POE switches and lightning

Pete Carah pete at altadena.net
Thu May 13 15:12:06 CDT 2010


On 05/13/2010 02:52 PM, Steven Bellovin wrote:
> On May 13, 2010, at 2:24 04PM, Daniel Senie wrote:
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>> While the equipment may well be affected by an EM pulse, if the gear returns to normal after a power cycle, then the equipment vendor didn't do their job fully developing the product. A product should be tested to take such pulses and should recover provided it has not suffered a catastrophic failure (and in fact it should contain sufficient protection to avoid such in most cases).
>>
>> In working on one particular router in the lab some years ago, I was verifying some software functionality and the hardware engineer I was working with reached over my shoulder and used a device that delivered a high voltage spike (simulated lightning) to a 10BaseT network port. After I peeled myself off the ceiling (and he stopped laughing), we set to work figuring out how to get the device to self-reset after such a strike. One component, an Ethernet hub chip, got into a confused state. I was able to detect this in software, so we adjusted the product design so that the software could yank the hub chip's reset line.
>>     
Luck.  I've needed that kind of reset a few times...
>> It's unfortunate that products, both hardware and software, receive minimal quality testing these days. Guess it's not a surprise, since buyers seemed to prefer products that were quick to market, with lots of bugs, rather than reliability and resilience.
>>     
That is certainly true (and not entirely modern; you can read about that
problem in old roman literature.  When was "Zen and the art of
motorcycle maintainance" written? - 1970's); however it is nearly
impossible to protect well against close-by lightning.
>>     
> It's not just a matter of "these days" -- lightning is awfully hard to deal with, because of how quirky the real-world behavior can be.  I had to deal with this a lot in the 1970s on RS-232 lines -- we could never predict what would get fried.  Of course, there was also a ground strikes very near my apartment, where the induced current tripped a circuit breaker, blew out a couple of lightbulbs, and and came in through the cable TV line to fry the cable box, fry the impedance-matching transformer, and fry the RF input stage on the television...
>   
I can second Steve in spades; I used to work for the power company in
Alabama...  There you learn a LOT more than you ever wanted to know
about lightning.  Consider that one hit can destroy the inside of a
>10Mw 66kv->12kv distribution transformer (I actually saw the strike
involved; it was less than a mile from my apartment at the time, and
dropped power to me; the apt was fed from an entirely different
company...  My power came back in a few minutes; the other load took
almost a week (they had a redundant feed; it was a hospital, but they
ran in a low-power mode till a BIG crane and big lo-boy truck came with
another transformer)); how are you going to protect any computer from
*that*...

-- Pete

> 		--Steve Bellovin, http://www.cs.columbia.edu/~smb
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