Re: Safety Questions (fwd)
---------- Forwarded message ----------
Date: Wed, 22 Jul 1998 11:53:32 -0700
From: Jim Lux <James.P.Lux-at-jpl.nasa.gov>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Safety Questions (fwd)
Tesla List wrote:
> ---------- Forwarded message ----------
> Date: Tue, 21 Jul 98 22:29:42 EDT
> From: Jim Monte <JDM95003-at-UCONNVM.UCONN.EDU>
> To: tesla-at-pupman-dot-com
> Subject: Safety Questions
> Suppose you want to discharge a capacitor and use a long insulator, say
> a piece of PVC tubing, to insulate yourself from the cap. You don't
> really know until after it's too late whether the insulator will be
> adequate. There may be conductive "junk" on the surface. It occurred
> to me that it would be much safer to ground the end of the tubing that
> you are holding. That way, the short is directly to ground rather
> than through you if the insulator fails. Is there a flaw with this
> reasoning? Is this typically done?
First off, you might give Ross Engineering in Campbell CA a call and ask
them for their literature on Personal Grounding Rods (it might even be
on the web by now), because that is essentially what you are describing.
Typically, you'll have a contact at the end of an insulated rod, and a
suitable wire from that contact (separate from the rod, except at the
end) to ground. The idea being that the wire provides a suitable path
for the current, rather than through you. Of course, you want make
really sure that the wire is grounded, that the rod is long enough so it
doesn't flash over the length, (or down the inside of a hollow tube).
Finally, you want to make sure that the grounding conductor can take the
energy you are going to dissipate. If you were discharging a 10 kJ cap
charged to 50 kV and you only had 16 gauge hookup wire as your grounding
lead, you will learn what an "exploding wire" really is.
For cap discharge use, a suitable(!) series resistor of a few tens or
hundreds of ohms to limit the current to 10-20 amps or so is a good
addition. It saves on the nerves by reducing the bang (unless the bang
is what you want) and prevents blowing off chunks of the terminal.
There is a difference between a suitable safety ground (which you leave
hooked up while you are working on the system, and which disspates any
charge that might accumulate) and a cap discharge ground (which is
designed to dissipate the stored energy). The usual personal grounding
rod is the former, not the latter.
BTW, a 6 ft length white PVC pipe filled with the foam in a can
insulation works fairly well as a cheap hot stick. If you get the #8
urethane or silicone covered car stereo power wire, it is fairly
flexible, compared with the usual #8 stranded wire which is pretty stiff
and always gets in the way.
> To prevent current from passing through a person's chest cavity, how
> about wearing two watches with spring-type metal bands, one on each
> wrist, and connecting them with a conductor tucked inside the person's
> shirt to keep it out of the way? I admit that this sounds a little
> strange, but it also seems like it might make a difference. Comments?
You should concentrate your efforts more on making sure that a situation
where the voltage is across your chest doesn't occur in the first place.
Good design(minimum exposed points, suitable clearance distances), good
work practice (one hand in the pocket), etc. is far more effective than
some after the fact fix.
You should talk to Brent Turner who does a "sitting on the tesla coil"
demo wearing chain mail gloves and wires across his body for just.
The "hot workers" who work on energized 765 kV power lines (something
you'll never catch me doing, no matter how much theory supports it) wear
a conductive suit to equalize the field across their body, because the E
field near the line is quite high (many kV/cm) and you could develop a
lethal current through your body just from the impressed E field.
> Jim Monte