GTH wrote:


Having a storage battery in the circuit avoids the situation too. The
battery, barring bad connections, will soak up the initial surge.



Yes, the initial surge. And then...?


The

overvoltage protection device is unlikely to trip in Ellipse's
scenario, because it won't need to.





?? Can't figure out that one. OV is OV, and the avionics pay no mind as
to the original cause : zap all the same.

It is a good thing to try to fool proof an architecture. But OV
protection is there to protect against even things that should not happen.
You'd be surprised at the number of things that happen in aviation, and
yet should not happen : empty tanks, wheels up landings, popped
breakers, fried radios, engine failures, accidents...

You may wish to have a peek at www.aeroelectric.com. The load dump
scenario has been covered extensively.



Regards,
Gilles Thesee
Grenoble, France
http://contrails.free.fr

Upon your advice, I peeked at the link. At
http://www.aeroelectric.com/articles/spike.pdf
Mr. Nuckolls makes exactly the same point as my previous post. Page 2,
last paragraph.

Mr. Nuckolls and I seem to have quite a bit in common. I, like him,
have been poking around aircraft avionics and electrical systems for
over thirty years. Currently I'm doing my best to assure the integrity
and reliability of avionics and electrical systems in a well-known
French-designed line of business jets.

I agree with you that overvoltage protection is necessary in an
electrical system using and alternator as a power source. I agree with
Mr. Nuckolls that most load-dump fears are unfounded, and that a system
using a well-designed regulator, effective overvoltage protection, a
well-maintained battery, and good wiring design and installation will
protect today's avionic systems quite well. Adding additional
protection devices or following Ellipse's recommendation carry their own
subtle hazards.

My point in my first post was that the battery would absorb the initial
transient in Ellipse's scenario. The system voltage would never reach
the level required to trip the overvoltage protection. If the battery
or its wiring were in poor condition, then the overvoltage protection
device could trip. Today's radios would not be threatened in either event.

Over the last thirty years, I've seen many avionics systems damaged by
lightning. I've seen a few damaged by reversed polarity, from improper
battery or alternator installation. I've never seen significant damage
from load-dump surges.