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On 8/21/06, Paul Weaver <iso@xxxxxxxxxxxx> wrote: > > > Big high current DC supplies to several devices have their own > > > interesting behaviours though, involving induction and possible high > > > voltages when things are switched. > > > > And, according to a number of electrical engineers I know, they're a > > much higher risk if they decide your body is a convenient conductor. > > But the chance of that happening is reduced. You can have upto 340V > potential with AC, or 680V if you have a PSU fault with acapacitor. > That's enough to break through your body fairly easilly. It's much > harder for a 50V potential to break through. > > Obviously transfering the same power means that if your body does > conduct, you'll have a much higher current, but then 1A or 6A, it's > pretty irellevent. 50V is considered the safe limit 50v and below no special precautions are recommended. Be that DC or AC. The problem with DC is the fact that if you accidentally touch a high voltage DC power supply you kind of end up stuck to it, with AC if the current enters your muscles they tend to pulse and you have a *chance* of getting off. The other thing with AC/DC is that AC current is controlled by reactance whist DC current is controlled by resistance alone. As reactance is a sum of the DC resistance and effects related to capacitance and inductance and frequency of the applied voltage it is likely to be significantly higher than DC resistance alone and hence have a lower current The safe limit is determined by looking at the average impedance of the human body, if you start considering high humidity, wet hands, good ground conduction then the potential needed to drive a fatal current is greatly reduced. Even at 50V current WILL flow through your body it is just likely to be very low and not harmful. As for the induction/ fly back problem, this is very real. Induced voltage is proportional the inductance and the rate of change of the current break. I have done it myself with a large electromagnet, it was running at 10A, by disconnecting the power supply suddenly a voltage some where around 1000V was generated and arked across the relay. -- Robin Cornelius http://www.byteme.org.uk -- The Mailing List for the Devon & Cornwall LUG http://mailman.dclug.org.uk/listinfo/list FAQ: http://www.dcglug.org.uk/linux_adm/list-faq.html