If the new circuit requires a type A (e.g. because it has sockets or a known load with electronic components) then protecting it with a type AC isn't compliant with BS 7671 amd2 - there's no leeway there. On the other hand, being plastic is not an issue.

But if an appliance becomes faulty with an earth fault, unless the system is TT, the overcurrent protective device will still disconnect the faulty appliance.

Z.

So? Normally RCDs are required for additional protection rather than fault protection. A nail in the wall contacting just the L conductor won't cause an overcurrent, but may still kill you if you touch it, unless an RCD trips. And hopefully that RCD isn't being blinded by DC leaks or faults.

Banging a nail into unknown territory is stupid. It is just like tombstoning, i.e. jumping into unknown water that may have hidden dangers. Fatality by banging the nail into a wall and connecting with a line conductor MAY kill somebody, but largely depends upon how well they are earthed. Probably if they are standing on an insulating wooden floor or carpet they will just get a reminder of how stupid they have been. There are not many reported deaths regarding this scenario.

SOLUTION. Don't bang nails into walls that may contain pipes or cables.

Z.

Banging a nail into unknown territory is stupid. It is just like tombstoning, i.e. jumping into unknown water that may have hidden dangers. Fatality by banging the nail into a wall and connecting with a line conductor MAY kill somebody, but largely depends upon how well they are earthed. Probably if they are standing on an insulating wooden floor or carpet they will just get a reminder of how stupid they have been. There are not many reported deaths regarding this scenario.

SOLUTION. Don't bang nails into walls that may contain pipes or cables.

Z.

You seem to be saying that we don't need to install RCDs for additional protection as only stupid people would be killed by their lack?