[c-nsp] Galvanic isolation for Ethernet?

[Michael Loftis]

On Tue, Aug 19, 2014 at 8:33 AM, Victor Sudakov <vas at mpeks.tomsk.su> wrote:
> Michael Loftis wrote:
>> No standard surge protector will help at all for you I'm betting.
>
> I have already guessed that therefore I was asking for something
> better than standard :-)
>
>> As
>> some have mentioned you're probably suffering significant (if
>> intermittent) common mode voltages...ethernet ports are already
>> "galvanically isolated" in the switch itself, and usually in the
>> device too, there's a transformer that'll remove the common mode
>> voltage component from the signal.
>
> Last time I looked at a NIC (and the inside of a toasted Catalyst) I
> don't remember seeing any transformers. But probably I did not look
> well enough, or they do not look as a typical transformer. Do you care
> to show a picture please?

Many RJ45 ports have integrated magnetics now (transformer) - external
options are made by pulse, bel, halo, and many others.  As for
external magnetics they're small - they don't need to be very large
because they're not designed to be carrying current.
http://www.pulseelectronics.com/products/lan has a dozen or more
examples.

>
>> If you've enough common mode
>> voltage to fault those out you may still be well short of "surge"
>> voltages that are protected by most equipment.  You might improve the
>> issue by re-terminating each end only leaving the "spare" copper pairs
>> connected at one end of the RJ-45 (the switch end
>> generally)...
>
> Sounds like magic but could you elaborate? I did not quite catch the
> picture.

You remove the extra (non 100mbit/10mbit) pins from the far end, don't
terminate them through, this sometimes is the path your stray ground
currents take.

>
>> ultimately I've found nothing cures these issues except
>> full optical isolation (using a media converter at each end of the
>> building to building runs...in some cases this meant colocating
>> another switch in each building) and not running copper between the
>> buildings.
>
> Well, there are different rugged industrial copper Ethernet solutions
> for outdoor use (e.g. from Siemens), what do they use?

Not to be a smartass...but...probably something from siemens.  And it
probably would have issues with this too.  Outside copper to ethernet
works a lot of the time, but, there's many, many, many times it
doesn't.  It wasn't ever designed with that use case.  I also doubt
I'd ever use anything by Cisco connecting to outside plant (they're
not hardened to this crap at all, they just meet the specs) - many
companies make or rebrand industrial hardened switches (Blackbox for
one) - you might want to try using one or more of those as a bridge
into your catalyst, let them take the outside plant.  They're not
cheap ($250 or so for 6 100mbit ports) but still generally cheaper
than the Catalyst....

Another option short of going to fiber is going to xDSL (see blackbox,
others) - xDSL *IS* designed for outside plant and will run over the
Cat5, just not as fast.

>
>>
>> The root cause is almost always bad grounding in one building or
>> another.  In a couple cases we discovered that ants and termites had
>> colonized around the grounding rod.  In another we discovered the
>
> Our electricians say that the grounding is correct, all grounding
> circuits on the site are interconnected and that they measure the
> parameters annually.  I am not the person to expose them to be lying
> (if they really are), I lack such qualification.

Have you checked to see how much potential difference is at one end or
the other?  (Disconnect equipment from one end, and measure voltage
between the various RJ45 pins and ground, should be 0.00) And how much
current is flowing? (set meter in mA and do same)  Worse your issue
could be entirely intermittent too.  Yes, you're dealing with
gremlins.  Just because the individual buildings are OK doesn't mean
they're OK with respect to eachother.  You can have grounding
potential differences in different floors of the same building or
different areas of the same building.

And all of this is assuming it's grounding potential
differences....not something like your bundle of Cat5 runs right by a
big industrial refrigerator, or a sodium light or something like that
which is inductively coupling to your Cat5 run.

>
>
> --
> Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
> sip:sudakov at sibptus.tomsk.ru



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