Re: Safety Questions (fwd)
---------- Forwarded message ----------
Date: Thu, 23 Jul 1998 17:12:31 +0300
From: Harri Suomalainen <haba-at-cc.hut.fi>
Subject: Re: Safety Questions (fwd)
>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?
It depends on situation. Sometimes tesla primary systems are floating
(ie. not grounded at any point). In that case there is no discharge path
even if you toughed directly one of the cap terminals (assuming
insulators etc. do not break down). In that case grounding the discharging
rod makes no sense.
If cap system is some way grounded you are in danger if you touch the
nongrounded cap terminal. In that case having a rod with grounded
end (or grounded middle point etc) will save you by directing the leakage
currents away from you as you assumed. This type of protection is
often used with eg. high-voltage metering probes. They usually have
grounded shields to take any arcs or grounded outer sides which you
Then there is still the question of how much protection is needed. I
rely on a 2" pvc insulator too much but with a rod of few feet long I'd
pretty safe after visually inspecting it is not dirty. At that case I
would see no
need for any grounded protection shields.
If you feel worried about flashovers you might also make better rods with
some disks to increase the lenght of path at the surface. They would also
make things much harder for any discharges along the rod due to the
field having the direction parallel to the rod helping the discharge go
Thats why most high-voltage insulators have such a form. Avoid sharp edges
at the insulating disks!
>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?
Grounding personel leads to other problems. That makes always new risks.
If a grounded person toughes something live the currents are really high
(in that situation for example from foot through chest to the wrist). It
has a risk with the grounded wiring. If such wire gets loose some way it
may tough something dangerous and make the accident.
In my opinion it is much safer and easier to just use enough overrated
rods to discharge caps of tough other live things. Only having a grounded
shield to take discharges would make sense. Preferrably that should be
arranges so that user does not touch it.
With caps you might also consider having bleeder resistors to do the stuff
automatically and use a discharge rod just as a safety check device to
make sure resistors have not broken. Safety issue with rods should not be
difficult to deal with.
Harri Suomalainen mailto:haba-at-cc.hut.fi
We have phone numbers, why'd we need IP-numbers? - a person in a bus