The consumer unit: how to meet 17th Edition requirements

Wouldn’t it be nice to buy a consumer unit off the shelf that was guaranteed to meet the requirements of the IEE Wiring Regulations 17th Edition? 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.

• 1. Socket outlets for general use must be protected by a residual-current device (RCD) (regulation 411.3.3).
• 2. All circuits in locations containing a bath or shower must be RCD-protected (regulation 701.411.3.3).
• 3. Unprotected cables buried less than 50 mm deep in a wall must be RCD-protected (regulations 522.6.6-8).
• 4. To prevent nuisance tripping, unnecessary hazards and minimise inconvenience, circuits should not be connected to one single upstream RCD (regulation 314.1).
• 5. Separate circuits must not be affected by failure of other circuits (regulation 314.2).

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

Because of this, I am only going to deal with consumer unit configurations that, depending on the installation design, will offer a fully compliant assembly (FCA) or partially compliant assembly (PCA). I will also cover pitfalls that can make an installation dangerous and render it a non-compliant assembly (NCA).

Let’s consider some practical examples. A dual RCD board (see figure 1) 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 1, 2 and 3 in the list above, but it is most unlikely to satisfy 4 and 5.

This is because, in the event of a fault on either set of miniature circuit-breakers, the associated RCD may also trip. This creates an unwanted disconnection of the MCBs where no fault exists, which is contrary to conditions 4 and 5.

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, with the arrangement shown, the smoke alarm circuit will be disabled, potentially putting the occupants of the house at risk. This arrangement must, therefore, be considered an NCA – it is dangerous and totally unsatisfactory.

Now consider the consumer unit arrangement shown as figure 2, a split load board with independent residual current-breakers with over-current protection (RCBOs). Once again, this satisfies conditions 1 to 3, but it does not fully satisfy conditions 4 and 5 as, for example, a shower fault may result in the disconnection of all socket outlets.

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

There is one other point that needs careful attention. As shown, all 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 to separate the upstairs and downstairs socket circuits.

Figure 3, with separate RCBOs for all circuits, is an example of an FCA that unconditionally meets all 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 figure 3 registers as the best consumer unit arrangement for meeting the requirements of the 17th Edition, pressure to keep installation costs to a minimum may mean that it is not 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, cables for burglar and smoke alarms are run in metal trunking, rather than buried in the walls, they do not require an RCD and can be fed from ordinary MCBs (see figure 4).

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 50 mm within the wall, again eliminating the need for RCD protection.

In these cases, the consumer unit can be considered an FCA, provided that there is RCBO protection 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 thought acceptable for a fault on the shower to interrupt the supply to the sockets (see figure 5).

But installers should 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 Wiring Regulations 17th Edition is still very new and the details of its implementation will become clearer over time.

For those who require more information, Moeller Electric has produced a free publication called Moeller Consumer Units and the 17th Edition Wiring Regulations. The item covers in more detail the examples I have used here.

This article was originally published in EMC February 2009 as Consumer demand