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Are you looking for a 17th Edition consumer unit? If so, you probably need to know that there is no such thing, says Colin McAhren of Moeller Electric.

However, as he explains, it is possible to match the configuration of the consumer unit to the wiring scheme so that the complete installation conforms to the 17th Edition.

Wouldn’t it be nice to buy a consumer unit off the shelf that was guaranteed to meet the requirements of the 17th Edition of the IEE Wiring Regulations? Unfortunately, despite what some suppliers are saying, you can’t.

The reason is very simple – the consumer unit has to be arranged to suit the installation’s wiring.

There’s no single arrangement that can guarantee compliance.

Let’s take a closer look.

The 17th Edition identifies five important conditions that must be satisfied in standard applications.

These are: Socket outlets for general use must be RCD protected (Reg 411.3.3); All circuits in locations containing a bath or shower must be RCD protected (Reg 701.411.3.3); Unprotected cables buried less than 50mm deep in a wall must be RCD protected (Reg 522.6.6-8); To prevent nuisance tripping, unnecessary hazards and to minimise inconvenience, circuits should not be connected to a single upstream RCD (Reg 314.1); Separate circuits must not be affected by the failure of other circuits (Reg 314.2).

Although they may at first seem very precise, the regulations are in practice open to a degree of interpretation, particularly regarding the division of installations, where acceptable levels of inconvenience resulting from a fault can be somewhat subjective.

Because of this, the remainder of this article discusses consumer unit configurations that, depending on the installation design, will offer a Fully Compliant Assembly (FCA), or a Partially Compliant Assembly (PCA).

It also discusses possible pitfalls that make an installation potentially dangerous and, therefore, result in a Non-Compliant Assembly (NCA).

Let’s consider some practical examples.

A dual RCD board (Example A) is often promoted as a fully compliant 17th Edition solution.

In reality, however, it can only be a PCA at best.

This arrangement satisfies conditions one, two and three in the list above, but it is most unlikely to satisfy conditions four and five.

This is because, in the event of a fault on either set of MCBs, the associated RCD may also trip.

This creates an unwanted disconnection of the MCBs where no fault exists, which is contrary to conditions four and five.

In addition, the smoke alarms are protected by the same RCD as lighting circuits and, when an incandescent bulb fails, it almost always trips the RCD.

Under these circumstances, the smoke alarm circuit will be disabled, potentially putting the occupants of the house at risk.

This arrangement must, therefore, be considered as an NCA – it is dangerous and unsatisfactory.

Now consider a consumer unit arrangement with a split load board with independent RCBOs.

Once again, this satisfies conditions one to three above, but it does not fully satisfy conditions four and five as, for example, a fault on the shower may result in the disconnection of all socket outlets.

Nevertheless, none of the fault conditions with this arrangement result in specific hazards, so it can be considered as a PCA.

There is one other point that requires careful attention.

As shown, all of the socket outlets are fed from one RCD.

Because of their in-built mains filters, many electronic devices, such as televisions and computers, have significant earth leakage even when turned off.

If too many devices of this type are plugged into the socket circuits, nuisance tripping of the RCD is very likely.

It would probably be a good idea, therefore, to separate the upstairs and downstairs socket circuits.

Units with separate RCBOs for all circuits, is an example of an FCA, which unconditionally meets all of the conditions listed earlier.

Where possible, the use of this arrangement is strongly recommended as it not only satisfies the regulations but also provides the best possible protection for life and equipment.

While the third example provides the best possible consumer unit arrangement for meeting the requirements of the 17th Edition, pressure to keep installation costs to a minimum may mean that it isn’t possible to use an RCBO on every circuit.

In these cases, looking at the wiring scheme carefully may allow economies to be made.

If, for example, cable runs for burglar and smoke alarms are run in metal trunking rather than buried in the walls, they do not require RCD protection, and can be fed from ordinary MCBs.

Likewise it may be possible to arrange for the feeds to the immersion heater and electric cooker to be surface-mounted in conduit or buried deeper than 50mm within the wall, once again eliminating the need for RCD protection.

In these cases, the consumer unit can be considered as an FCA, provided that RCBO protection is provided for each of the remaining circuits.

This gives a good balance between safety, convenience and cost.

It is also possible to configure a consumer unit with even fewer RCBOs if it is accepted that faults on certain circuits will affect others.

For example, if one RCD feeds the shower and sockets throughout the house, it may be considered acceptable for a fault on the shower to interrupt the supply to the sockets.

Installers should, however, be clear that an arrangement of this type does not satisfy the regulations in their current form and, for that reason, it cannot be recommended as best practice.

The 17th Edition of the Wiring Regulations is very new and, no doubt, the details of its implementation will become clearer over time.

Hopefully this article will have provided a few useful pointers and, for those who require more information, Moeller Electric has produced a new free publication, ‘Moeller Consumer Units and the 17th Edition Wiring Regulations’, which discusses in more detail the examples used in the article.

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