Back in the 80s I was an installation inspector for the local electricity board and before fitting a meter we had to check that the earth loop impedance was below 0.8/0,35 ohms.
You should always design to the maximum stated value as the DNO may reconfigure their network without notifying the customers.
A typical example is the PSCC the 16kA is based on you being right next to a substation with a 3 meter long service cable. You may know that the substation is a 300 meters away the DNO will say that they may but a new substation right next the house in 10 years time.
Some DNOs allow you to reduce the 16kA if you know the distance between the point of supply and the nearest footpath, again even if you know the main cable is across the road, it may be moved in the future. Appendix 14 says you do not need to measure PSCC in a domestic installation if the DNO declare it at 16kA.
My understanding is that you can use 6kA mcbs as long as they are in a type tested enclosure backed up by the DNO fuse, cant remember where that is written down though.
You should always design to the maximum stated value as the DNO may reconfigure their network without notifying the customers.
A typical example is the PSCC the 16kA is based on you being right next to a substation with a 3 meter long service cable. You may know that the substation is a 300 meters away the DNO will say that they may but a new substation right next the house in 10 years time.
Some DNOs allow you to reduce the 16kA if you know the distance between the point of supply and the nearest footpath, again even if you know the main cable is across the road, it may be moved in the future. Appendix 14 says you do not need to measure PSCC in a domestic installation if the DNO declare it at 16kA.
My understanding is that you can use 6kA mcbs as long as they are in a type tested enclosure backed up by the DNO fuse, cant remember where that is written down though.