Re: O/T Question - What are the purpose of lightni

Posted by KEB on April 06, 2007 at 09:49:56 from (192.31.106.35):

In Reply to: Re: O/T Question - What are the purpose of lightni posted by RAB on April 06, 2007 at 00:21:13:

In the case of lightning, the voltage is sufficient to force a given amount of current through whatever the lightning stoke hits. The power dissipated in say a wire, is given by the square of the current times the voltage drop along the wire (i^2*r, derived from ohms law). The voltage between cloud & ground is immaterial, the only thing that matters is the voltage drop across whatever the current is flowing through. My comment above refers to the cloud to ground charge differential voltage, which as I said is immaterial in determining how much energy is deposited in the material the current is flowing through.

Lightning analysis uses a term called the "action integral", which is the current squared integrated over time, i.e., the i^2 part of the the equation above. This integral times the real part of the impedance (resistance) gives the total energy (typically in watt-seconds) deposited in the material.

Explaining all the nuances of lightning interaction with materials would require a textbook. If you want a more detailed explanation, do a seach on the web for MIL-STD-464, Electromagnetic Effects Requirements for Systems, and read the rationale for the lightning protection requirements.

Keith

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