestic household installations, this would be with 30mA RCBOs to meet the requirements for additional protection. This provides a compliant design and ensures that all healthy circuits remain in service and unaffected by faults on other circuits, which prevents hazards, dangers or risks to occupants from unwanted tripping.
Design the installation so that the total PE current from any group of circuits connected to an RCD cannot be more than 30% of the rated trip current, i.e. no more that 9mA for a 30mA RCD. But how easy is that to achieve? The designer will need to know the PE current values for each item of equipment and each circuit in order to be sure of providing a compliant installation. This information may not be easy to acquire.
The consumer unit (pictured) uses 30mA Type A RCBOs throughout for each individual circuit so that unwanted tripping is prevented.
Types of RCD
Within Chapter 53 regulation 531.3.3 provides additional information on some of the types of RCD that are available. It requires the designer/installer to select and install the appropriate type of device for each application. Although correct selection of devices is not a new consideration, this additional guidance will no doubt prove very useful as an aid to selection in the design process.
Four types of RCD are mentioned: 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 when selecting RCDs; other regulations also apply, such as:
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)
For PV installations Type B or AC could be appropriate
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