The certificate states that the Nature of Supply Parameters can be obtained by measurement or enquiry, National Grid guidance says:

Maximum prospective short circuit current

The maximum prospective short circuit current is used to determine the short circuit rating requirement of electrical equipment.

Low voltage connections (up to 1000V)

The following values can be assumed at low voltage connections:

• Single phase connection; 19.6kA*
• Two phase, split phase or three phase connection; 25.9kA

https://connections.nationalgrid.co.uk/information-for-electrical-installers/

So you can write 25.9 kA on the certificate without testing, but that may be problematic.

But on the load side of a suitable 'death or glory' fuse to limit the energy let-through, even that 25.9kA becomes rather more manageable, as the  maximum damage is reduced to being the equvalent of a much lower PSSC.

Which is how 10kA breakers are fine downstream of almost any supply that a 100A B1361 can protect, almost regardless of the pre-fuse pssc.

Of course the loop tester  does not 'see' the fuse and has no means to allow for it's  energy limiting action so may well over-read the danger.

(that cast iron box by the incomer does something.)

Mike

But on the load side of a suitable 'death or glory' fuse to limit the energy let-through, even that 25.9kA becomes rather more manageable, as the  maximum damage is reduced to being the equvalent of a much lower PSSC.

Which is how 10kA breakers are fine downstream of almost any supply that a 100A B1361 can protect, almost regardless of the pre-fuse pssc.

Of course the loop tester  does not 'see' the fuse and has no means to allow for it's  energy limiting action so may well over-read the danger.

(that cast iron box by the incomer does something.)

Mike