[sdiy] diy HV probe?

[harrybissell]

I once got zapped (literally -tossed across the room...) by 25KV.  I was doing a
high-voltage
experiments on some bean (yes - bean, really :^) plants. I noticed some necrotic
spots on one
leaf.. and decided to lift the leaf with the non-conductor in my hand (a plastic
BIC pen).  You
would think that was an insulator, right ?

wrong.  The 25KV was full wavr rectified 60Hz... which gives it quite a few
volts/us slew rate, yes ?
Capacitive coupling, through the liquid ink... then into my hand... had enough
oomph to toss me.

It could have been done SAFELY. I could have turned the power off.

If you work on LIVE high voltage you are at risk.  Even if you make your own
divider (which our company
used to do, back in the 60's when high voltage probes were less readily
available...) you still have
to watch out for "hidden weenies" like parasitic capacitance.

they say "enter at your own risk"

H^) harry

"J. Larry Hendry" wrote:

> Thanks Doug.
> Liberally "snipped:"
>
> > ----- Original Message -----
> > From: The Peasant <ecircuit at telus.net>
> > I strongly recommend that hobbyists NOT diy their own high voltage probes.
> Of course maybe I'm just a little paranoid, but I'd sure hate to lose any
> members of this list over a $60 tool. :-(((  Mr Hendry, what do you think?
>
> As a HV professional, I have no choice but to say, "I agree."
>
> >  Yes it can be done fairly easily, and would probably work just fine, but
> unless you really know what you are doing you are risking your safety and
> possibly your life. There are many special considerations when working with
> high voltage that
> don't normally apply at lower voltages, and they will bite you when you
> least expect it.
>
> I'm not sure I could say anything that could sum this up in fewer words.
> When you rely on the voltage drop of a large series resistance to protect
> yourself from high voltage, you are exposing yourself to the potential (pun
> intended) problem if there is no current in your circuit.  Yes, surely the
> resistors will limit the current, and even possibly to a safe level.
> However, what is "safe" is so hard to quantify.  That current that tingles
> my skin might kill the next guy.
>
> >  I personally have many years of professional experience with high voltage
> electronics, and I diy virtually everything I own, but yet I will not make
> my own HV probe. There is no way that you will find me on the other end of
> some tape-coated straw full of resistors with 30KV on the other end!!!
>
> Funny enough, we actually have a procedure we use on high voltage circuits
> up to 138 KV where we connect a 15 KV meter directly to 69 KV and 138 KV
> conductors through resistors, relying on the voltage drop of the resistors.
> Voltage between conductors is used to determine relative phase angle where
> voltage step down transformers are not available.  Of course, we do this at
> the end of insulated fiberglass sticks with several hundred thousand volts
> of insulation.
>
> When running this test about a year ago, one of the 15 KV leads came into
> contact with support steel.  The the current through the 15 KV dialectric
> lead was limited to such a small amount, I could not even detect the
> insulation failure.  But, the voltage measurement read near zero even when
> voltage was present.  We proved this by moving the wire away from the steel
> and watching the meter read.
>
> I guess the purpose of my story is that this kind of arrangement has many
> pitfalls.  Doug's statement, "bite you when you least expect it is" so true.
> If I were making a HV probe,  It would be more than a bunch of series
> resistors. I would have one resistor to ground that would connect first.
> That resistor ground connection at the meter end would be opened ONLY after:
> 1. I verified I have a closed circuit through my meter.
> 2. I had separated myself from that meter.
>
> There are a lot of things we can do and "get by" with.  But, that does not
> mean we should.
> Larry