Most linear supplis have about +20V before the "pass element",
transistors.

And if a transistor shorts C to E, then ~+20V will appear at the
+12V output. A good OVP/crowbar is a must have!

But if an otherwise good supply lacks an OVP I would add my own.
The first link I gave has a diagram for a very good, in fact better
then
any I have ever seen built iinto any PS. The Areotech diagram specs 1%
resisitors. I used 5% with no decrease in functionality. Given the
orignal
use was to be in aircraft, the slight performance edge offered by 1%
ressitors was well worth it, that is a "life critical" mission. While I
value
my radios, I am more then willking to settle for a little less absolute
performance. I built the first one with teh specified 1% resisitors,
I built another unit with 5% resisitors, and as far as I can measure,
they both trip in about ~50mS. That is 0.05S. Thats for a jump of
13.69V
to +20V.

I use 13.69V because that is the most commonly suggested voltage to
keep lead acid gell cells "float charged". I added a 120V AC relay to
lift,
open, the + output from the PS so during power failures, my radios
keep running.

And as a rule, the larger a supply, the cooler it runs, and cool
equates
to LONG life.

Terry