Posted By Jane Blunt
There is no strict definition of high voltage, as you may guess from the two previous correspondents.
You need to decide why you are defining a high voltage - if your definition is to be used to help divide equipment, etc, into those items unlikely to pose a threat of death, and those that are potentially lethal, then the first correspondent is nearer the mark. Ordinary mains voltage in the UK, 230/240 V AC can be positively lethal, and yet it comes under the IEE description of low voltage.
AC and DC have different effects on the human, and we can withstand a somewhat higher DC voltage, in most cases.
Other definitions of high and low voltage appear to be more to do with the difficulties in insulation, switching, and problems with arcing, etc. That is, technical problems, not directly concerned with the safety of human beings.
When looking at the safety aspects of electricity, do not forget that even a very low voltage source, if it is capable of delivering a lot of current (e.g. a power supply for an electromagnet), can be a source of fire.
The effects of electricity are complex, because there are the effects of both the voltage, which shocks, and the current, which can heat things up. We can be killed by either, I believe. Electricity also has some bad habits, because it is possible for it to paralyse our muscles so that we cannot let go, and cannot breathe.
Regards
Jane