[TowerTalk] "Faraday Shield" for Coax and Control Lines

[Joe Subich, W4TV]

> NESC rule:
> As I understand the rules, ALL facility grounds must be 
> tied together with a minimum size conductor back to the 
> service entrance.  Maybe someone can qualify this, but I 
> don't 'think' there is a distance limit.  This is a power 
> frequency safety issue, NOT a lightning safety design issue.

Failing to bond the power line entrance ground (safety ground), 
tower grounds, "entry window", and telephone/satellite/cable 
grounds together is a prescription for equipment damage.  

USB interface chips are particularly sensitive to reverse 
voltage.  If the station ground (tower/entry window) moved 
even a few volts above the power (safety) ground - the 
"ground pin" on the computer power supply - and the shield 
connection on the USB cable is questionable you're likely 
to lose a USB UART.  

The same problem occurs with cable modems, telephone 
modems, DSL modems, network switches/hubs/firewalls, 
etc. if the telephone and cable grounds are not bonded 
to the power ground.  

Unless everything enters your home at ONE location, install 
a perimeter ground system and bond everything to that. 

73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: towertalk-bounces at contesting.com 
> [mailto:towertalk-bounces at contesting.com] On Behalf Of jimlux
> Sent: Sunday, August 09, 2009 10:39 AM
> To: K1TTT
> Cc: 'Towertalk e-Goups'
> Subject: Re: [TowerTalk] "Faraday Shield" for Coax and Control Lines
> 
> 
> K1TTT wrote:
> > 
> >
> > NESC rule:
> > As I understand the rules, ALL facility grounds must be 
> tied together 
> > with a minimum size conductor back to the service entrance.  Maybe 
> > someone can qualify this, but I don't 'think' there is a distance 
> > limit.  This is a power frequency safety issue, NOT a 
> lightning safety 
> > design issue.
> 
> 
> No length limit.  All systems must be bonded together. There are some 
> really funky schemes you might see in a factory with large motors fed 
> from 3phase (where the center point of the Star/Wye is tied to ground 
> with a resistor).
> 
> 
> > 
> > Next, on the separation of the ground... there are 2 
> important cases 
> > here. First, the near miss case:
> > 
> > As I stated above from simple physics, the bigger the loop 
> the higher 
> > the induced current.  For conductors above ground the air 
> filled loop 
> > is easy to see.  The current induced in the loop created by raised 
> > cable runs and the ground is relatively easy to calculate 
> and can be 
> > used to predict the peak voltages at gaps or arresters in 
> the loop.  
> > Simple rules are: the higher the cable is above ground the 
> higher the 
> > voltage/current, and the longer the loop the higher the 
> > voltage/current... so grounding long cable runs at intermediate 
> > support posts would reduce the voltages seen at the ends from near 
> > misses.
> 
> 
> There's also an interesting phenomenon where the transient 
> propagating 
> down the above ground line flashes over at an intermediate 
> point (e.g. 
> at a protective gap), and the magnetic field from THAT new 
> transient (as 
> the spark discharges the "charged" transmission line) induces a 
> transient in the down stream line (the field is smaller than the 
> original lightning strike, but it's a LOT closer to the victim loop). 
> This is why series transient protection is recommended in some cases.
> 
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> 
> 
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