Fuse Boards

The new 17th edition IEE Wiring Regulations (BS 7671: 2008) call for more extensive provision for RCD (residual circuit protection) of both domestic and non-domestic dwellings. The practicalities of meeting these requirements for the protection of cables buried in plaster or enclosed within partition walls means that most circuits now need 30mA RCD protection. It is also a requirement that all 13A sockets up to 20A for use by ‘ordinary persons’ should normally be protected by a 30mA RCD and in bathrooms, for example, RCD protection is now required for all circuits.

There are a number of approaches to fuse board design meeting the new 17th edition Regulations. Some of these utilise traditional RCD main switch consumer units and split load consumer units, while others rely on the new type of dual RCD protected consumer units. In general, there is a greater use of RCDs and RCBOs and a growing trend towards larger consumer units with more ways available.

Outgoing Circuits

An RCD with a residual operating current not exceeding 30mA is the recognised device for providing additional protection in the event of:

  1. Failure of the basic protection
  2. Electrical fault
  3. Carelessness by the user or operator.

Such RCDs should not be used as the sole means of protection and do not avoid the need to apply one or more of the protective measures detailed in the 17th edition Wiring Regulations. Under the new Regulations, an installation is required to incorporate one or more RCDs depending on the individual circumstances. These include:

  • All socket outlets not exceeding 20A but with certain exceptions. One such exception would be a circuit labelled, or otherwise suitably identified, socket outlet for connection of a particular piece of equipment (e.g. freezer).
  • Mobile equipment with a current rating not exceeding 32A for use outdoors.
  • Electrical circuits installed in special installations and locations as defined in Part 7 of the Regulations (e.g. swimming pools and saunas).
  • All circuits, including shower and lighting circuits, in rooms with a fixed bath or shower (e.g. bathrooms and bedrooms with en-suite).

Protection of Cabling

In addition to the needs of the outgoing circuits described 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 50mm from the surface, it must be provided with additional protection by means of a 30mA RCD unless earthed mechanical protection is included (e.g. metal trunking or conduit). This applies even when it is installed in a Safe Zone.

Whilst it may be desirable to have one or two circuits which do not have RCD protection (e.g. clearly labelled circuit ‘freezer’), the installation of the wiring may still dictate that the circuit must be protected by an RCD.

The protection of a circuit by means of a 30mA 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 earthed mechanical protection. This is to avoid damage to the cable during installation or subsequent alteration.

Solutions

The most comprehensive solution is to provide individual RCBO protection on each outgoing circuit backed up with a 100A double pole main-switch isolator.

The second option would be to use an isolator-controlled dual RCD consumer unit supplying two RCD protected sections. This consumer unit also includes a 3-way busbar to allow a small number of ways to be protected directly from the isolator and not protected by either RCD for the supply of exceptional circuits (e.g. freezer) as described above.

The third option is a standard split load consumer unit protected
firstly by a 100A DP isolator with the RCD protecting an amount of outgoing ways. The non-protected section can then have RCBOs protecting
individual circuits.

The 17th Edition IEE Wiring Regulations regarding RCD protection

Regulation 411.3.3

Additional protection by means of a 30mA RCD is to be provided for all socket outlets with a rated current not exceeding 20A for use by ordinary persons. The only exceptions allowed are for socket outlets for use under the supervision of ‘skilled’ or ‘instructed persons’, e.g. some commercial/industrial locations or a specifically labelled socket provided for connection of a particular item of equipment, such as a freezer.

REGULATION 710.411.3.3

In specific locations such as those containing a bath or shower it is now a requirement to provide RCD protection on all circuits, including lighting and shower circuits.

REGULATION 314.1 & 2

Every installation should be provided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault. Designers are required to reduce the possibility of unwanted RCD tripping due to excessive protective conductor currents but not due to an earth fault.

Separate circuits may be required for parts of the electrical installation which need to be separately controlled in such a way that they are not affected by the failure of other circuits. The appropriate subdivision should take account of any danger arising from the failure of a single circuit, for example an RCD trip causing the disconnection of an important lighting circuit.

REGULATION 522.6.7

Much greater use of RCDs is required to protect wiring concealed in walls or partitions, even where this is installed in previously defined Safe Zones. This effectively means that all concealed wiring at a depth of less than 50mm from the surface now requires protection by a 30mA RCD, unless provided with earthed mechanical protection.