Finally out of the closet, eh, Dave?
I'd imagine the SPG discussions here may take on a somewhat tone,
in the wake of your post.
Jim/N2EA
------Original Mail------
From: "K1TTT" <K1TTT@ARRL.NET>
To: "'Towertalk e-Goups'" <towertalk@contesting.com>
Sent: Sun, 09 Aug 2009 12:13:33 +0000
Subject: Re: [TowerTalk] "Faraday Shield" for Coax and Control Lines
Note, I play the part of an amateur on here, but in real life I write
simulation software for designing HV power line lightning protection
systems
and have participated in several high voltage and high current
experiments
at the lab where I work. This has included high current impulse
testing of
full size ground rods and radials. We have an impulse generator that
can
create about 25KA or 5MV pulses with several microsecond durations for
simulating lightning.
Some simple rules for induction:
Currents are induced in loops. The bigger the loop the more
current. The
closer together the conductors are the smaller the loop and the less
current
will be induced IN THE LOOP.
Observations about lightning impulses and ground:
Because of the fast rise time and finite propagation velocity of
currents in
the ground the farther apart ground connections are, the higher the
voltage
difference between them.
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.
Lightning physics:
When lightning strikes the ground or an object connected to the
ground it is
not 'dissipated'. There is always an opposite charge on the ground
that has
been attracted by the charge in the cloud that the lightning is being
attracted towards. When the downward leader and the upward stream
meet the
charges cancel each other starting at the connection point and
propagating
up the lightning channel and downward and outward along the ground at a
finite velocity (approx .3c).
For perspective:
A conductor in the ground has a velocity factor of about .3. This
gives a
speed of about 100m/usec. So in a 1usec rise time of a typical
lightning
stroke hitting a tower, the wavefront can travel 50m to the shack and
back
and still affect the voltage on the tower. One way to interpret this is
that in the 1usec rise time at 50m the voltage will just start to
rise at
the shack end when it is near the peak at the tower. This has little
application for amateur designs, but helps scale the problem as far
as speed
and distance.
How these apply to the topic at hand:
When properly installed the coax shield is connected to ground at the
tower
and at the SPG. While this doesn't legally qualify as an NESC ground
connection, it IS connecting those two grounds. If you don't
parallel the
coax shield with a REAL conductor it WILL try to carry some portion
of the
lightning current away from a struck tower.
This does NOT depend on the distance to the shack. Because of the
fast rise
time there is not enough time for the wave to propagate to the shack
end of
the cable and back within the rise time... and even if it did that would
make it worse since when the reflection returns it would INCREASE the
current in the coax shield because of the ground on the shack end.
So short
coax would carry more current, but during the critical rise time even
long
coax could get significant currents depending on how many other paths to
ground there are at the tower.
Conclusion 1: YES, put a large conductor between the tower ground
and the
shack ground, no matter how far away it is.
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.
In the case where there is a 'ground' conductor on the pole with the
current
carrying conductors it is still the area of the loop that is the
critical
factor. The closer the ground conductor is to the current carrying
ones the
smaller the induced voltages will be. This is independent of it
being above
or below the other conductors... though there is the secondary effect of
voltages induced on the ground conductor itself that makes it harder to
evaluate.
For buried conductors the near miss calculation is not so easy
because the
cables are in a partially conductive media that is also a
dielectric. And
the 1-3' range normally used by amateurs to bury them is usually well
within
the skin depth of normal soil. But generally because of the surrounding
soil the currents induced in loops in the ground are greatly reduced
from
what they would be in the air. So while there may be some small
advantage
to keeping the conductors close to each other, from an induced voltage
standpoint it is not as critical. My instinct on this would be to
ignore it
for coax shields since they should be connected to ground at both ends
anyway. It would be different for low voltage signal conductors like
rotor
cables. I would not count on the ground attenuation to protect them so
arresters should be used at both ends.
For the direct stroke to a tower:
This is more problematic because the currents are much higher. In
this case
I would worry most about the initial rising edge where the currents and
voltages are the worst.
There are 3 things I would consider here:
1. In the design of high voltage transmission lines it is well known
that
you improve the protection of the current carrying conductors by
keeping the
overhead shield wire close to them. This is because the current
flowing in
the shield wire induces a voltage in the nearby power conductor that
reduces
the voltage difference between it and the tower. This is easily
calculated
and can result in 10's of KV's changes in voltages over separations
of many
meters. While this effect may also be reduced in a buried situation
I would
expect it to still be significant because of the high currents and
closer
physical proximity. For a ground buried next to the cables we are
talking
inches instead of many meters... on the other hand hopefully only a
small
percentage of the lightning current is present in the buried ground as
opposed to the high percentages seen in high voltage line overhead
shield
wires.
2. Electric fields from the buried ground in the soil would also help
prevent punctures of the outer coax jacket. Because the buried
ground is
conducting some of the lightning current it is changing the electric
field
in its vicinity. This should be propagating at about the same speed
as the
wavefront on the coax shield and would result in reducing the voltage
difference across the outer insulation. If the coax were buried
separately
the full voltage difference between the shield and the soil would be
higher.
3. If 2 conductors are buried in different trenches between a tower
and SPG
there is a strong likelihood that the propagation velocity may be
slightly
different in each of them because of depth, soil conditions, vicinity of
other conductors, etc... It is also likely that they may not be as
near the
same length as they might be in the same trench. While this may be a
small
effect in amateur sized installations, our voltages are much lower than
utility lines also so the difference in arrival time of the two
wavefronts
at the far end of the conductors may be significant.
Another possibility to consider, though likely a smaller probability
event
because of the relatively short length of buried amateur cables and the
proximity of towers and buildings... it is well known that lightning
strokes
to the ground can penetrated several meters through the soil to strike
buried power lines, pipelines, etc. There is even a name for the
fused soil
channels left in the ground after these events.(Fulgerites sp?) In
these
cases having a good ground conductor above, or at least near, the signal
cables 'may' provide some protection by intercepting the penetrating
stroke.
The combined effects of the above make it hard to recommend
separating the
ground in a different trench from the coax/control cables. There is an
advantage to be gained from the induced currents coupling together the
various cables, and the construction is easier to start with... and
with the
possible bad effects of propagation differences and the added cost it
just
doesn't make sense.
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