[geeks] Whee! Lightning strikes, AGAIN!

[gsm at mendelson.com]

On Wed, Jul 29, 2009 at 10:20:18AM -0600, Barry Keeney wrote:
>  If EMP was a likely cause for damage we would see more damage here at
>work from lightning and we have 10,000+ phones. We would be replacing more
>phones/fax machines/modems/etc after storms and we don't see that pattern.
>We're also in the state with the second highest lightning strikes in the
>US and at a size of 40+ square miles it's not like we don't get hit
>either. I've seen a few lightning hits on site in the last week alone.

No, you would not. Phones are rung by placing 20Hz 90 volt ac across 
the line. In order to prevent the phone/modem/fax from exploding 
when the phone rang, it would have to be protected already.

Usually it's just a large capacitor and an inductor which shunts the current
to ground with enough resistance that it does not seem to be a short circuit.

Old phones used the inductors as coils for mechanical ringers, more modern
phones just tap off enough signal to trigger an electronic ringer.

I remember 1990 vintage modems having gas discharge tubes across the phone
line, and cheap ones using neon bulbs. :-)

You can tell by looking at the REN (ringer equivalence number). An REN of 1
means 1 amp draw at 90 volts, equivalent to the bell in a 500/1500/2500 series
telephone.

Digital phones, which use no ringing current, are probably also protected
so that if someone accidently hooks them up to an analog line, they don't
explode. If they were not, or not protected from spikes on the phone line,
they would have all be returned the first time the "new" phones died and
the old ones did not. :-)

Geoff.

-- 
Geoffrey S. Mendelson, Jerusalem, Israel gsm at mendelson.com  N3OWJ/4X1GM