17th Edition - To RCD or not RCD?


So far there has been little or no press comment about the implications of the 17th edition for commercial projects.  Yet it is for these installations where the electrical designer has some difficult decisions to make about the final circuits and distribution argues Paul Collins, technical engineering manager for Hager 

Everybody, it seems, has jumped on the 17th edition bandwagon when it comes to advice about domestic installations. Here the case is clear-cut, the most cost effective way to comply is to protect all or most circuits with 30mA RCDs.

Unfortunately for commercial installations things are little less clear. To explain we need to understand some definitions from Part 2 of the Regulations.  This categorises people into three groups:

- A skilled person is someone with technical knowledge or experience to enable him/her to avoid the dangers which electricity may create.
- An instructed person has been adequately advised or supervised to enable him/her to avoid the dangers which electricity may create - for example  facilities manager.
- An ordinary person is someone who is neither skilled nor instructed, typically the general public.

So 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 provide an RCD not exceeding 30mA for socket outlets up to 20A that are for general use by "ordinary persons". 

Exceptions are permitted where:  the use of socket outlets is under the supervision of someone "skilled" or "instructed" or if they are specifically labelled or identified for a particular item of equipment.

For large installations, which are owned by a single organisation, you can certainly make the case that an instructed person will have such control. You must then ensure that  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 large 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 of 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 16th Edition the 17th, in section 314, calls for the installation to be divided to:
a:  Avoid hazards and minimise inconvenience in the event of a fault
b:  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, that you should consider installing three, four or perhaps even more.
Consideration also needs to be given to the final distribution board, clearly these circuits will need protecting by different RCDs - perhaps by using RCBOs.
Other points to consider include cables buried in walls.
Surface cabling is far more common in commercial installations so this may not apply. However if cables are buried in walls at a depth of less than 50mm, or where metal partitions are used, then 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 and the usual solution is to install cables in a dedicated safe zone. If however an installation is not under the supervision of someone skilled or instructed then regulation 522.6.7 applies.
This regulation states that if the only protection for the cable is to install it in a safe zone then it must have additional protection by an RCD not exceeding 30mA.
So to comply with the 17th edition, electrical designers and installers must make some decisions.  Not least is to define who will be responsible for ongoing electrical safety.  If it is an instructed person then you must ensure that they are fully briefed and probably even get them to sign a document to state that this is the case. Where this is not the case you must give some careful thought to the final circuit layout and protection.

Hager is running a series of training seminars about the 17th edition on behalf of the NICEIC.  For further information visit www.hager.co.uk

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