[geeks] Whee! Lightning strikes, AGAIN!

[Barry Keeney]

On Wed, 29 Jul 2009 gsm at mendelson.com wrote:

> 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.

  Sorry forgot, almost all are ISDN, not analog (except fax/modem/etc).
 
> 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.

  All above is for analog, which can handle more stress, but doesn't apply
here.

> 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. :-)

   But if the power (EM field) from lightning was strong enough to damage
devices with high voltage/current subsystems, *2* CRT TV's, it would
likely cause some damage to much lower power devices like unshield
digital phones. With 10,000+ phones we'd see more failures after a storm
and we don't. We have 1-3 phone failures a week, most are user or simple
wear related problems, handset cable bad, the "X" button doesn't work
anymore, someone spilled coffee on it, etc.

   The CRT TV's would have RF signal protection so it wouldn't have come
from the antenna/cable line, it would have to come from something inside
the TV not able to handle the EMP. 

   With all the damaged devices sharing the power grid, it's more likely
the surge came from there. Now if just network devices were damaged, maybe
the case for EMP generating a surge in the wires as the cause would be
stronger, maybe.... 

Barry Keeney
Chaos Consulting
email barryk at chaoscon.com

"Rap is Square Dancing gone terribly, terribly Wrong...." 

"Michael Jackson has touched so many, and I'm not just talking about the children......"