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

Mon Jan 11 23:39:28 EST 2016

If you are going to use any cable outdoors or where it may be exposed to
the elements, getting cable rated for the desired outdoor use may be as
important as its speed rating.  You don't want the cables getting water in
them, causing interesting shorts and ground currents.

Cables routed inside of walls/vents/etc. also often have to be one of a few
special types for fire safety and other reasons.

Note that these cables types are not as flexible as the cords running into
your computer, so make sure not to bend them too hard.

If you can afford it, it might be worth splurging for a few Ethernet
switches which support fiber connections.  That way there is no electrical
link on the data line between buildings.

On Mon, Jan 11, 2016 at 11:05 PM, Adam Holt <holt at laptop.org> wrote:

> 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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