Electricity: measuring and testing – Testing potential in specific environment
Reexamination Certificate
2000-11-09
2002-06-18
Gutierrez, Diego (Department: 2858)
Electricity: measuring and testing
Testing potential in specific environment
Reexamination Certificate
active
06407539
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the measurement and testing of structures for adequacy of lightning protection. In particular, the present invention provides an apparatus and method for accurately determining theoretical energy levels that a lightning strike causes within a tested structure and the need for appropriate remedial measures to protect personnel and critical assets.
2. Description of Related Art
In general, there is no systematic, reliable and accurate quantitative method available to determine the degree of lightning protection afforded by a structure, an external lightning protection system (LPS) for that structure, or for the combination of the structure and the lightning protection system.
Mardiguian,
Grounding and Bonding
, pps. 10.27 to 10.29, and Plumey, et al., “High Frequency Harmonic Input Impedance Of An Antenna Embedded In A Conducting Half-Space”, Electromagnetic Compatibility, 1983 Proceedings, pp. 45-50, both discuss grounding lightning transients and refer to measuring ground impedance at frequencies above dc. The measurement is made using a grounding rod under test, an auxiliary grounding rod providing a current return, a reference grounding rod, a surge generator for injecting a current impulse through a matched resistor between the grounding rod under test and the first auxiliary grounding rod, and an oscilloscope or spectrum analyzer for measuring the voltage transients across a voltage divider network connecting the ground rod to be tested and the second auxiliary grounding rod. The impedance is determined from the ratio v/I of the probe to earth voltage to the injected current transient.
As a modeling methodology, this method suffers from the problem that how lightning strikes affect structures was not well understood until recently. Recent testing has demonstrated that a substantial amount of the energy of a lightning strike hitting a structure is dissipated in the metallic support elements of the structure, rather than the external lightning protection system.
Because the flow of lightning energy through a structure can damage or destroy critical assets such as sensitive electronic equipment or explosive materials within the structure, additional analysis has proven necessary.
Indeed, the most commonly used prior art lightning protection system measurement and testing method requires the measurement of the continuity and ground resistance from an air terminal to surrounding earth using the fall-of-potential method with an alternating current ohmmeter. The resistance value must be less than 10 to 25 Ohms, depending upon the governing regulation. A rigorous visual and mechanical inspection of all of the air terminals, conductors, fasteners and ground rods is necessary to assure the mechanical robustness of the external lightning protection system. No quantitative protection level can be calculated from such a method. Other versions of this ohmic technique require disassembling various individual components and conductors and measuring the continuity of key elements in the external lightning protection system, as well as the resistance to ground.
Prior U.S. Patents that deal with testing of a lightning protection system generally involve either direct current injection of a test signal or magnetically inducing a current in elements of the lightning protection system. For example, U.S. Pat. No. 4,142,143 teaches a method of measuring the admittance (conductance and reluctance) of a conductor to determine its continuity within a larger lightning protection system without the disassembly of the structure. Another example is U.S. Pat. No. 5,654,641, which teaches a method that measures the current division within a lightning protection system to qualitatively estimate whether protection is affordable in the vicinity of the particular conductor. U.S. Pat. No. 5,929,625 teaches a method of monitoring a lightning protection system with a device that continually monitors the lightning protection system using a magnetic circuit.
None of these methods provides accurate, quantitative measurements that can be used to calculate the expected energy levels within the structure from a lightning strike or recommended modification of either locating critical materials within the structure or ways for modifying a structure or the combination of the lightning protection system and the structure to prevent damage when a lightning strike occurs.
Other prior art U.S. Patents that deal with automated testing of grounding systems at frequencies above dc using automated control apparatus include U.S. Pat. No. 5,365,179. This reference teaches a method and apparatus for measuring ground to earth impedance at frequencies above dc using a computer controlled instrument to implement a three point measurement or a fall-of-potential method using three ground paths and a plurality of discrete frequencies in the range of from 5 Hz to 200 MHz. The complex impedance vector array for each frequency selected in the range may be determined for identifying antenna effects to improve the quality of the ground and improved ground path selection for an electrical system. This patent does not describe methods of determining the lightning protection effectiveness of a structure and its lightning protection system, nor does it recognize methods for deriving a transfer function for a structure, the characteristics of a lightning strike and how it interacts with that structure or problems resolved by the instant invention.
One of the problems these prior methods represent is that they do not model accurately a structure with metallic elements or take into account that these metallic structural elements dissipate most of the energy incurred by a lightning strike. The present invention provides a direct, accurate and quantitative measurement and determination of protection of a structure by deriving a collection of transfer functions at various locations within the structure and using modeled excitation functions in the frequency spectrum of lightning that are convolved with these transfer functions to predict the affects of a worst case lightning strike. This information provided useful standoff distances for minimizing harm to personnel and damage to critical assets located within that structure.
BRIEF SUMMARY OF THE INVENTION
Objects of the Present Invention
The present invention provides an apparatus and measurement methodology that allows accurate, safe and practical determination of the degree of lightning protection for a structure having metallic structural elements. This is particularly useful for a structure that requires zones of safety for personnel or critical materials, such as computers, explosives or other critical assets. The apparatus in cooperation with frequency domain analytical method yields a quantitative protection level for a structure. The method of the invention includes:
i) determining the test locations, such as an air-terminal (lightning rod) or other metallic conductors at the top of the structure, for the measurement of the transfer functions of the structure;
ii) injecting a low-level test current into the test locations at multiple test frequencies and allowing the injected test current at each test frequency to flow through the metallic and other partially-conductive elements of the structure, and flow to the surrounding earth beneath the structure;
iii) measuring the electromagnetic fields (electric field, magnetic field, or both); The method of the invention includes:
i) determining the test locations, such as an air-terminal (lightning rod) or other metallic conductors at the top of the structure, for the measurement of the transfer
iv) calculating and synthesizing the transfer functions for each test location within the structure; and,
v) determining the internal energy levels that lightning strikes would cause inside the structure using probable models of a lightning event at these various test points.
The collection of transfer functions from the various injection points of measurement locations c
Jessee John C.
Khowong George K.
Lee Henry H.
Magnotti Philip J.
Struck Jacob K.
Beam Robert
Moran John
Nguyen Vincent Q.
The United States of America as represented by the Secretary of
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