One needs to measure the loop impedance to have more then 6kA simple ohms law, 230/6000 = 0.04 Ω (0.07 phase to phase) unlikely with a domestic supply,
We've been through this a good few times before.
I certainly agree that it is virtually inconceivable that the PSCC in a domestic installation will ever be anything even approaching 6kA.
The issue (in terms of BS7671) seems to arrise because of it's Ap[pendix 14, which says:#
..... In domestic (household) or similar premises, where a consumer unit to BS EN 61439-3 is used and the maximum
prospective fault current declared by the distributor is 16 kA, it is not necessary to measure or calculate prospective
fault current at the origin of the supply.
Even though it makes little/no sense in relation to domestic installations, It appears that DNOs generally
do "declare the PFC" as being 16kA, which at least some people take to mean that one has to work with that figure. If one has that belief, then it means that, without the dispensation regarding type-tested CUs in 536.4.201, domestic DBs.CUs would have to use 16 kA devices.
If I recall, this discussion initially arose because someone (I think BAS!) suggested that, even though my installtion is 'under the control of an ordinary person' (myself), and even though all my DBs are ('type-tested') CUs, I could not invoke the dispensation of 536.4.201 to have only 6 kA devices, because my supply was not "a 230 V single-phase supply rated up to 100 A" (it is a 3-phase supply) - such that all my devices should be 16 kA ones!
I do find the Approved documents do seem to embellish on the actual law,
The do not claim or intend to be presenting the law. Rather, they are guidance as to
one way of satisfying the law.
.... I would not worry about fitting a three phase distribution board in my house, but one I am not classed as an ordinary person, and two not until a court case is fought do we really know what the courts would rule.
The courts can only rule on the basis of laws and, despite your speculation, I personally very much doubt that there is any legislation that prevents the installation of a single-phase non-type-tested DB or a 3-phase DB in an installation which is 'under the control of an ordinary person'.
Kind Regards, John