[Elecraft] Low Voltage Supply Danger

[Tom Hammond]

At 08:55 AM 6/28/2005, Ron D'Eau Claire wrote:

>That's a legitimate concern with our present 13.8 supplies as well. Twenty
>or thirty amps can be very dangerous if you get a piece of jewelry across
>the contacts. If you're unlucky enough to fail to make good enough contact
>to trip the crowbar or blow a fuse, you'll be frozen there as the ring (or
>bracelet) melts into your flesh.

You're certainly right there, Ron... UN-limited current from a big honkin' 
battery, or power supply really CAN be extremely dangerous.

Fortunately (as I can attest, from several almost unfortunate experiences) 
most of the 'modern' AC power supplies offer foldback current limiting 
which, if the output is shorted, will reduce the output current (and 
voltage) to nearly ZERO, helping to protect against such catastrophes.

UNfortunately, this is not the case for storage batteries, and I'm sure, 
some older power supplies as well, which will supply almost unlimited 
current until they either run out, blow up, or melt through the power 
cable. In fact, a local friend just very recently managed to short out the 
power cable on his fully-charged 80AH gell cell... not only melted almost 
all the insulation on the DC cable, but actually began melting the THICK 
LEAD terminals build into the battery itself. Fortunately, there was no 
harm done to my friend... but he got a really QUICK education is the power 
of a low internal impedance current source.

As you noted, ALL DC power sources should be adequately (and appropriately) 
fused, and fairly close to the DC source itself, if at all possible... it 
doesn't have to be the far end of the DC cable that shorts out... could be 
anywhere along its length that becomes accidently stripped of insulation... 
BAD THINGS CAN HAPPEN.