OK Bryan,
Yes I can answer your questions, and, yes I can back them up with pointers to appropriate legislation and guidance.
However, I need to understand your problem or question first.
Some of your OP is somewhat confusing.
There is no statute law requirement for any qualification or certification for PAT, nor any other electrical qualification, this is one HUGE anomaly, IMHO of statute law and other guidance
The requirement is under several pieces of legislation and it is for competence, not for a specific qualification.
In the same way there is no statute law requirement for PAT.
The “requirement” comes under EAWR, to ensure that the equipment is safe for use, etc.
Now if you are talking about extension leads/cables then the requirements under the IET CoP (hence the C&G exam in PAT) as you are talking about qualifications are identical regardless of operating voltage.
In fact, you are only talking about two voltages wrt true earth when you are discussing 110/230/400V “extension leads”.
There is no such nominal voltage in the UK anymore which is referred to as 415V in standards.
That is if the 110 is derived from a CTE supply (i.e. big yellow transformer, or the like), this gives 55V to earth.
230 & 400V leads are both 230V to earth.
The current carrying capacity is irrelevant as the procedures for I & T, are the same.
Mistakes and incompetence are easier to spot in higher current cables and devices, during or after use, however, the fundamental procedures for termination, inspection, testing, and the physical science and engineering are identical.
Now when you come to plant, that is where things get much more complex.
The “PAT”, “qualification” does not really cover much 3ph equipment, and that which it does is VERY simple.
If a “person” has only undertaken the “PAT” qualification and has no further experience and qualifications to draw upon to understand the testing and inspection of 3ph “plant” then they are not competent to do so.
100 yrs of testing experience on 230V toasters and kettles (for example) following the “PAT” course will never make them competent to assess the safety of even simple 400V plant and equipment.
Now BS7671 covers the fixed installation, extension leads, and “plant” are not covered by BS7671, some fixed equipment could be, but that needs serious consideration.
“Plant” and “machinery” are outside the remit of BS7671, and covered in general by the machinery directive, if they are stand alone.
So, I need to understand a little more about these items of “plant”, and what you mean by “plant”.
Finally, for now. BS7671 is on the 17th Ed, AMD 3, the IET CoP for “PAT” is on Ed 4, so what are you referring to when you state 9th Edition qualified?
The 9th edition of BS7671 was published in 1927, and the 10th edition in 1934, so these are somewhat outdated now!