[Elecraft] Dangerous Low Volage (WAS: Power pole for K3?)

[Robert Tellefsen]

Hi Ron
One other precaution I've always used is to ensure
that all hot terminals are always female with no
exposed metal to accidentally allow a short.  I know
this is stating the obvious but it may not be to those
who are new to this.
73, Bob N6WG

----- Original Message ----- 
From: "Ron D'Eau Claire" <ron at cobi.biz>
To: <elecraft at mailman.qth.net>
Sent: Saturday, April 19, 2008 8:10 AM
Subject: [Elecraft] Dangerous Low Volage (WAS: Power pole for K3?)


I see you got plenty of answers about the APPs, Bill. You will need to
provide your own suitable connectors for the power supply end of the
cable.

You mention having high-capacity batteries and a 70-amp Astron supply.
It
sounds like you have experience with high-current supplies and
understand
how dangerous they can be, but there are a lot of readers here who may
not
have that background.

I mention it because many Hams think 13.8 VDC is safe to handle. It's
*NOT*.
It's not the voltage, of course, but the current. A short often
produces a
flash of molten metal that may continue for a while if the resistance
is
enough to avoid tripping a breaker or crowbar circuit.

It can take off a finger or a hand, or very severely injure a person
in an
instant should a ring or watch band get across the supply line,
immediately
transformed by the current into red-hot metal burning its way through
flesh.
In my days working in the field as an aircraft radio/radar tech I knew
some
who were missing fingers because they neglected to take off a wedding
band
when working. It's not a nice way to lose a hand or finger. It's not
always
quick. It can take a while, with the ring or watch band welded solidly
to
the terminals by the current as the hot metal eats its way through a
finger
or wrist.

I've seen tempered steel screwdrivers literally explode into a spray
of
molten metal blobs when dropped across a high-current, low-voltage
supply -
blobs of molten steel that splatter the face and eyes of the unwary
worker
not wearing protective gear. Less severe shorts can set wiring, and
the
shack, on fire in short order, all without opening a breaker or
tripping the
crowbar protective circuit in the power supply. Even a more moderate
35 Amp
supply can start a fire or cause a severe burn under the right
conditions.

Working on shipboard and aircraft systems using 12 to 28 VDC at high
currents, I've learned to treat such supplies with the same respect I
do a
3KV high-voltage supply. The concerns and procedures needed to be safe
are
different, but either a high voltage or a low-voltage, high-current
supply
is dangerous in its own way.

I use soldered-on ring terminals on my Astron supplies that provide
large,
low-resistance contact areas with the lugs and which cannot be pulled
loose
without physically breaking the wire, and I make very sure there are
no
metallic objects (cabinets, connectors, other equipment, etc.) located
where
they might be accidentally moved and make contact with exposed supply
terminals anywhere. And then I make sure the terminals on my power
supply
(and anywhere else) are carefully covered and insulated from
accidental
contact, just in case.

Ron AC7AC


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