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

[jimlux]

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.