[Power] Lightning kills underground Ethernet too; PoE wiring/voltages non-trivial!

[Adam Holt]

Just a quick digest below for microdeployments -- some summary excerpts for
those who missed both interesting/longer discussions today:

   - Power-over-Ethernet (PoE) might seem like magic cure (when installing
   Wi-Fi Access Points across classrooms not having nearby electrical outlets)
   but PoE is tough to get installed properly, as Tim has discovered!
   - Burying Ethernet cabling in a shallow trench between nearby buildings
   seems rather popular "to the garage" (quite a number of personal
   experiences are documented online) however this does NOT eliminate the
   lightning problem.  What kind of trenching/equipment mitigates lightning
   risks and to what degree seems to be open for debate across different web
   sites (if you don't have El Chapo's tunneling team on your side, avoid
   these risks using underground fiber or directional wireless/repeaters
   instead ;)


From: Anish Mangal <anishmg at umich.edu>
Date: Mon, Jan 11, 2016 at 10:32 PM
Subject: Re: [XSCE] RE: [UKids] Ethernet Cable
To: xsce-devel <xsce-devel at googlegroups.com>

Interesting insight Terry.

FWIW, I did some rough testing of a homemade cantenna a couple of months
ago. (Rough, because it was not a perfect Line of Sight)

http://people.sugarlabs.org/anish/IMG_20150910_222141.jpg

The measured gain over a omni 5dbm antennas was around 7-8 dbm, which would
put the overall gain at about 12-13 dbm. Not bad for a tin can antenna made
essentially from spare parts :)

In theory, two of these could form a pretty stable link over a kilometre or
so.


---------- Forwarded message ----------
From: T Gillett <tgillett at gmail.com>
Date: Mon, Jan 11, 2016 at 10:29 PM
Subject: Re: [XSCE] RE: [UKids] Ethernet Cable
To: xsce-devel <xsce-devel at googlegroups.com>

Underground cabling doesn't add a lot of protection against lightning
strike as George has related.

One problem is that when a lightning strike occurs, there are large ground
currents flowing from the point of the strike outwards.

This produces voltage differences between different points on the ground -
the further apart, the greater the voltage.

If you stand at one point, you experience no problem. But if you straddle
two points some distance apart, then you are in trouble.

If you introduce a copper cable between two distant points, when a strike
occurs you will see a large voltage between the end of that cable and local
ground (to which the equipment is essentially connected). And your gas
arrestors just try to tie it all together and so carry the current in the
copper conductor.

The voltages and currents produced in lightning strikes are truly awe
inspiring.

This is just one mechanism of failure that occurs. A lightning strike is
really like a bomb going off. You can describe what happens in general
terms, but the detail is extremely variable and difficult to predict with
any certainty.

Prevention is generally considered the best strategy.

On Tue, Jan 12, 2016 at 1:18 PM, Adam Holt <holt at laptop.org> wrote:
On Mon, Jan 11, 2016 at 10:14 PM, T Gillett <tgillett at gmail.com> wrote:

> Long network cable runs are a magnet for lightning problems, particularly
> if they are run between buildings.
> And no amount of expensive protection equipment will save you from a
> nearby strike.
>
> General guideline for lightning prone areas is to keep network cables
> short as possible and within a single building.
>
> If you need to network to other buildings, consider a wireless technique.
>

Is shallow/quick trenching also viable?
Or will rats/worms/frost destroy the cable quasi-annually?

On Tue, Jan 12, 2016 at 1:08 PM, Anish Mangal <anishmg at umich.edu> wrote:
>
>> fwiw, a simple google search reveals that these are prohibitively priced.
>>
>> http://www.blackbox.com/Store/Detail.aspx/Power-over-Ethernet-Surge-Protector-60-Volt/SP075A
>>
>> On Tue, Jan 12, 2016 at 8:36 AM, Anish Mangal <anishmg at umich.edu> wrote:
>>
>>> Perhaps this is slightly offtopic
>>>
>>> Talking to a school a few days ago up in the mountains, they had a
>>> lightning strike that knocked out a bunch of electrical equipment. Are
>>> there any surge inhibitors for ethernet cables, would using PoE create more
>>> of these issues (i.e. the surge inhibitors probably wont work with PoE
>>> stuff)? Sure inhibitors are easily available for AC power sockets.
>>>
>>> On Tue, Jan 12, 2016 at 3:10 AM, James Cameron <quozl at laptop.org> wrote:
>>>
>>>> PoE can be loads of fun.  The products vary considerably; both
>>>> injectors, cables, and routers.  Most problems can be solved through
>>>> measurements, ohms law, and power calculations.  Most problems can be
>>>> pre-empted by choosing industry best practice equipment, but that
>>>> raises costs.
>>>>
>>>> To avoid all the PoE, there's no substitute for an electrician running
>>>> nice thick cables for AC.  Because these cables must meet a code, they
>>>> are way thicker than they need to be for a router.  It depends on
>>>> where you want to put your money.
>>>>
>>>> --
>>>> James Cameron
>>>> http://quozl.netrek.org/
>>>>
>>>
>>>
>>>
>>> --
>>> Anish
>>>
>>>
>>>
>>
>>
>> --
>> Anish
>>
>>
>>
>


-- 
Anish





-- 
Unsung Heroes of OLPC, interviewed live @ http://unleashkids.org !
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