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Consumer Unit Guide to the 17th Edition - 2 - Consumer Unit arrangements

Consumer Unit Guide to the 17th Edition - 2 - Consumer Unit arrangements

  2009-01-23         sparksdirect         Advice » Consumer Units Advice,   Rules and Regulations

After laying the "theoretical" foundation, and talking about the rules and regulations concerning the sockets, the cables or installations in the bathrooms, we can go on to see more about some practical consumer unit arrangements.

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

Consumer Unit Arrangements Not Permitted

A consumer unit with a 30mA RCD main switch would not be suitable for 3 main reasons:
  • The Fire detection circuit and the socket outlet circuits share a common RCD. This could reduce the reliability of the mains supply to the Fire detection circuit as appliances and portable equipment are likely causes of RCD tripping.
  • The cumulative effects of electronic equipment in the modern home, is such that some current is likely to flow in the protective conductor. A 30mA RCD will trip between 15-30mA. This could cause unwanted tripping, regulation 314.1 (iv) refers.
  • Any fault would result in the loss of all the lighting, this could in itself cause a hazard and the lack of power to the fridge/freezer circuit for example would be very inconvenient. Regulation 314.1 (i) asks the designer to consider this eventuality.
A consumer unit with a 30mA RCD main switch should not be used to protect all the circuits

Consumer Unit Arrangements Option 1 Main Switch with RCBO’s On All Circuits

A standard main switch disconnector controlled consumer unit could be used with every circuit having individual RCD protection at 30mA. This could be achieved by selecting RCBO’s for every outgoing circuit instead of the usual MCB’s. A fault on any circuit would not affect other circuits and hence all relevant regulations would be met by such a design.
Selecting RCBO’s for every outgoing circuit meets all relevant regulations.

Consumer Unit Arrangements Option 2 Split Load Twin RCCB plus Dedicated RCBO

This arrangement provides a dedicated 30mA RCBO for the smoke detector circuit, but combines the rest of the circuits across two further 30mA RCCB’s. Careful arrangements of the circuits can reduce the likelihood of nuisance tripping, thereby limiting the inconvenience or potential hazards that a loss of supply can cause by limiting the number of circuits affected.
This arrangement provides a dedicated RCBO for the smoke detector circuit.

Consumer Unit Arrangements Option 3 Split Load 3 RCCB Board

This arrangement provides a 30mA RCCB for the smoke detector circuit which could also supply other circuits e.g. lighting, and combines the rest of the circuits across two further 30mA RCCB’s. Careful arrangements of the circuits can reduce the likelihood of nuisance tripping, thereby limiting the inconvenience or potential hazards that a loss of supply can cause by reducing the number of circuits affected.
This arrangement provides a RCD for the smoke detector circuit which could also supply other circuits e.g. lighting.

Consumer Unit Arrangements Option 4 Split Load Twin RCCB

This arrangement provides two separate 30mA RCCBs with the circuits spread across both. The design of the circuit arrangements ensure the smoke detector is not fed from the same RCD as socket outlets to improve the reliability of the mains supply to the Fire detection circuit as appliances and portable equipment are likely causes of RCD tripping. Careful arrangement of the other circuits can reduce the likelihood of nuisance tripping, thereby limiting the inconvenience or potential hazards that a loss of supply can cause. However with all socket outlets being supplied from one RCD certain compromise must be accepted.
One option is for the smoke detector not to be supplied from the same RCD as socket outlets.

Hager VML933H 6 Way 3+3 Split Load Consumer Unit 80A Switch 2 x 80A 30mA RCCB 18th Edition Design 10 Surface Mounted Hager Fuseboard

Consumer Unit Arrangements Option 5 Split Load Twin RCCB plus unprotected circuit

Under the 17th Edition requirements it is still possible to install some circuits in domestic premises that are not fed via an RCD. Different wiring systems would need to be used.

The cost of installation could rise considerably if most circuits were installed using armoured cable or earthed metal conduits.

The smoke alarm circuit could be installed in such a way to negate the need for RCD protection, this may be possible by using one of the other wiring methods described in 522.6.6 for the length of run that the cable is in the wall (use of earthed metal conduit for example).

Or depending on the layout of the property there maybe an attached garage for example where surface wiring might be possible. The requirements of that regulation are therefore not applicable.

The level of compliance with the Regulations would therefore be the same as option 2 Split Load Twin RCCB plus Dedicated RCBO.

If the smoke alarm circuit is not to be protected by an RCD it must be installed using a method from (i) to (iv) of regulation 522.6.6.

Types of Residual Current Devices normally used in Consumer Units

  • RCD – Residual Current Device. A generic term for devices providing earth fault protection.
  • RCBO - Residual Current Operated Circuit-Breaker with Integral Overcurrent Protection A mechanical switching device designed to make, carry and break currents under normal service conditions and to cause the opening of the contacts when the residual current attains a given value under specified conditions. In addition it is designed to give protection against overloads and/or short circuits and can be used independently of any other overcurrent protective device within its rated short circuit capacity.
  • RCCB - Residual Current Operated Circuit-Breaker without Integral Overcurrent Protection A mechanical switching device designed to make, carry and break currents under normal service conditions and to cause the opening of the contacts when the residual current attains a given value under specified conditions. It is not designed to give protection against overloads and/or short circuits and must always be used in conjunction with an overcurrent protective device such as a fuse or circuit-breaker.

This is the second part of the full Consumer Unit Guide to the 17th edition, by Hager, the first part can be found here

You can buy online consumer units, either domestic consumer units (MK or Hager) or industrial consumer units (Hager, Ryefield boards, or isolators and switch fuses) by visiting the Electrical Items category at sparksdirect.co.uk.