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Yes most of the time even at 250v IR faults would show but your not appreciating dialectric properties of materials here, insulation breakdown is not linear therefore you cannot say that 250v IR test would expose a issues proportionally as that of a reading using 500v. It may be the case that the issue can only be detected at say 300v and remain undetected below that, this could lead to a false positive if testing on 250v, the value may show lower than expected but well within concerning values but at 500v you may see a significant breakdown in IR.
Even though many IR issues can clearly be identified at both test voltages due to the severity it remains the case that you can have a breakdown occuring at only close to peak voltages, now in the days before rcd's were thrown at every situation this would just be a matter of time before the breakdown got worse to cause noticable problems and a sparky called in, rcd's now have such sensitivity that adding general background leakage of devices its possible for the rcd to be sensitive to say 5-10mA of a fault where the rest is made up of electronics goods and other designed leakages, this in itself can make for a device able to pick up peak only issues that cannot be detected at a lower test voltage, also to note is the conductor temp' - because you are not really putting any current down the cable the conductors will not be subject to the same conditions as when loaded so the 500v also compensates for this too.
That is not practical or indeed necessary. Why would you need to note conductor temperature and compensation for this particular fault in a home. The cable is not loaded, it is dead and won't reset. That is totally OTT and you don't need to for this one. You should appreciate that the fault impedance we are talking about here is the region of 7666 ohms. Initially, 250V range would pick this fault up, and you can identify faulty circuit and subsequently the fault itself far quicker and without the issue of isolating vulnerable equipment. You have even said yourself above "Yes most of the time even at 250v IR faults would show" so why even bother with all this, it is just overcomplicating and confusing for the OP, who I also appreciate is probably long gone now