QUOTE(Richard @ Dec 9 2016, 09:41 PM)
A simply "path of least resistance" is an apt explanation ..
Resistance is irrelevant. Impedance is the significant parameter.
For example, a 12 meter copper wire might be 0.2 ohms resistance. That same wire is something like 120 ohms impedance.
Other factors also apply. If a wire has numerous sharp bends, resistance does not change. But impedance increases significantly.
From an engineer at WXIA-TV:
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Since my disasterous strike, I've been campaigning vigorously to educate amateurs that you *can* avoid damage from direct strikes. The belief that there's no protection from direct strike damage is *myth*. ...
The keys to effective lightning protection are surprisingly simple, and surprisingly less than obvious. Of course you *must* have a single point ground system that eliminates all ground loops. And you must present a low *impedance* path for the energy to go. That's most generally a low *inductance* path rather than just a low ohm DC path.
From the book "Safe Coordination Between Power and Communication Surge Protection":
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1.4 Role of grounding in lightning protection systems
In the case of direct strikes, a low impedance path from the lightning protection conductor to the ground is essential to keep the inevitable voltage-rise within safe limits, when currents of large magnitudes are conducted by the lightning protection system. ... While the normal power system ground is designed primarily to provide a low resistance path to ground, in the case of grounding systems of lightning/surge protection systems, it is the impedance which is of importance. As we shall see later, a surge gives rise to voltage and current pulses having extremely fast rise times. Any inductance in the grounding circuit obstructs the flow of surge currents and produces a voltage drop. This drop is a function of the inductance and the rate of rise of the current. Remember that even a piece of wire has its own self-inductance, which is sufficient to cause an appreciable voltage drop while conducting a lightning surge if the length becomes excessive. Thus the grounding conductors of a lightning protection system (including the ground connections of surge protection devices) must be as short as possible and without any avoidable bends.
From Smiths Power application note entitled "AC and DC Power Protection at Communication Sites":
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The amount of current the protector will conduct to ground depends on the equipment load impedance, the inductance of the ground conductor on its way to earth ground, and the fall of potential resistance/impedance of the earth ground system.
From QST Magazine (by the ARRL) entitled ""Lightning Protection for the Amateur Radio Station"
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The purpose of the ground connection is to take the energy arriving on the antenna feed line cables and control lines (and to a lesser extent on the power and telephone lines) and give it a path back to the earth, our energy sink. The impedance of the ground connection should be low so the energy prefers this path and is dispersed harmlessly. To achieve a low impedance the ground connection needs to be short (distance), straight, and wide.
From Mike Holt's "Grounding vs. Bonding":
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2.3.1 Grounding
An effective, low-impedance ground path is critical for the successful operation of an SPD.
From Electrical Engineering Times entitled "Protecting Electrical Devices from Lightning Transients":
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Another aspect of impedance ... of a wire is predominately related to its length and weakly related to its diameter. ... The length of the cable increases the impedance dramatically. ...
... wire should have at least a 10 inch or 30 cm radius.
Wire thickness affects resistance. Impedance is defined by wire length and other parameters. No soundbyte (apt) explanation of science exists. Sound bytes (simplistic answers) are a first indication of junk science reasoning. Professionals cite impedance - not resistance. A protector is only as effective as its earth ground. That means single point earth ground AND a low *impedance* connection.
That 6kV protection is effective only when a low impedance connection to ground exists. Then a surge that might create 6000 volts instead creates near zero volts. How close to zero? How low is that impedance?
Dec 10 2016, 01:19 AM
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