Yup, wind will do it, though in a rainstorm I'd expect that the water
on the surface of the jacket would provide a good enough path to ground
to dissipate the charge.
I would also expect that if your UTP cable is directly attached to the
metal tower structure at either end, you wouldn't have any problems with
flashover at the ends of the cable as described by Mike Ford. But if
there are any long sections of cable that aren't directly touching the
tower structure, I suppose it would be possible for the static charge to
jump through the cable insulation and reach the ethernet conductors. If
you have a lightning protector at either end of the cable, this wouldn't
matter much as the energy would go to ground through this instead of
through the ethernet chipset in your radio or switch.
Patrick Shoemaker
Vector Data Systems LLC
shoemak...@vectordatasystems.com
office: (301) 358-1690 x36
http://www.vectordatasystems.com
On 8/26/2010 11:08 PM, Greg Ihnen wrote:
Mr. Shoemaker,
So the effect you are describing below is the result of wind rather
than induced energy from a nearby lightning strike?
When I was a maritime radio officer I witnessed very strong static
electricity on non-dc grounded antennas during storms. I've seen
repeated rapid static discharges across PL-259 connectors that went on
for minutes as a rain squall blew past. I assume you're talking about
this effect.
If plain old non-shielded ethernet cable is wired tied to antenna
support, down the tower and to cable tray at the bottom of the tower
doesn't that give the static charge on the outside of the jacket a
place to go?
Greg
On Aug 26, 2010, at 9:21 PM, Patrick Shoemaker wrote:
Mike ford is describing electrostatic charge buildup on the outside
of the cable jacket. This occurs because the friction between air
molecules moving by and the surface of the cable jacket causes a
charge separation on the surface of the jacket. In conductive
materials this charge would simply bleed off, but a good plastic
cable insulator will retain the charge on the surface of the jacket
until the voltage is so high that it exceeds the breakdown voltage of
the jacket. Then the charge dissipates through the nearest path to
ground, which could be through a sensitive electronic component in
the radio. Shielded cables and connectors will solve this problem by
dumping the charge to ground once it builds up enough to jump through
the cable jacket.
This is different than the usual EMI scenarios we look at as
WISPs--namely induced currents in ethernet cables due to strong
nearby electric fields.
Patrick Shoemaker
Vector Data Systems LLC
shoemak...@vectordatasystems.com
office: (301) 358-1690 x36
http://www.vectordatasystems.com
On 8/26/2010 9:25 PM, Scott Carullo wrote:
Mike Ford says in this post:
http://ubnt.com/forum/showthread.php?t=22612&page=3
===========
Hello,
Using STP with AirGrid/NB give buildup of ESD along the Ethernet
cables plastic jacket a place to go other then the radio. The
Ethernet cables plastic jacket is the major cause of damage to our
devices if they are not grounded at the base of the tower or have no
provisions for ground. Once the charge on the outside of that jacket
passed 25-30kV it has enough potential to ark OVER the top of the
plastic RJ45 connector and onto the Ethernet pins of the Ethernet
lines. If you have a grounded cable, that is properly grounded at
the base of the tower, this will provide the path of least
resistance for the ESD buildup on the Ethernet cabling. The whole
goal is to prevent the static from ARcing to the Ethernet lines
damaging the devices.
===========
I always thought the ESD was picked up and transferred through the
metal components on a cable. I never considered the plastic jacket
the source or transmission medium for a high-voltage static
discharge. This would seem to indicate if you lose the jacket and
had a naked cable that you would eliminate the ESD problem - but
somehow I don't buy that either.
Those of you electrical gurus please enlighten me. Either something
isn't right (or needs further explanation) or I just learned
something I never knew... maybe both :) Thanks
Scott Carullo
Brevard Wireless
321-205-1100 x102
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