To RCD or not to RCD?

Everyone seems to have jumped on the 17th Edition bandwagon when it comes to advice on domestic installations. Here the case is clear cut: the most cost-effective way to comply is to protect all or most circuits with 30 mA residual-current devices.

Unfortunately, commercial installations are less clear.

To explain, we need to understand some definitions from Part 2 of the Wiring Regulations. This categorises people into three groups.

A ‘skilled person’ is someone with technical knowledge or experience to enable them to avoid the dangers that electricity may create.

An ‘instructed person’ has been adequately advised or supervised to enable them to avoid the dangers that electricity may create – for example, a facilities manager.

An ‘ordinary person’ is someone who is neither skilled nor instructed, typically a member of the general public.

First, the electrical designer must decide whether commercial installations will be under the control of a skilled or instructed person. Talking to several consultants, it is clear that there are many commercial installations that will not be under such control. Also, how far does such control extend?

This decision will affect the new requirements for protecting circuits supplying socket outlets. Regulation 411.3.3 requires that you have an RCD not exceeding 30 mA for socket outlets up to 20 A that are for general use by ‘ordinary persons’.

Exceptions are permitted where: the use of socket outlets is supervised by someone ‘skilled’ or ‘instructed’, or if they are specifically labelled or identified for a particular item of equipment.

For large installations owned by a single organisation, you can certainly make the case that an instructed person will have such control. You must then ensure you have fully instructed this person.

For example, that person should have a written policy that equipment not owned by the company, such as a phone charger, is not plugged into any sockets. Also, there should be separate socket outlets that are RCD-protected for use by an ‘ordinary person’ – a cleaner, for example.

If there is no such control on the use of socket outlets, you should provide RCD protection. Such applications might include hotels, small shops and some offices.

Even some large installations will be affected. Take a big commercial office installation, which is going to have multiple tenants not an unusual situation. Here you would be advised to take a ‘safety first’ approach and assume that not all these tenants will have an ‘instructed person’ responsible for electrical safety.

In this instance, all socket outlets will typically need RCD protection. Clearly, even in a small office there will be multiple PCs, printers and other equipment likely to have a small earth leakage, which of course may lead to nuisance tripping.

Just like the earlier 16th Edition, the 17th Edition, in section 314, calls for the installation to be divided to:

• avoid hazards and minimise inconvenience in the event of a fault.
• reduce the possibility of unwanted tripping of the RCD due to excessive protective conductor currents.

This has implications for planning and installing the final circuits. It may mean that, instead of installing just one circuit for the socket outlets, you should consider installing three, four or even more.

Consideration also needs to be given to the final distribution board. Clearly, these circuits will need to be protected by different RCDs – perhaps a residual-current breaker with overload (RCBO).

Other points to consider include cables buried in walls.

Surface cabling is far more common in commercial installations, so these may not apply. However, if cables are buried in walls at a depth of less than 50 mm, or where metal partitions are used, you must again consider who will be responsible for electrical safety.

The regulations provide five options, which are generally the same as in the 16th Edition. The usual solution is to install cables in a dedicated safe zone. But if an installation is not under the supervision of someone skilled or instructed, regulation 522.6.7 applies.

This regulation states that, if the only protection for the cable is to install it in a safe zone, it must have additional protection from an RCD not exceeding 30 mA.

So, to comply with the 17th Edition, electrical designers and installers must make some vital decisions. Not least is to identify who will be responsible for ongoing electrical safety.

If it is an instructed person, you must ensure they are fully briefed, and get them to sign a document to state that they have been instructed.

Where this is not the case, you must give some careful thought to the final circuit layout and protection.