Mike Marlow wrote:


"J. Clarke" wrote in message
...
Mike Marlow wrote:


"Mark Jerde" wrote in message
news:yawld.1482$N_5.603@trnddc03...


I got the idea from (IIRC) the "grounding straps" sometimes used when
repairing computers. It's my understanding these all have a resistor
in
them to limit the current flowing through the repair-person in the
event
of
current-to-ground.

Techs don't ground themselves when working on live power which is the
only
way that current could flow. That would be dangerous. Grounding is
used
to
dissipate static electricity so that circuits aren't damaged. These
are
circuits that are being worked on with no power. It would serve no
purpose to put any kind of a current limiting resistor in the ground
path
to
dissipate static electricity. In fact, it would be counterproductive.

First, only a very tiny current through the heart is likely to be fatal.
You
don't need megaamps, or amps, 60 milliamps can do it.

I'm not sure I understand why you pointed that out. It wasn't a point
under discussion.

Safety was being discussed. Terms like "megaamps" were being thrown around.

Second, while techs
don't ground themselves when working on high voltage, they do ground
themselves while working inside computers, and the grounding devices
(wrist
straps are one kind, but there are others) always have resistors in the
cord specifically to prevent such an accident.

I was a tech for a lot of years before making career changes and we used
direct ground when working on computers. I've worked on mainframes where
we
literally climbed inside the frame all the way to common PC's. I have
seen the resistive wrist straps, but they are far from the only grounding
technique. When concerned for static discharge, which is the primary
concern for computer techs, there is no need for the resistive component.
A
direct ground path is equally effective. The accident that the tech is
concerned for when working on a circuit board is the accidental discharge
of
static electricity through the board. No need for a resistive element to
address that issue.

Be kind enough to tell me where one can obtain a wrist strap that does not
have a resistor installed, and if the purpose of the resistor is not safety
then what is its purpose? Yes, there are other devices with other safety
measures that go with them. But they all have some means of ensuring that
you don't get a low resistance path to ground.

As a kid I was badly shocked by 1960's era tools (*) and I admit I may
be
overly concerned about a direct circuit to ground (especially since
the ground wire of a standard 3-prong outlet does the same thing if I
understand
home wiring correctly). But suppose I'm concerned about doing
something
really stupid like grabbing the DC grounding wire in a sweaty hand,
then
biting through the cord of my Skilsaw. Would a properly-sized
resistor between the DC grounding wire and ground help in this
situation?


No resistor would help that kind of foolishness. BTW - your body is
more
of a resistor than the wire, so in effect there is already a current
limiting
resistor in the circuit.

Not good enough to prevent death though.


Did you read what he posted? With the type of foolishness he suggested,
there is nothing that can prevent death.

This is true, but beside the point, which is that one's body is not a good
enough resistor to prevent electrocution.

--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)