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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