In message , w_tom
writes

[top-posting corrected]

Richard Herring wrote:
In message , w_tom
Again an assumption that something will stop or block a
destructive transient. Kilometers of air could not stop the
transient.

I think you need to be more quantitative here. Kilometres of
air could not stop what, exactly? They certainly do a good
job of protecting me from lightning strikes in the next
village.

Lightning is a connection from cloud to earth borne
charges. Electricity travels through kilometers of
non-conductive air.

No, current travels through kilometres of conductive ionised air. First
you need a strong enough potential difference to produce a strong enough
electric field to cause that ionisation. Then there's a complicated
process by which the ionisation spreads to produce a complete channel.

Why would some millimeters of air between
two RCD contacts stop same electricity? 1,000,000 mm of air
did not stop lightning. Why would 4 mm of RCD air stop what a
million mm could not?

Because the potential difference across _that_ gap, and hence the field
strength, may now be insufficient to produce ionisation. Never mind how
many megavolts there were between cloud and earth, the question is how
many there are across that contact gap.

That's why I asked you to be quantitative. Qualitative language like
"same electricity", "surge" and "transient" is not helpful here. Do you
mean a current, a potential difference, an electric field, or what?

The bottom line is this. Nothing is going to stop
lightning. Lightning protection is about shunting - also
called diverting, redirecting, or electrically connecting - to
earth ground. Any protector that claims to stop or block
lightning (such as in the RCD question) is simply promoting a
myth. And yet that is exactly what many people do - promote
the myth - when they recommend plug-in protectors.

Now you're confusing two different things. Most plug-in protectors that
I've encountered do not rely on open contacts (as in the RCD) but use
some form of voltage-dependent resistor and/or spark gap to shunt the
excess potential difference to local ground. To be sure, there may be so
much inductance in the system that they are ineffective, but that's a
different issue.

In response to the OP's question. 4 mm of air inside the
RCD (open switch contacts) is not going to stop a potentially
destructive surge.

It _will_ stop a "surge" or even a "transient" up to some threshold
potential difference. What's that value, and how is it related to what's
going on in the sky outside?

--
Richard Herring