On 10/25/2014 3:43 PM, Bill Turner wrote:
(Please excuse the one handed typing errors)
You missed both points Bill.
The grounds were installed by a licensed electrician and inspected at
the end of the job.
The soil is highly acetic. The bolts on the bronze (Believe it was
bronze) clamps were steel.
After a couple of years in damp/wet, acetic soil they were a mess and
loose. I've seen this quite a few times over the years in this area.
Yes, it makes a difference that the bolts are torqued down (code
requires it), but in acetic soil, they will not last forever and
sometimes, not for long.
The point with the resistance to ground was not lightening jumping a
gap. There was no gap. Thousands of amps for a few milliseconds across
that small resistance allows the house electrical ground to rise
thousands of volts above ground. (at this peak current, even a half ohm
is important) This would be fine if "everything were grounded/earthed
at that point and all leads were the same length (the same distance from
the strike. The farther, the later the peak so all these different peaks
have to discharge through the same point), but rarely does a
home/building have a true single point ground. The goal is to have
everything at any specific location rise to the same potential at the
same time. Who care that there may be thousands of volts difference
between ends of the house or different locations in the house? The lower
we can keep that peak and the sooner we can bleed it off, the better.
In my case it would not only be impracticable, but probable ineffective.
I have a large "ground system" for the towers and antennas. I have
multiple coax cables and control cables that run to two different
buildings. All of that comes in at it's own SPG (at each building). It
gets a lot messier as the CAT5 network ties to two buildings with
different electrical feeds and services. The antenna ground system ties
to both buildings, essentially tying their service grounds together
through a network of over 600 feet of bare #2 cad welded to 32 or 33 8'
ground rods. To tie them all to the entrance ground would greatly
increase the length of the station ground system.
But neglecting the messy part, whether a nearby strike, or direct hit
there is a propagation time so it may be a SPG for the electrical
system, but different parts of the house will be at different potentials
at the same instant. That half ohm between clamp and ground will allow
the voltage to rise much higher for a longer time due to the different
potentials throughout the house and the added discharge time. Each
location has to discharge through that half ohm. The time is still
small, but due to the higher voltages and differences may allow phone
line, cable, and electrical system to reach much different voltages.
This happens normally, but now we are looking at higher voltages with
greater differences for a longer time.
Will it make a difference in the end? We all know there is no such
thing as 100% protection from lightening in a house. When we allow a
reduction in ground effectiveness we reduce what protection we do have,
so we do increase the odds against us. However we are playing the odds
and decreasing those that are in our favor or increasing those against
us, so I would think that resistance would be important let alone,
ground clamps that come loose.
73
Roger (K8RI)
------------ ORIGINAL MESSAGE ------------(may be snipped)
On Fri, 24 Oct 2014 17:22:58 -0400, Roger wrote:
Seeing actual clamps that can be lifted off the ground rods after a
couple years is no old wives tale. It was that way on our house.
Corrosion even raising the contact
resistance to a half ohm leaves little protection from lightening.
REPLY:
With all due respect, those two statements defy common sense.
First, properly tightened bolts do not come loose except perhaps with
vibration, which of course is not present here. If anything, corrosion
makes things tighter, not looser. I suspect the original clamps were
installed but never tightened at all. Simple human error. It happens.
Second, a lightning bolt cares nothing about a half ohm connection. It
has already traveled through millions of ohms of air, another half ohm
in series is nothing. And of course, when the lightning arcs through
the half ohm, ionization makes the resistance go to an even lower
value.
73, Bill W6WRT
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73
Roger (K8RI)
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