POE switches and lightning
pete at altadena.net
Thu May 13 20:12:06 UTC 2010
On 05/13/2010 02:52 PM, Steven Bellovin wrote:
> On May 13, 2010, at 2:24 04PM, Daniel Senie wrote:
>> 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
> --Steve Bellovin, http://www.cs.columbia.edu/~smb
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