>From my reading on the subject, EN 60950 has different Safety Extra-Low Voltage
(SELV) limits for alternating current (AC) and direct current (DC) because the
human body reacts to them differently. AC makes your muscles contract, so you
tend to hang onto the source of the electric shock. DC makes you push away,
removing contact, but you may fall or otherwise hurt yourself as you jerk away
from the source of the shock. I have heard hams (amateur radio operators) tell
of picking themselves off the floor, clear across the room, after accidently
touching the plate supply of a tube radio.
I found an article in Electronics magazine, published between 1940 and 1945 (I
can't find the article right now), on a study that was done on "let-go" current.
In this study the subjects (something like 100 young males) would grab a 1/4"
wire with one hand, and put their other hand on a copper or brass plate. The
experimenter would apply a voltage between the wire and the plate, giving the
subject a shock. Then the subject would try to let go of the wire. If they
couldn't, they could open the circuit just by lifting their hand from the plate.
If the subject could let go of the wire, the experimenter would increase the
voltage and they would try the experiment again. As I recall the experiments
were done mainly at 50 and 60Hz, with some done at DC and low frequencies, and
others up to 10kHz.
The results of the study were that let-go current was lowest in the 40-100Hz
range, and ranged from 15mA up to about 100mA. (I got the impression that some
of the young men were trying to show how macho they were...) The
let-go current increased as the frequency increased above 100Hz, or decreased
below 40Hz. For DC the subjects had trouble trying to hold onto the wire, and
instead of a shock they felt a heating effect.
I have not seen any studies on how much AC superimposed on DC changes the let-go
effect to a hang-on effect, and I don't plan to find out for myself if I don't
have to...
John Barnes Advisory Engineer
Lexmark International
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