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17th Edition Wiring Regulations and Fuse Boards

 

The new Wiring Regulations BS 7671 published on the 1st January 2008 and came in to force at the end of June 2008, call for much more extensive provision for RCD (residual circuit protection) of both domestic and non-domestic dwellings.  The practicalities of meeting those requirements for 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.  In bathrooms for example RCD protection is now required for all circuits. 

There are a number of approaches to fuse board design to meet the new regulations, some of which 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 the use of RCDs and RCBOs is now greater and there is a trend towards larger consumer units with more ways available. Image of the Chint NX2-18DSL-A

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 RCD’s 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 Regulations. 

Under the new Regulations, an installation is required to incorporate one or more RCDs depending on the 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 in to account. 

Where a cable is concealed in a wall or partition at a depth of less than 50mm form the surface, if earthed mechanical protection is not included (e.g. metal trunking or conduit), it must be provided with additional protection by means of a 30mA RCD.  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 so as 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.  A typical solution would be a Chint NX2-14M 12 way consumer unit with 100a double pole mainswitch isolator.

  Up stDown stOutsideBurglar Immersion KitchenUp st Down St  
  LightsLightsLightsAlarmHeaterSocketssocketsCookersocketsShower 
  6a6a6a6a16a32a32a32a32a32a100A
SpareSpare30mA30mA30mA30mA30mA30mA30mA30mA30mA30mAMain 
  RCBORCBORCBORCBORCBORCBORCBORCBORCBORCBOIsolator

 

The 2nd option would be to use an isolator controlled dual RCD consumer unit supplying two RCD protected sections, like the Chint NX2-18DSL-A 12 way High Integrity consumer unit. 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 (Freezer) as described above. 

  Kitchen ImmersionDownstairs     UpstairsUpstairs Downstairs 
 CookersocketsHeaterLights     LightsSocketsShowerSockets 
63A      63A      100A
30mA40A32a16a6aSpareSpare30mASpareSpare6a32a32a32aMain 
RCDMCBMCBMCBMCB  RCD  MCBMCBMCBMCBIsolator
            ****  
 

The 3rd option is a standard split load consumer unit protected firstly by a 100a DP isolator and then the RCD protecting an amount of outgoing ways.  The non-protected section can then have RCBOs protecting individual circuits.  An example of this is the Chint NX2-18H Splitload Consumer Unit with 14 outgoing ways protected by a 100a double pole isolator and 7 ways protected by an 80a 30ma double pole RCD. 

  Fridge  ImmersionDownstairsUpstairsOutsideBurglar KitchenDownstairsUpstairsGarage  
  FreezerShowerCookerHeaterLightsLightsLightsAlarmSocketsSocketsSocketsSockets  
80A              100A
30mASpare16a40a32a16a6a6a6a6a32a32a32a32aSpareMain 
RCD MCBMCBMCBMCBMCBMCBRCBORCBORCBORCBORCBORCBO Isolator

 

The final choice of consumer unit will depend on cost versus convenience and level of protection required.  The 1st solution, providing RCBO’s on each outgoing circuit can be seen as the simplest and most convenient both in terms of installation and isolation of a fault should a failure occur while still providing full protection to the rest of the installation.

The Regulations affecting RCD protection 

Rule 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. 

Rule 710.411.3.3

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

Rule 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 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 and 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.

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