K4SAV wrote:
> R. Kevin Stover wrote:
>> And this is why you don't attach copper anything, strap or wire,
>> directly to a galvanized (zinc) surface.
>> Clamp some stainless sheet to the leg then clamp your strap or wire to
>> to the stainless.
>>
>> Scott MacKenzie wrote:
>>
>>> One good reason is that copper is anodic to zinc and steel. You will get
>>> preferential corrosion of the zinc and eventually of the steel - eventually
>>> corroding the steel until it is unable to support its weight. At least that
>>> is what I was thinking....
>>>
>>> Scott, AKA KB0FHP
>>>
>>>
> Yes, and so is stainless steel, and it can be even more anodic depending
> on the exact composition. So how do you justify putting stainless steel
> next to the zinc?
>
> And while I'm at it, I may as well throw out another unknown that bugs
> me. Why is the industry recommended method of connecting lightning rods
> to use a rope-lay copper wire. That is twisted such that individual
> strands take a dive into the center of the conductor very often. That
> is very close to the way braid is made and should have the same high
> impedance properties at RF frequencies once the strands obtain a little
> corrosion.
>
For lightning protection, there's several things driving the selection
of the conductor. Interestingly, the absolute lowest RF impedance is
not one of them.
1) You need to be big enough not to melt. However, that's pretty small
wire, considering the current pulse is only a few tens of microseconds
long. It might be amazing, but AWG 16 wire can carry many kiloamps for
a 50 microsecond pulse without melting.
http://home.earthlink.net/~jimlux/hv/fuses.htm has the equations
2) The real issue is mechanical stresses, in two forms.
a) First, lightning protection conductors are outdoors, and go through
lots of temperature cycling, so you want something that can take the
mechanical loads of the contracting and expansion, as well as the
regular slings and arrows of being outdoors and subject to mechanical
impacts.
b) more important is the electromagnetic stresses. This is where the
"don't bend the conductor too tightly" rule comes from and also why we
don't use AWG 16 wire (even though it can carry the current). An
inductor, even a single turn loop, tends to want to expand if there is a
transient pulse through it. The confusing thing is that folks describe
the "no turns and bends" rule as "because it reduces inductance", which
it does, but that's not really why you do it (other than inductance and
mechanical forces go together). The forces go as the *square* of the
current, so they can be quite large. A half a dozen turn 1" diameter
coil wound with AWG 10 wire will explode from the hoop stress with a few
kiloamps put through it (google "quarter shrinker" for pictures). The
forces get fairly high even for incomplete turns (like a 90 degree
bend), particularly if the turn is abrubt. (Draw a picture of the
magnetic field lines to see why).
3) The increased voltage drop from a slightly increased resistance or
inductance isn't that big a deal. We're not talking about dropping the Q
of an inductor from 500 to 200 here. Inductance varies a very small
amount with diameter of the wire or it's shape. A good rule of thumb is
about 1 microhenry per meter, regardless of diameter or shape.
Rosa at NBS in 1908 gave this equation for AC inductance of a wire.
Lac = 2L[ln(2L/r) - 1.00]
(where the frequency is high enough that skin effect has moved all the
current to the surface)
You can see that the inductance per unit length depends on ln(L/r),
which varies pretty slowly. The ln() term at 1mm radius for 1 m is 7.6,
going to 10mm drops it to 5.3, a 30% change. That diameter change is
like going from AWG 30 to AWG 10.
If you are building high frequency inductors or transformers (for PWM
DC/DC converters for instance) and looking to get the highest possible
efficiency, or use the lowest cost of materials, then the difference
between ribbons, tubes, litz wire, stranded wire, etc, all make a
difference. For lightning protection.. nope.. A 20% change in the
inductance for a few foot run of cable isn't going to change whether the
house burns down.
Way, way down on the list might be things like interstrand resistance, etc.
I note that in electrical substations, and the like, you'll also see
much heavier ground conductors than strictly needed for lightning
protection. This is because you might also be carrying a very high
fault current from an energized power line. Unlike lightning, which
only lasts a few tens of microseconds, if you get a flashover from a
power line to your grounded structure (perhaps originally triggered by
lightning), the continuing current could last for tens of milliseconds,
until the upstream circuit breaker or fuse trips. Likewise if you have
a physical line failure, and an energized conductor falls on the
grounded structure. It could take seconds for the current to be turned off.
(By the way, this is the real reason that the electrical code requires
antennas to be grounded with a certain size wire. Accidental contact
with the 10kV sort of primary distribution lines is much more common
than other transients like lightning. )
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|