BarryB wrote:
I'm not an electrician or an electrical engineer. Is there a amperage below which there is no shock hazard for under 50V?
70E uses voltage as does the standards that it comes from. However if you really must know...this all comes from the work of Charles Dalzeil. He was the first one to study the effects of electricity on organisms. His work proved two things. First that electrical shock effects are proportional to body weight and second he was able to determine the thresholds for various effects such as fibrillation based on shocking animals (thus the need for the correlation...not enough available human subjects for some of the more severe testing!)
There are more details out there if you google his name but essentially mammals are highly susceptible to shocks at around 50-60 Hz (below or above this point is less of a hazard). If we are not considering tissue damage then below 0.08 s, there is little danger because the pulse doesn't last long enough to interfere with normal heart rhythm and above 5 seconds if heart fibrillation does not occur, it won't occur.. For human size subjects at around 100 mA or less, fibrillation will not occur after 5 seconds.
Now we need to get from here to a voltage. IEEE standard 80 specifies this as 1,000 ohms while the corresponding IEC standard uses a variable with a minimum of 650 ohms. Taking 100 mA as fatal and using Ohm's law (V=IR), we arrive at a fatal threshold of 65-100 VAC. In practice there are no known fatalities due to electrical shock below 50 V except for some very questionable data from a couple cases in China which were not even reported as specific values but a range of values.
50 V is pretty much the standard everywhere and it is extremely pervasive. Telephone and data systems use 48 V including Power-over-Ethernet (PoE) which uses 48 V as the standard voltage. Most DC batteries for vehicles are 6, 12, 24, or 48 V.
The only place I know of where 50 V is not the accepted standard is New Brunswick province in Canada which uses a 28 V standard (albeit with a huge number of implementation problems) although the rest of the country is 50 V.
There have been some potential medical problems reported by welders with open circuit voltages in the range of 50-150 V that purport to cause all kinds of strange neurological disorders but I haven't seen anything conclusive from any of it.
Also a word about DC is in order. With DC there is not a fibrillation issue and roughly 1 A is necessary to cause tissue destruction. Instead of a fibrillation hazard DC standards are usually 100 VDC. The concern in this case is that there is a severe amount of pain caused when de-energization occurs and the pain threshold varies from one individual to another but has been somewhat arbitrarily set at 100 VDC.