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.
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.