Re: [ccd-newastro] conductive painting of a fiberglass dome
- You can get some static protection that way, but you can't get lightning
protection. Lighting is extremely powerful, and needs nearly a quarter
inch of metal in order to even have a prayer of conducting away the
strike. So a conductive coating would mean nothing for a strike.
Getting rid of the corona charge, on the other hand, is a very good idea.
We are putting a lightning protection system on a 50' dome. The standard
we are using is that the contactor for the point where the rotating dome
meets the building must be at least 8 square inches in area, and the
contactor must press against this with at least 200 pounds of force (to
prevent it being kicked loose in cases of a strike or surge).
You would need to design a custom contactor adequate to route the static
charges you encounter to ground. How much will those charges be? Good
question - that would be hard to find out, in reality.
Every lightning/static protection system is unique, it seems. For
example, how conductive are your soils? That would determine how you do
your grounding. What is your exposure (e.g., are you on a mountaintop,
and a strike is a real threat, or not?).
You should spend a good deal of time learning the ins and outs of
lightning before you jump in with a solution. It's not a simple subject.
> I just received a beautiful 3 meter fiberglass dome from Astrodomes in
> Australia, and have not yet assembled and mounted it. I have been
> researching lightning protection for my dome and am realizing how
> complicated and potentially expensive this topic is. I have seen a
> couple of references to painting a dome with conductive paint to help
> with protection and am curious as to how this works; would it reduce
> the chance of being struck by reducing the charge corona and making
> the dome less of a target, or will it actually keep the strike out of
> the dome? I would assume for the latter that the dome skin would need
> to be suitably grounded- and how does one achieve this with a rotating
> dome (Astrodomes rotate from observatory floor level)?
> I would appreciate any insight on this.
> Thank you.
> Scott Mortenson
> Yahoo! Groups Links
New Astronomy Press
- I would not use triboelectric effect to describe what's going on,
however. This refers to rubbing (fr. Greek "tribos'), and thus the
transfer of charge due to polarity differences specific to given
materials - the greater the inherent polarity difference, the greater
the static charge from the touch/separation action. Wind can move
charge, affecting electrical fields, and there is always plenty of
charge in the air. Moving wind provides a very large effective cross
section, plus an increase in charge variations, and a lot of potential
can build up in a moving wind. This is usually referred to as
wind-induced static charge or flow-induced electrification, although
field-induced charging (charging without contact via field/field
interactions) is also involved.
On the other hand, I would not discount triboelectric effects either.
Even the lowly dust devil can produce very high electric fields, and
that is produced simply by dust particles rubbing together. The lighter
particles tend to pick up a negative charge, and the heavier particles a
positive charge. Since the lighter particles will rise higher and faster
than the heavier ones, a separation of charge of as much as 80KV/meter
can be produced (and has been measured.)
It would not be a stretch to extend this effect to planar winds picking
up dust, separating charges, and transferring the charge to an exposed
surface, such as a dome.
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