The value of 100 milliohms has been considered a standard, ( in fact it was THE standard then) since 1989, as was the touch voltage of 10 millivolts.
Standards evolve continually to match the 'state of the art'.
The value of 100 miliohms for mains socket-outlets originated from the calculation to keep the simultaneous voltage between accessible conductive parts below 25V a.c. on medical IT systems and made the assumption that a 20A type B MCB was fitted. This assumes a fault in another item after a first fault to earth has already occurred, since the IT system then becomes a TN system.
The value of 10mV was only ever mentioned in guidance documents or informative notes and never mentioned in a official standard. This was just as well as it made a false assumption that the electrical installation earth needed to meet the CF applied parts touch leakage limits. Clearly this was quite flawed, as even a CF rated medical device is allowed 100mV touch leakage (measure across a 60601-1 human body model). Also the medical IT systems allowed 500uA leakage current so making a mockery of the 10mV (10uA) limit. Measuring a voltage using a high impedance voltmeter, as often suggested, will not give a true idea about the leakage current that may affect a patient, especially for parts isolated from Earth and when using long measuring leads, due to induced pick up.
Currently there is no requirement in the UK to measure any touch leakage for the electrical installation, such as mains socket-outlets. This can only be applied to medical equipment in accordance with BS EN 62353.
