OP, You make the mistake of assuming the 5seconds has much to do with the limits of human withstand, it does not, the 0.4 seond disconnection time is based on protecting persons, it used to apply to just cirucits suppling socket outlets, but over the years, as gradually been increased in scope. In line with wat Geoff said, we are worried about equipment the user might be grasping hold off and unable to let go of if they get a shock, AC is slightly better than DC, but it doesn't follow that you'll be able to throw the metal cased drill away if it comes live! (DC makes your muscles contract and not let go, AC you are there twitching with no control....) The 5second disconnection time is somewat arbitrary, legend has it that a wiring regs commitee member did a fag packet caluclation of what could be reasonably achieved in an installation he had knowledge of, and 5 seconds is what it ended up being. You arn't likely to remain in contact with fixed equipment that has given you a shock, you quickly move your hand away from the live light fitting, or worst case fall off your steps, etc
Touch voltage is not always going to be 50% of supply voltage either, if its twin and earth with a reduced CPC , it'll be quite a bit higher, if the cirucit is in MICC, however, it'll be lower
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