[TowerTalk] Fwd: Grounding of tower

[K1TTT]

>>You can kind of get away with pouring a bit of household 60hz into the 
>>ground, but don't count on it to work for a primary on a power line 
>>either!

>Actually, we CANNOT use the EARTH as a conductor for AC power either  --
the resistance is far too high to carry enough current.

Well, actually it can be done, but only for relatively low current
densities...  my point was that the amount of current you can pump through
the ground without causing a large potential rise is limited.  For example
look up some videos of distribution primaries dropped on the ground bouncing
around and arcing away for long periods... the ground potential in the area
of those fallen wires can be quite high and obviously there isn't enough
current to blow the line breaker or fuse.   But don't get too close because
the step potential (that is the voltage between your feet on the ground) can
be quite high.  And if they fall on your metal fence don't touch, even
though the fence is 'grounded' the potential between the metal of the fence
and your feet a short distance away can be high.

The only real reason for requiring ground rods for electrical distribution
is because one part of the system is grounded, so if there was a fault and
your house entrance wasn't grounded you could have a shock hazard between
anything plugged in and the local earth.  At least if you have ground rods
the electrical system ground will be closer to local earth potential, even
if that is not at 0v with respect to a distant ground.

Consider this case... the Navy takes the exact opposite approach in
submarine power distribution, nothing is 'grounded' to the hull of a
submarine.  One purpose of this is to prevent stray currents from
magnetizing the hull, but a big reason is for safety.  As long as no current
path is available through the hull back to a power source you could
theoretically touch a piece of equipment that has a power line short to the
case and not get shocked (assuming of course the case wasn't properly
connected to the line neutral, which would be a 2nd failure).  It takes a
fair amount of effort to maintain this isolation, but since the submarine is
essentially a close electrical system it is worth it.  

The same approach could be used for land based power distribution, but with
all the exposed equipment it is impractical except for some cases where it
has some local advantages.  So the opposite method is used, connect
everything to the local earth so that at least if there is a fault somewhere
else there won't be a potential difference between earth and the power
system ground.   A common misconception is that the ground rod at the house
entrance protects you from faults in the house... while there may be some
odd cases where it helps a bit for that, it is really the green wire, which
is only connected to the earth through the ground at the entrance panel,
that saves you most of the time... it's function is to provide an alternate
path to the neutral in the panel to blow the fuse or breaker, that is what
saves you from electrical faults most of the time.  As noted above, if there
is a double fault, say the green is broken and there is a short to the case
and you touch it while standing in a puddle there won't be enough current to
blow the fuse, though it may be enough to blow your fuse.  The real purpose
of the house entrance ground is to keep your ground at the same potential as
the system ground so any fault in the transformer or on the line doesn't
show up at your doorstep.

Consider this example... my neighbor had a broken ground in the wire
overhead from the transformer to his entrance panel.  There are ground rods
at the pole and at his entrance as required.  When a big load on one side of
the line started, like a refrigerator, that leg browned out but the other
leg of the 220 would get up to about 200v and blow out stuff on that circuit
due to the overvoltage.  Obviously there wasn't enough return current
through the ground, even over that relatively short distance, to keep the
supply balanced.

However, I just had a lightning strike REALLY close to the house, it may
have even hit one of the towers.  But because all the equipment is well
connected to the common earth connection when the ground potential rose due
to the current from the strike, all the equipment rose together and not even
the dsl dropped out.  If everything wasn't connected the typical fault would
be caused by a radio connected to an antenna but not to earth having a
flashover from the case to the power line, that is, the equipment becomes
the path to ground... not a good thing.

David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net