On Sat, 13 Aug 2011 05:52:34 -0700 (PDT), DrYattz
wrote:

Which tells me that if the incoming current exceeds 230V,
.... if the incoming voltage exceeds...
it dumps up to 1000V and 6000A to ground. Right?
Sort of (voltage does not dump, so to speak).

Hi Rees,

The voltage in excess of 230V sustains the dumped current. The 230V
is still there, all the same.

That is to say that the resistance of the gap is (for all practical
purposes) infinite at voltages below 230V. For voltages applied that
exceed 230V, the resistance of the gap is 230V/2300A = 0.1 Ohm.

I chose 2300A both for ease of math, and as being representative.

Hence the gap undergoes a astronomical change in resistance in a very
short time (about a microsecond or less).

This is great fortune to the house's protection. This is poor fortune
to the radio's first transistor which resides in an environment of, at
most, 9V to 12V as a supply for gain, and 0.5V as typically the
highest voltage seen at the input. Transistors are rated to sustain
higher voltages, typically in the 10s of volts, sometimes higher.

230V to 1000V surge (as guaranteed) offer a speedy death. Of course,
of the original source being orders of magnitude greater, lightning
boosts speedy death to instantaneous (within nanoseconds).

Let's take a scenario closer to experience, and one that could easily
be likened to a near death experience at that. Open the hood of your
car, grab an unconnected spark plug lead with the end just a tenth of
an inch from the engine block (a common test of the ignition system).
You observe a spark. Dare you hold the metal clip where that spark is
jumping from it to the block? Dare you even hold the lead on its
insulation? The gap guarantees a certain voltage, and the system
guarantees a certain current (otherwise you would be forever stalled
in the driveway).

Why the hesitation in holding this lead, IF ALL the voltage and
current is dumped into the block? Experience will inform you of why
you hesitate, and why the transistor fears elevated potential.

An antenna invites access to elevated potentials from many sources
other than lightning. Even on a clear day, a dipole can accumulate
enough charge to make the spark gap sizzle. This would be
extraordinary circumstances in some parts of the US, and typical in
other parts. Yet and all as this may be commonplace, radios still
play and life goes on. There are many other factors to consider
insofar as what the input transistor has to suffer or enjoy.

73's
Richard Clark, KB7QHC