[Elecraft] Local Noise Problem

[Guy Olinger K2AV]

There were rules when I worked in the big 24 volt power supply cave
underneath the AT&T building at 13th & G in Washington DC.  No rings,
no watches, nothing hanging on your neck, no tools that were not
wrapped, etc.  Long sleeve shirts out in the locker.  I always brought
tee shirt to work in there.  Hot place to work.

One of the 10k ampere 24 volt supplies used four 1 by 6 inch copper
slabs in parallel as a "lead". The positive and negative were side by
side with about four inches in between.  A Western Electric guy was
working above with an unwrapped 8 inch crescent wrench and dropped it
down on the bus bars.  The story I heard was that the middle of the
wrench dropped through the space between the bus bars without even
slowing down as both ends exploded into a room full of  tiny
copper-steel bb's.  It took us two days to clean up the place.
Western guy wasn't hurt, but I never saw him in our building again.
Might have been fired, but I don't know that.  There was a kind of
"dent" on the top of the bar, but they never replaced it.

It did NOT blow the master fusible link, so it was not "working hard"
to evaporate the wrench.

Shorts on large conductors will definitely eat things even at 24 volts.

73, Guy.

On Mon, Apr 4, 2011 at 6:25 PM, Ron D'Eau Claire <ron at cobi.biz> wrote:
> In the 24 volt aircraft systems I worked with power was typically routed
> into the cockpit via 1/2" diameter copper rods that ran along behind the
> breaker panels. Breakers for each circuit were tapped into the rod with
> threaded holes and screw.
>
> That's what it takes to avoid excessive I^2R losses for a system that draws
> significant current.
>
> And never lose sight of the danger of a low-voltage high-current system.
> First, if you have an inductive load, the pulse that occurs when it's
> disconnected can run into many hundreds of volts - enough to do real damage
> if you're holding the two ends of the circuit as you disconnect it.
>
> But more commonly is the danger of a short circuit. Things turn red-hot
> instantly and even thick bus-bars can melt. I saw a guy drop a screwdriver
> into an aircraft 24-volt bus once. I was glad I was 50 feet away in the next
> plane. It looked like a bomb went off as the shaft of the screwdriver melted
> so fast it literally exploded with a huge bang, showering the poor guy with
> molten steel and setting the plane on fire.
>
> I also knew techs missing fingers after getting a ring across a low-voltage
> source. The rings heated so fast it welded to the terminals before he could
> pull away and the hapless fellow had to suffer the melting metal burning his
> finger off until he was able to jerk his hand away without the finger. I
> heard of one fellow who wore a metal watch band and, yes, he reportedly lost
> his whole hand.
>
> To this day, I remove any jewelry when I'm working around a high-current
> low-voltage source like an automobile battery.
>
> Ron AC7AC
>
>
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Edward R. Cole
>
>
> Regarding high-current dc distribution:  I ran into that issue lots
> of years ago when I wanted to run some ham gear outside my travel
> trailer and use the 12v system as power source.  I quickly found that
> 30-feet of #14 or #12 wire would not do.  I finally used a 12v
> battery and floated it from the trailer power.  And I was only
> running 150w on 2m.
>
> At one time I was considering solar power for a two-story log-cabin
> in the bush, but the wiring gets expensive (so do the panels).  The
> cabin now has 120vac utility service so its a moot issue (and I sold
> it in 2003).
>
> My present shack has 13.8v provided by  50A Astron which is connected
> via a 12-foot run of #6 welding wire to the main 30A buss fuse.  This
> connected to a heavy barrier terminal strip to provide power to all
> the amplifiers and high-current equipment.  I see about 13.5v at the
> main fuse when running loads; unloaded it shows 13.75v.
>
> Two power leads run from there to two more barrier strips:  one is
> #12 solid house wire and the other #8.  Both about 3-foot long.  My
> new 144-28 50w transverter runs off the strip fed by #8 wire and
> voltage runs at 13.2 under load.  My K3/10 is fed from the #12 wired
> strip.  There are only two other items connected there: panel lights
> for my manual antenna tuner and the main station controller (fused at 5A).
>
> Many of my new amps require 28vdc.  I have a 8A and two 50A supplies
> for 28vdc.  One 50A PS runs the 16-foot dish rotator-motors and will
> run 432-100w and 1296-300w PA's at the dish.  I run #12 romex out
> there for 120vac power (remotely switched).  The other 50A PS may go
> to the base of one tower where I may install some big amps for 900
> and 1296 MHz.  To place the amps at 50-feet will require #6 wiring up
> the tower.  Installing a switching PS at tower-top becomes attractive
> (not the 90-lb. 50A PS).
>
> Ohms law rules dc-wiring!
> Ed
>
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