The origin of a supply

by | 1 Dec 2016 | electrical

The origin of a supply

by | 1 Dec 2016 | electrical

The origin of a supply

by | 1 Dec 2016 | electrical

Non-conbustible material

In effect we are saying that all boards should now be manufactured and installed as metal units. There does not seem to be a rush by manufacturers to develop other non-combustible materials and why re-invent the wheel anyway.

BS 7671:2008 Amendment 3 (IET Wiring Regulations Seventeenth Edition), came into effect on 1 July 2015. This requires that all electrical installations designed and periodically inspected comply with the updated regulations.

Contractors are expected to hold a copy of BS 7671:2008. The Amendment included a new regulation requiring consumer units and similar switchgear assemblies in domestic premises to have a non-combustible enclosure.

BS EN 61439-3, mentioned in the regulation above, is entitled Distribution boards intended to be operated by ordinary persons (DBO).

This is how we interpret the regulation as pertaining to domestic dwellings.

Premises

Regulation 421.1.201 uses the term ‘premises’. The question could therefore arise of whether the requirements of the regulation apply to a consumer unit or similar switchgear assembly within an outbuilding such as a garages or shed, or mounted on the outside of a building.

However, the Regulation was introduced to cover the interior of a household building and any garage or other outbuildings integral, attached, or in close proximity to that building. It is not intended to apply to a consumer unit or similar switchgear assembly that is not within a building, such as a unit mounted outdoors on the outside of a building.

The significant factor for the electrician is in appreciating the regulation requirement for areas outside the main premises. Although ‘premises’ is not defined explicitly, a shed at the bottom of the garden which may contain flammables and has a supply in it, would most certainly be considered part of the premises in consideration of the new regulation.

In addition, at that shed, the electrician may want to install an RCD housed in its own enclosure before the tails arrive at the consumer unit. The first RCD would of course have to also be enclosed in metal. The RCDs ratings are selected for discrimination, i.e. 100A at first RCD and 80A at CCU RCDs, as an example.

The use of metal enclosures in outer sites like sheds has typically been used to provide greater mechanical protection. But in recent years the trend has been to use thermoplastic enclosures as they would appear to be just as tough these days and needless to say much cheaper.

However, the current requirement to use non-combustible enclosures places emphasis on the consideration of an electrical fault occurring between the incoming cables and the RCDs at the source of the sub-installation at the shed.

If interpretation of other wiring regulations have resulted in our shed utilising a TT earthing system, then faults occurring on the upside of the RCDs will not have a valid fault path to earth unless the cable armour is independently connected to the earth bar at the origin of the main supply.

But back at the shed, if a live conductor is damaged at cable entry at the 1st RCD enclosure or the 2nd RCD enclosure at the sub-board, then both metal enclosures could be live.

The two earthing arrangements need to be kept separate and so earthing the armour to the main supply earth is done to protect the cable length to the shed.

In this example where the SWA arrives at the 1st RCD enclosure at the shed, it must not be made off with the standard metal banjo and gland, otherwise it will also be connected through the metal enclosure to the TT earthing arrangement.

Now the real issue arises at the TT side. With a plastic enclosure the exposed conductor would not liven the plastic, but of course with metal, if it should become live, both the 1st and 2nd tier RCDs will not trip because the fault current will not be high enough due to the increased impedance of TT systems.

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