[Beowulf]Infrastruture planning for small HPC 40/100 gigabyet eyhernet or Infiniband?

Perry E. Metzger perry at piermont.com
Tue Jul 29 12:33:16 EDT 2008


Eugen Leitl <eugen at leitl.org> writes:
> On Sun, Jul 27, 2008 at 07:19:56PM -0700, Jim Lux wrote:
>
>> bear in mind that ordinary ethernet both coax and twisted pair is  
>> galvanically isolated.
>
> This is strange, because I've seen (small) sparks and received (mild)
> shocks from both, in two different locations. 

Ground loops are a real phenomenon in UTP Ethernet. For example,
*NEVER* run UTP between buildings. If the grounds in the two buildings
are at a different relative potential, and they often are, very bad
things can happen.

The building complex I live in ran Cat 5 between buildings in
underground ducts. They were very surprised when lightning strikes
some distance away regularly blew out the switches. Changing to fiber
eliminated the problem, of course.

> In any case I'll have an electrician diagnose the problem.

If you're seeing sparks, as you say, I suspect you do indeed have an
AC supply problem. Ground loop, or something worse. (The Electrical
Wiring FAQ describes several problems that qualify as "worse"...)

-- 
Perry E. Metzger		perry at piermont.com
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