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

[AJSOENKE at aol.com]

Another low voltage power source that is in common use is the lead-acid  
storage battery used  on cars and especially the deep cycle marine battery  
favored by hams for backup.  Usually a ring or watchband is not a problem  because 
of the terminal spacing at the battery, but the danger is very much  present at 
the other end of the cables where it may be connected to a terminal  strip 
etc. 
 
The added danger of these batteries is they can deliver as much as 800 amps  
depending on the internal resistance of the battery and cables.  If the  
internal resistance is as little as 1/10th Ohm at 14 volts that is 140 amps. The  
power generated in the battery under these conditions is 140X140/10 or 1960  
Watts.
 
2KW heating the acid contents of a battery can cause it to explode from  
steam pressure in a matter of seconds, thus spraying everything in the area with  
sulphuric acid and hot lead. When this happens in a car the result is usually 
a  totalled vehicle and seriously injured mechanic and passers-by. 
 
I never use a wrench  which has exposed metal surfaces long enough to  short 
a battery, and I always disconnect the negative terminal (- ground assumed  
here) and cap it before working around them.
 
Al WA6VNN
______________________________________________________________________
 
In a message dated 4/19/2008 8:11:26 A.M. Pacific Daylight Time,  
ron at cobi.biz writes:
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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