TechArchive

Original article date: April 1996

What are forms of separation and why are they important? Paul Markham product manager for low-voltage devices at Merlin Gerin provides the answers.

National Annex NC to BS EN 60439-1 the new British Standard for switchboards is all about forms of separation.

But why separate the components making up a switchboard? Why not just mount the incoming and outgoing devices the busbars and the terminals in one big enclosure and shut the door? There is of course no fundamental reason why this can’t be done and indeed this type of all-in-one construction is sometimes adopted. It is perhaps worth noting that merely putting the equipment in an enclosure provides a basic degree of separation by preventing accidental contact with live parts at least while the enclosure door is shut!

In most applications however there are substantial benefits to be gained by providing additional separation between components. For example dividing the enclosure into separate compartments may allow fuses in one compartment to be replaced safely without the need to shut down the whole; and if the outgoing terminals are separated from other components it may be possible to connect new circuits while the board remains in service.

These are not the only reasons for providing separation in switchboards. Another feature is useful protection against the development of arc faults. Separation also protects against the passage of foreign objects from one part of the switchboard to another preventing for example a loose screw from finding its way into the magnet assembly of a contactor.

From these simple examples it might seem that the more separation the greater the benefits – but beware! More separation often means a larger and more expensive switchboard and it is quite likely that some of the additional “benefits” may not be useful in the intended application. Specifying maximum separation is not therefore a default or safe option. In every case the requirements of the application should be looked at carefully and the level of separation chosen to suit.

Clearly however if every switchboard manufacturer supplied products with slightly different levels of separation or even if they used the same types of separation but described them differently from their competitors switchboard purchasers would face severe difficulties in drawing up specifications and evaluating tenders. Fortunately BS EN 60439-1 provides the answer. The standard itself defines four basic forms of separation while the National Annex NC further splits these forms into a total of fourteen subdivisions.

Without explaining the subdivisions in detail they are largely concerned with separation between terminals and other elements and with the way that separation is achieved. They make a distinction for example between separation provided by metallic barriers such as the enclosure’s internal walls and that provided by insulating barriers.

But there are two important points which need to be made concerning forms of separation. The first is the popular belief that the higher forms of separation particularly Form 4 specify that the assembly is safe for live working. This dangerous belief is entirely incorrect. In fact the standard states categorically that no assembly which it covers is ever considered safe for live working!

The next point is that the forms of separation and more particularly the subdivisions defined in BS EN 60439-1 and National Annex NC are not to be confused with those relating to an earlier standard BS 5486 and the EIEMA guide relating to this standard. Unfortunately the designations are very similar but the detailed requirements differ. It pays to ensure that suppliers’ literature and documentation specifically relates to the current standard.

Switchboard users cannot fail to benefit from matching the forms of separation which they specify to the requirements of their applications. The uniformity imposed on manufacturers by BS EN 60439-1 makes this matching easier but specifiers still need to have a clear understanding of the standard and its implications. Those needing more detailed information are recommended to attend one of the seminars currently being presented by Merlin Gerin which provide a complete and straightforward explanation of BS EN 60439-1 and the National Annex. The seminars also cover type-testing another important switchboard-related topic about which there is considerable confusion and even misinformation. They are hosted by senior engineer Tom Mennell who is a member of the committee which drafted BS EN 60439-1. Full details of seminar dates and venues are available from Dan Varnam on (01952) 290536.

• Merlin Gerin
• Tel: 01952 290029
• Fax: 01952 290534

April 1996