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Re: Terrified Parents
Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net>
> Original poster: "Stephen Mathieson" <s.mathieson-at-charter-dot-net>
>
> Always have a safety "observer," there was no reason for the assistant to
be
> holding the meter. Also, "floating" a scope is an unacceptable practice
when
> it comes to safety, make a HV measurement differentially using two
channels.
Floating a battery powered instrument is a perfectly safe way to make a
measurement. I'd venture to say that you'd have a tough time measuring
millivolt level signals riding on 20 kV with a differential measurement.
Modern HV practice often has a whole lot of digital and low level analog
circuitry floated at the HV deck. This is particularly so in tube circuits
where, for instance, the cathode is floated at -voltage so the anode is near
ground. The grid and filament supplies are referenced to the cathode, with
the control signals passed across the LV/HV barrier by optos or
transformers. Hard to measure grid current (milli and microamp) and voltage
waveforms (measuring fractions of a volt out of a few hundred) in such a
situation without a floated instrument.
Mind you, precautions need to be taken that the operator doesn't touch the
instrument, etc., for which we often put it in a plastic box. We also have
HV isolated IEEE-488 GPIB interfaces. Some of the new instruments have IR or
wireless RF interfaces (particularly the PC based ones, you can just get a
wireless keyboard/mouse)
Oddly, our biggest problems are fighting off the ESD protection folks, who
want everyone to wear a ground strap, which IS hazardous around HV.
And, of course, the incident with the Simpson did occur over 50 years ago,
and standard industrial practice has changed quite a bit in the intervening
years. I was watching a TV program about linemen, and they were commenting
on how hard it was to get the guys to wear a (insulating) hardhat, even
though the vast majority of injuries were from dropped tools, etc., and most
of the electrocutions were from the head getting too close to a MV feeder.