Phil Williams gets your consumer unit arrangements up to scratch for the 17th Edition

The advent of the 17th Edition of the Wiring Regulations presents new challenges to installers and manufacturers of electrical installation equipment. One challenge is how to satisfy the demand for greater use of residual current devices (RCDs).

It is well known that most circuits feeding 13 A socket-outlets now require RCD protection, but equally important are requirements for additional protection of wiring concealed in walls or partitions, which includes lighting circuits and their concealed switch wiring.

This means that the vast majority of home power and lighting circuits now require 30 mA RCD protection. There are also crucial requirements to minimise nuisance tripping and collateral risks due to a tripped RCD affecting other circuits, such as lighting.

Outgoing circuits/loads protection

An RCD with a residual operating current not exceeding 30 mA is the recognised device for providing additional protection in the event of failure of basic protection, electrical faults or carelessness by users. Such RCDs should not be used as the sole means of protection and do not obviate the need to apply one or more of the other recognised protective measures detailed in the regulations.

Under the new regulations, an installation must incorporate one or more RCDs, depending on circumstances. These include:

  • Socket-outlets not exceeding 20 A, with certain exceptions. One exception is for a specifically labelled, or otherwise suitably identified, socket-outlet for connection of a particular piece of equipment (for example, a freezer);
  • Mobile equipment with a current rating not exceeding 32 A for use outdoors;
  • Electrical circuits installed in “special installations and locations” as defined in part 7 of the regulations (for example, swimming pools or saunas);
  • Circuits, including shower and lighting, in rooms with a fixed bath or shower (for example, bathrooms and bedrooms with ensuite facilities).

Protection of cabling

In addition to the needs of the outgoing circuits and loads outlined above, the requirements of the installed cabling must also be taken into account.

Where a cable is concealed in a wall or partition at a depth of less than 50 mm from the surface, if it is not provided with earthed mechanical protection – metal trunking or conduit – it must have additional protection by means of a 30 mA RCD. This applies even if it is installed in a ‘safe zone’.

While it may be desirable to have one or two circuits that do not have RCD protection (for example, a clearly identified, dedicated freezer circuit), if the wiring is installed under the conditions described above, the circuit must still be protected by an RCD.

The protection of a circuit by means of a 30 mA RCD is also required where cables are concealed in walls constructed with metal stud partitions, irrespective of the depth from the surface, unless provided with protection in the form of earthed metallic covering, trunking, conduit or other mechanical protection so as to avoid damage to the cable during installation or construction of the wall.

Circuit arrangements

Regulations 314.1 and 2 require that every installation must be divided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault. Specifically, any subdivision should take account of the risks arising if the tripping of an RCD causes unwanted failure of, for example, a lighting circuit, thereby exposing users to potential hazards.

Regulations 314.1 and 2 also call on the designer to take steps to reduce the likelihood of unwanted RCD tripping due to excessive protective conductor currents, other than earth faults. Typical situations would include installations involving IT equipment, telecommunications and electronic equipment with certain types of radio-frequency interference suppression, and some heating elements.

Such equipment, by nature of its operation, can have an inherent earth-leakage current.

Consumer unit solutions

The practicalities of meeting the requirements for protection of cables buried in the plaster surface of walls or enclosed within partition walls will mean that most circuits will require 30mA RCD protection. Even if this is not the case, the requirement for RCD protection of socket-outlets up to 20 A, for use by ‘ordinary persons’, means all circuits feeding 13 A socket-outlets must normally be protected by 30 mA RCDs.

There are a number of approaches to consumer unit design to satisfy these requirements, some of which use traditional in-line and split-load consumer units while others rely on new types of dual-RCD unit. In all cases RCDs and RCBOs (residual current breakers with over-current protection) will be used more widely, and the trend will be towards units with more ways.

The precise solution will usually involve a trade-off between levels of protection, convenience and cost. Figs 1-4 illustrate the principal options available.

The optimum solution in terms of safety and convenience is for every final circuit to be protected by an RCBO (Fig 1). The isolator-controlled dual-RCD configuration (Figs 2 and 3) would provide the most cost-effective solution.

However, the final choice will depend on whether the end-user is willing to pay for the convenience and safety of individual circuit protection by RCBOs, as opposed to the group protection afforded by RCDs controlling multiple circuits in a split-load consumer unit.