What are the requirements for a consumers main switch on a TPN 200A new supply ? Can the tails from the metering go straight into a panel board with a 4 pole 200A MCCB as the main switch, or do you have to install a separate isolator

Does the size of the supply matter?

Given that a separate isolator is not required on the domestic scale, I assume not.

However, it is convenient to install one, because it then leaves you free to do anything inside the panel board, or even replace it.

I would have thought it was a BS 7671 matter rather than the DNO's (their responsibility mostly stops at the cut-out terminals) or supplier's (after metering). The only time I can think of where DNO specifications are relevant is where you're depending on their equipment - e.g. their main fuse for overcurrent protection - for which they tend to specify things like min c.s.a. and max length of conductors and such like.

As with many things, BS 7671 allows designers quite a bit of latitude to do things how best suit the particular situation and their customer's requirements. There are advantages to having a separate isolator upstream of the main board - e.g. it allows the main board to be conveniently isolated for periodic inspections & tests, alterations or even replacement. It is always possible to use the DNO's cut-out for isolation, but that usually has to be done by DNO or supplier's personnel and at their convenience so tends to cost the customer both time and money when it's necessary. In the domestic world where a single call out can be in excess of £100 and a DP "REC" isolator available for about £20 it's a bit of a no-brainer to fit one at the first opportunity to my mind, but it's not a requirement and many a supplier don't fit them by default.

   - Andy.

Just spotted this - apologies for the late reply.

I have seen installations in the UKPN area exactly as you describe - panel board with MCCB main switch fed directly from the cut-out (CT metering). Cables in 100x100 metal trunking for neatness and protection.

Having an isolator in between obviously has some benefits as Andy said. Disadvantage is that an 200A isolator is large and expensive, also it's another possible point of failure.

This is a classic example of Requirements/Regulation versus Engineering Judgement and yes Designers should have and use Engineering Judgement.

Others here have stated some of the Pros and Cons but you need to also consider maintenance and future proofing as well as safety.

On the safety side
If the isolator is inside the CU/DB and you remove the cover to work there is a possibility of contact with Live parts.  Some CU/DB will not allow you to lockout before removing the cover due to lack of physical space.  By having an isolator upstream you can safely lockout/lockoff and be in CONTROL for whatever task is required.  Looking into the future a CU/DB change becomes a more simple procedure.  As would the connection of Renewal energy which in effect is a second supply into the CU/DB.

Thus personally I think an upfront isolation system should be included even if CURRENTLY they are not a REQUIREMENT.  Look to the future and consider you are the person/people who will come back and work on this install and have to maintain it when commercial/financial/Duty holder implications apply.  (DNO/DSO appointments for isolation are not always punctual)