[BC] Saving Your Own Life /was/ Tech. competence

[Phil Alexander]

On 12 Oct 2005 at 8:00, Robert Meuser wrote:

> please see in line
> 
> WFIFeng at aol.com wrote:
> > In a message dated 10/12/2005 5:01:53 AM Eastern Daylight Time, 
> > reader at oldradio.com writes:
> > 
> > 
> >>There is a big difference. Do you "believe" a transmitter is off just 
> >> because the meter says there is no plate voltage? You have the faith 
> >> that you can just open the door and dive in?
> > 
> > 
> > No. That's what the shorting stick is for. Do you have "faith" in it that it 
> > will ground-out the hazardous voltages before you start work?
> 
> No a shorting stick needs to be checked at least by an ohm meter which has been 
> checked.
> 
> > 
> > This is not a smart-a** comment, here, this is a serious question: How the 
> > heck do you ever get *any* work done? Do you trust your own voltmeter? Do you 
> > trust the shorting stick? Do you trust the breakers in the transmitter, itself? 
> > Where do you draw the line, how do you draw it, and then how do you know you 
> > *can* work on the bloomin' thing?
> 
> 
> Trust nothing believe nothing if death is the alternative

Since I changed the topic, I left more of the previous than Barry
might like ...

HOWEVER - This is a very important topic IMHO, and Rob's point is VERY
well taken.

Blind faith in anything just before you open a cabinet - even a cabinet
with "low" voltages invites FATALITY - YOURS. IOW, the ultimate FATAL
ERROR, for which there is no reboot or recovery. ZAP - GAME  O-V-E-R.

As a part of normal inspection, j-sticks, bleeder resistors, interlocks
should be inspected for integrity on a frequent schedule. When in doubt,
use your own EXTERNAL j-stick. IOW a j-stick with a long, non-conductive
handle, well insulated cable and a LARGE bulldog clamp on the end of it.

Remember IT IS THE CURRENT FLOW THROUGH YOUR BODY THAT KILLS YOU, not 
the voltage. All the voltage does is break down skin resistance and
cause current flow. In theory, if you have open cuts on both hands and
contact them with a flashlight battery, IT CAN KILL YOU. If you touch
an incompletely discharged 200 V supply in a solid state box while
touching ground, unless your skin resistance is high, the odds of DEATH
are not something I'd ever put to a test of "faith" or any other kind
of test.

So, what to do?  Before you open the box, kill ALL power disconnects
to it, and make very sure you have ALL of them. I've seen some with
as many as three AC feeds going into them. For example, consider an
old box originally with crystal heaters, now with a newer exciter,
and a main HV primary supply.

The first time you open a box, use an external j-stick that you KNOW 
is a short in the milli-ohm range or lower. At the very least, check 
it with your ohmmeter on its lowest scale for a reading that is the 
same as the one you get when you cross the ohmmeter test probes, IOW 
< 0.5 ohms and preferably < 0.1 ohms, but don't stop there. Connect your 
external j-stick to bare metal on the frame of the box and measure from 
the end of the stick (the "j" end) with your ohmmeter to another bare 
metal spot on the frame. AND make very sure neither of those bare spots 
is anywhere near anything the could be "hot" if the box were energized. 
This is better done BEFORE you open any door or panel, but not always
possible. Unless the reading is < 0.5 ohms, you need to find another 
spot to clamp your portable j-stick.

Carefully, and slowly crack the door of the box about 1/2 and inch,
go away and have a cup of coffee or stand there and look at your
watch for 5 minutes - whatever works for you - to give the built in
safety protection (grounding switches, bleeder resistors etc.) time
to do their thing. AFTER, and only after that, use your external
j-stick. Hit every obvious HV terminal first, and then every wire
you can see. ALWAYS touch a wire with a j-stick first, and ALWAYS
work with one hand in your pocket.

Once you have confirmed the box is dead and discharged, the first
thing to check is the internal j-stick(s) - first visually, then
with a wrench to make sure all connections are tight, then with your
ohmmeter. Do this at least once a month as your first maintenance
activity.

Do all of the above, any you may be able to go to church next Sunday
and practice your faith if that is your preference. Don't do it,
and you may not have that opportunity. 

Always remember that ANY TIME you open a box, you have the chance of
becoming yet another proof of Kirkoff's Law with YOU as part of the 
circuit. In it's simplest form, Kirkoff says that given the chance, 
current will flow (equally, etc etc) and you don't want to be the 
conductor that closes that circuit.

If taking precautions doesn't give you time to get your work done,
consider that you may be in the wrong line of work.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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