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"bud--" wrote in message
.. .
Don Kelly wrote:
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"Tony Hwang" wrote in message
news:dncTj.112858$rd2.31639@pd7urf3no...
wrote:
In alt.tv.tech.hdtv Michael A. Terrell
wrote:
| Bull****. Like ALL charges, it simply seeks a complete circuit to
| flow. You have absolutely no grasp of the basic concepts, yet you
| continue to spout your ignorance and lies.
Not true.
When you close a switch between a power source and a pair of wires that
go
out yonder, the electrical energy does not "know" whether the circuit
is
complete or not. If it refused to flow, it would not be able to find
out.
It will flow, whether the circuit is complete or not. What happens
after
that depends on what is at the other end, which could be an open
condition,
a short circuit, or some kind of resistive or reactive load.
You've claimed to have worked in broadcasting in an engineering role.
So
you should understand what happens at the end of an open transmission
line.
The electricity flows to get to the open end. Yet it is not a
"complete
circuit".
Hmmm,
You seem to be confused between current flow(energy) and
voltage(poential) Nothing flows in an open circuit. If not we have to
rewrite Ohm's law. Show your credential to make a stamement like that.
Shameful.
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Actually, you are showing some confusion. Phil is right in that he is
bringing out a point that normal lumped RLC circuit theory doesn't handle
because it essentially treats the speed of propagation of electrical
signals as if it were infinite- which isn't true.
.
2)Also, on energizing a line whether it is open or closed, there is a
current flow as the applied voltage "sees" the characteristic impedance
of the line (wire or whatever) so a current will flow-even on an open
circuit- until there is a modifying reflection from the termination. For
a house the distances are such that this may be of the order of 0.1-0.2
microsecond. After all such reflections at terminations have ceased or
are negligable, conventional circuit theory is applicable.
In these situations, you are dealing with wave propagation rather than
conventional circuit theory.
This is the regime that is of interest in considering "surge protectors"
The last standards for simulating typical surge waveforms I have seen
(IEEE) were
1.2 us rise time, 50 us duration
8 us rise time, 20 us duration
a ring wave with a frequency about 100kHz.
All are long relative to 0.2 microsecond, so wave propagation should not
be relevant for household circuits.
----------------------------------------
Your point is true- the time interval is so small that for practical
purposes it can be ignored. I am not denying that. Obviously I gave that
impression- sorry for that.
I was simply pointing out that phil had it right in theory and Tony had it
wrong.
After this time for the wave to travel to the end and be reflected (and
other re-reflections die out) then conventional circuit theory is
applicable. The fact that the time is extremely small simply means that we
can pretend that it doesn't even exist.
While Matzloff is right in the time for a round trip is of the order of
200m, it is also dangerous to assume that one can ignore waves for shorter
distances. For example, a stroke to a tower of an EHV line (a lot less than
200m) will go down the tower, meet ground resistance and be reflected.
Such reflections have been found to be more likely to cause flashover than
direct strokes to the line (EPRI). Similarly, the practice in substations
is not "whole station" protection (where this is applicable, it must be done
considering a number of factors- quite interesting ) and putting specific
protection as near as possible to the protected apparatus-definitely within,
say, 10m. - It's not just the time to peak that is the critical factor. Do a
lattice diagram approach or use Bergeron's method (Hermann Dommel did a lot
of work with this at EPRI and has a lot of papers in IEEE- more dealing with
switching surges than lightning).
It's been a long time since I did any calculations in this area so I would
have to brush up.
Now - is this all germane to household protection? You say not and I agree
with you- because household equipment can ride through - at worst- doubling
of the clamped voltage for a very short time even though the clamped voltage
is relatively small compared to the peak of the incoming surge. --
Don Kelly
remove the X to answer
A favorite article from w_ also uses a "8x20 us impulse as a very rough
representative pulse" with most harmonic content from 20kHz to 100kHz.
Martzloff, using the shorter rise time, has written: "For a 1.2/50 us
impulse, this means that the line must be at least 200 m long before one
can think in terms of classical transmission line behavior."
What reason is there to believe wave propagation is relevant to house
circuits?
As to the advantage of "whole house" vs local surge protection, "whole
house protection depends on distances to all "protected" items being
small.
Longer distances make the system more subject to effects like direct
induction from lightning into the wiring. I don't see why, in general,
the distance has to be small.
--
bud--