[sdiy] diy HV probe?

[J. Larry Hendry]

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