Consumer unit wiring - 17th Edition.

The introduction of the 17th Edition of the IEE Wiring Regulations on the 1st January 2008 has major implications for all electrical contractors, designers and consultants. From 1st July 2008 all new electrical installations will have to be designed to comply with the new regulations.
There are five aspects that the 17th Edition identifies as requiring consideration for installations that are not under the supervision of skilled people (e.g. qualified electricians) or instructed people (e.g. facilities managers) – i.e. most domestic installations, but also some commercial installations. These are:

• Socket outlets for general use in domestic installations must have the additional protection of an RCD not exceeding 30mA (regulation 411.3.3)
• All circuits in locations containing a bath or shower must be protected by an RCD not exceeding 30mA (regulation 701.411.3.3)
• Cables buried in a wall or partition at a depth of less than 50mm, and not mechanically protected by appropriate earthed metal, must be protected by an RCD not exceeding 30mA (regulations 522.6.6–8)
• To prevent nuisance tripping, unnecessary hazards, and minimise inconvenience, circuits should not be connected to a single upstream RCD (regulation 314.1)
• Separate circuits shall not be affected by the failure of other circuits (regulation 314.2)
  

The following CU arrangements, each with their own benefits, can be considered by the installation designer:

Example A - Dual RCD split load board.
Example B - Split load board with independent RCBOs.
Example C - Main switch with RCBOs on all circuits.
Example D - Main switch with RCBOs on critical circuits.
Example E - Split load board with RCBOs on critical circuits.

A dual RCD split load board will meet the 17th Edition requirements for the following:

• Socket outlets for general use in domestic installations must have the additional protection of an RCD not exceeding 30mA.
• All circuits in locations containing a bath or shower must be protected by an RCD not exceeding 30mA.
• Cables buried in a wall or partition at a depth of less than 50mm, and not mechanically protected by appropriate earthed metal, must be protected by an RCD not exceeding 30mA.

However, depending on the installation design, it is unlikely to satisfy the regulations on:

• To prevent nuisance tripping, unnecessary hazards, and minimise inconvenience, circuits should not be connected to a single upstream RCD.
• Separate circuits shall not be affected by the failure of other circuits.

A split load board with independent RCBOs will meet the 17th Edition requirements for the following:

• Socket outlets for general use in domestic installations must have the additional protection of an RCD not exceeding 30mA.
• All circuits in locations containing a bath or shower must be protected by an RCD not exceeding 30mA.
• Cables buried in a wall or partition at a depth of less than 50mm, and not mechanically protected by appropriate earthed metal, must be protected by an RCD not exceeding 30mA.

However, depending on the installation design, whilst this layout takes into account the danger arising from the failure of a single circuit (such as lighting) it does not fully comply with:

• To prevent nuisance tripping, unnecessary hazards, and minimise inconvenience, circuits should not be connected to a single upstream RCD.
• Separate circuits shall not be affected by the failure of other circuits.

A standard main switch disconnector controlled consumer unit with RCBOs for every outgoing circuit instead of the usual MCBs, will fully comply with the 17th Edition regulations. A fault on any circuit will not impact on other circuits, and so all aspects of the regulations are satisfied.

• Socket outlets for general use in domestic installations must have the additional protection of an RCD not exceeding 30mA.
• All circuits in locations containing a bath or shower must be protected by an RCD not exceeding 30mA.
• Cables buried in a wall or partition at a depth of less than 50mm, and not mechanically protected by appropriate earthed metal, must be protected by an RCD not exceeding 30mA.
• To prevent nuisance tripping, unnecessary hazards, and minimise inconvenience, circuits should not be connected to a single upstream RCD.
• Separate circuits shall not be affected by the failure of other circuits.
  

By looking closely at the wiring scheme in an installation, the assembly in Example C can be made more cost effective by using MCBs to protect some circuits rather than RCBOs, without compromising compliance with the 17th Edition. For example, at the point of cable entry to the consumer unit, incoming and outgoing cable runs are all encased in suitable ‘metal trunking’ or buried deeper than 50mm, so do not require RCD protection. As none of the cables are in the walls, the smoke alarms similarly do not require RCD protection. The burglar alarm system may have its own trunking or metal covering for the supply to the main control circuit. If it does, and all the other devices are low voltage or cable runs in the ceiling, then this too could be MCB protected. This will depend on the wiring scheme employed for this ancillary piece of equipment. You could also decide that the immersion heater cable and cooker point (without socket) are in areas where they can be surface mounted and shrouded inside a short length of conduit, or buried deeper than 50mm in the wall.

By reviewing the wiring scheme employed with the split load assembly proposed in Example B, the cost of the finished assembly can be reduced, and partial compliance with the regulations achieved.
The split load board laid out here will meet the 17th Edition requirements for the following:

• Socket outlets for general use in domestic installations must have the additional protection of an RCD not exceeding 30mA.
• All circuits in locations containing a bath or shower must be protected by an RCD not exceeding 30mA.
• Cables buried in a wall or partition at a depth of less than 50mm, and not mechanically protected by appropriate earthed metal, must be protected by an RCD not exceeding 30mA.

However, depending on the installation design, it is unlikely to satisfy the regulations on:

• To prevent nuisance tripping, unnecessary hazards, and minimise inconvenience, circuits should not be connected to a single upstream RCD.
• Separate circuits shall not be affected by the failure of other circuits.

Article based on Moeller Consumer Units and the 17th Edition Wiring Regulations
www.moeller.co.uk

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