Hi Ted:
> The electrician's rule of thumb when working on domestic wiring was to
> always check the wire is dead, even though you isolated it, in case of
> bridging or faulty connection. The technique used was to brush the
> exposed conductor lightly with the back of the fingers. Any AC present
> will be noticeable as a tingle. DC would cause contraction of hand
> muscles which would cause the hand to withdraw from the conductor.
The technique of brushing lightly avoids a low-
resistance connection to the body which would
occur with a tight or solid connection.
The back of the hand has more hair, fewer sweat
glands, and higher resistance than other parts
of the body. So, this, too, tends to avoid a
low-resistance connection.
Both ac and dc would cause a tingle; in this
situation, the protection is by means of a high-
resistance connection which limits the current,
and the electrician maintains control of his
muscles regardless whether ac or dc.
> The same teacher explained that this precaution saved lives. Should the
> hand be allowed to grasp a live conductor the paralyzing effect of DC
> would be more likely to cause injury, shock or death because the effect
> of the current on the hand muscles when grasping would increase the grip
> and prevent the hand being opened to break the circuit. In this case it
> is precisely tetanus of the body which is dangerous because it
> interferes with the ability to move away from the hazard.
Dc may cause a single contraction, but not
continuous contraction. In my experience, the
dc tends to throw you away from the source.
Charles Dalziel reports in his experiments that
his subjects chose not to let go of dc (and they
could!) because of the severe pain at the moment
of letting go.
I studied Reilly (again) and found no reference
to tetanization due to dc. Reilly says that
"As sinusoidal frequencies approach dc, it
becomes quite difficult to stimulate peripheral
nerves. Stimulation may be elicited at the
onset of direct current as with any rectangular
pulse. However, following the onset of direct
current, additional stimulation of... skeletal
muscle is not so easily achieved..."
> Power is hazardous to health in all its forms.
I would say that ENERGY is the key parameter, not
power. Defibrillators are calibrated in joules,
and essentially apply a corrective pulse to the
heart to contract the heart muscle and then let it
resume normal rythyms.
Because we can easily do so, the limits in safety
standards are in terms of voltage and current which
can be sensed. Sensation levels are very much below
the injury level (where energy becomes important).
Best regards,
Rich
ref: Reilly, J. Patrick
Applied Bioelectricity from Electrical Stimulation to Electropathology
ISBN 0-387-98407-0, Springer-Verlag, New York
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