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

James Cameron quozl at laptop.org
Tue Jan 12 01:53:35 EST 2016


No, because the physics mechanism of inducing ground currents is based
on magnetic fields.

PVC pipe does not block magnetic fields.

When a strike occurs, there is a strong current following the path of
the strike, and then into the various parts of the earth around you.

Imagine an upside down flat umbrella; current down the handle, then
spreading out radially.

The strike is a DC current, which sets up a strong magnetic field, and
as the field grows to full strength and then collapses it induces a
current into other metals and conductors.  Structural metals aren't
harmed.  But telephone and ethernet cable will pick it up fine.

A PVC pipe can help keep water out of ethernet cable, and provide a
barrier to first shovel strike.  It may also briefly contain a cable
if a direct strike occurs, but expect plastic shrapnel.

The schools near me all use fibre between buildings, inside PVC pipe.
Keeps the callout costs down, and prevents fires.

On Tue, Jan 12, 2016 at 12:08:27AM -0500, Sora Edwards-Thro wrote:
> Here in Lascahobas, Ben splurged and hired professionals to install solar
> panels back in 2014. The roof of the building with the battery/computer
> charging room was not stable enough, so they installed them on the roof of a
> building ~10 feet away. They ran a PVC pipe from the solar panel and through a
> hole in the charging room wall, and they placed the wire inside that. I think
> they may have also grounded (is that the correct term?) the solar panel itself
> by connecting it via wire to one of the metal rebar prongs sticking up out of
> the roof.
> 
> Would a PVC pipe protect an ethernet cable that ran between buildings too?
> Sounds like if not, an ethernet switch (to prevent electrical links, like Sam
> suggests) or a wireless repeater (to eliminate the need for cables of any kind)
> are necessary.
> 
> Thanks for bringing this issue up, Anish! 
> 
> On Mon, Jan 11, 2016 at 11:39 PM, Samuel Greenfeld <[1]samuel at greenfeld.org>
> wrote:
> 
>     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 <[2]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 <[3]anishmg at umich.edu>
>         Date: Mon, Jan 11, 2016 at 10:32 PM
>         Subject: Re: [XSCE] RE: [UKids] Ethernet Cable
>         To: xsce-devel <[4]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)
> 
>         [5]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 <[6]tgillett at gmail.com>
>         Date: Mon, Jan 11, 2016 at 10:29 PM
>         Subject: Re: [XSCE] RE: [UKids] Ethernet Cable
>         To: xsce-devel <[7]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 <[8]holt at laptop.org> wrote:
>         On Mon, Jan 11, 2016 at 10:14 PM, T Gillett <[9]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 <[10]
>             anishmg at umich.edu> wrote:
> 
>                 fwiw, a simple google search reveals that these are
>                 prohibitively priced.
>                 [11]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 <[12]
>                 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 <[13]
>                     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
>                         [14]http://quozl.netrek.org/
>                    
> 
>                     --
>                     Anish
> 
>                
> 
>                 --
>                 Anish
> 
>            
> 
>        
>        
> 
>         --
>         Anish
>        
> 
>         
> 
>         --
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>        
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> References:
> 
> [1] mailto:samuel at greenfeld.org
> [2] mailto:holt at laptop.org
> [3] mailto:anishmg at umich.edu
> [4] mailto:xsce-devel at googlegroups.com
> [5] http://people.sugarlabs.org/anish/IMG_20150910_222141.jpg
> [6] mailto:tgillett at gmail.com
> [7] mailto:xsce-devel at googlegroups.com
> [8] mailto:holt at laptop.org
> [9] mailto:tgillett at gmail.com
> [10] mailto:anishmg at umich.edu
> [11] http://www.blackbox.com/Store/Detail.aspx/Power-over-Ethernet-Surge-Protector-60-Volt/SP075A
> [12] mailto:anishmg at umich.edu
> [13] mailto:quozl at laptop.org
> [14] http://quozl.netrek.org/
> [15] http://unleashkids.org/
> [16] http://unleashkids.org/
> [17] mailto:unleashkids+unsubscribe at googlegroups.com
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-- 
James Cameron
http://quozl.netrek.org/


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