tube prototyping safety

[J. Larry Hendry]

> From: jhaible <jhaible at debitel.net>
> I was wondering about safety in building tube circuits,
> especially about grounding.
>  -------------- snip ----------------
> But what are you doing while prototyping, when you're
> working on the open circuit ? 

I know this may sound simple, but it is a very effective personal safety
precaution that I learned in school (many) years ago when working on tube
circuits.  Once the high voltage circuit is energized on the bench I use
the "one hand in the pocket rule."  If you have only one hand on the bench
with your meter or scope probe, and the other in your front pocket, you are
much less likely to ever get yourself in trouble.  You easily get adjusted
and find that one hand is enough when the circuit is hot.

Another thing to consider are special gloves.  Recent changes in OSHA
safety rule in the USA have caused even utility workers to wear certified
protective gear when working on what we consider low voltage (under 600
volts).  The result is that the companies that make our high voltage rubber
goods (20KV gloves) are now also making 600 volt gloves.  Unlike their big
20 KV brothers, the 600 volt gloves are lightweight and comfortable.  They
are very thin rubber and have a soft leather protector that prevents the
rubber from getting damaged from sharp edges.   I use my 600 volt gloves
more off the job that I actually do at work.

I have been working in the high voltage industry for more than 23 years.  I
can say without reservation that the two things that are most responsible
for preventing injury are protective personal equipment and following good
procedures.  When working with high voltage supplies, make yourself some
good common sense rules that you believe will best protect yourself and
equipment.  Then stick to them.  For me, that is the one-hand rule.

You do also bring up a good point concerning the neutral or ground.  It is
just as important to insulate one's self from the power ground.  Even at
very high voltages, this procedure is always used to provide that extra
buffer of safety should a high voltage contact be made.  Even if your bench
probing does put you in contact with a few hundred volts, if you have no
contact with the return path (my hand is still in my pocket), your chances
of being injured are greatly reduced.

Sorry to ramble, but I consider myself a bit on an expert on this subject. 
Part of my job responsibility is to commission all new equipment going in
service.  In 23 years, I can assure you that I have "let the smoke out" of
more that one piece of gear.  Stuff does go wrong and planning for that
possibility is good safety practice.

Larry Hendry