Cable shield fault locator

Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location

Reexamination Certificate

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Details

C324S522000, C324S523000, C324S525000, C324S528000

Reexamination Certificate

active

06281685

ABSTRACT:

BACKGROUND OF THE INVENTION
Large electronic systems rely on cables to carry electrical power and signals between units and locations housing such electronics units. The single most important element in the maintenance of system protection from electromagnetic pulse effects and various noise sources is the integrity of the shielding of signal and power cables (coaxial and multiconductor) and any metallic tubing that provides such shielding. These system elements are prone to degradation over time from various environmental sources such as, for example, corrosion, oxidation, and mechanical stress. Such degradation effects may lead to loss of protection, compromise of security, or to intermittent equipment malfunction. Previous inspection methods and apparatus required partial disassembly of the cables. Conduits to be inspected had to be disconnected for testing, and the analysis of the test results required trained and highly skilled technical personnel for accurate detection and diagnosis.
Previous inductance/resistance test sets did not allow cables and conduits to be inspected in-situ without disconnection. The present invention can be used by relatively unskilled personnel, and without disconnecting the cables. Environments where the cable shield fault locator would be used include fixed and mobile communications facilities, military and commercial aircraft, naval; ships, and combat vehicles (i.e., tanks).
Flaws in the shields of cables can often be traced to connector connections. U.S. Pat. No. 5,189,375 includes a method and apparatus which can be used to inspect for resistive joints that may occur in ground/grounded connectors on cables, including shielded cables. Resistive joints are not the only flaws in cable shielding integrity, however. These flaws can result from an improper bond between cable shield and connector back shell, mechanical stress, metallic oxides, and the like, which introduce resistances in series with the shield and reduce the effectiveness of the shielding by introducing electromagnetic flux disturbances in the shielding path. When resistive flaws at connectors were found to be present, they could sometimes be detected by direct measurement of the cable shield resistance with, for example, a milliohmmeter.
Such shielding integrity flaws as occur at points along the length of the cables, however, have been more difficult to detect.
U.S. Pat. No. 5,391,991, also shows a resistive shielding flaw detector. The U.S. Pat. No. 5,189,375 also shows a method and apparatus for locating shielding integrity flaws by resistance techniques. The presence of a flaw in the shield will be indicated by an increase in the transfer impedance (ohms per unit length) or shield resistance (ohms) above a precisely established maximum permissible value. Typically acceptable values of cable shield resistance in working systems normally range from about ten milliohms (0.01 ohm) to several tenths of an ohm, depending upon cable parameters as length, diameter, characteristics of the shield material, and allowable junction resistances.
It is not always desirable to attempt detection of cable shield flaws by measurements of transfer impedance or shield resistance by standard techniques. The techniques described in U.S. Pat. No. 5,189,375 require that the equipment terminating the cable under test be disconnected for the resistance test. When measurements are made on a cable disconnected from its terminating equipment, a serious flaw may go undetected. Disconnection may relieve the mechanical stress that caused the defect, or may eliminate a resistive junction between a cable connector and the equipment connector. The movement may also create an additional defect that may easily be traced and corrected, leaving the original problem uncorrected. Additional serious degradation in cable shield protection may or may not be detectable with these techniques. The defects may reside in the cable shielding per se rather than in the connectors. Shield defects may exist between the equipment connector and the equipment itself. Thus, inspection for cable shield flaws should be performed with all the cables installed so that all sources of shield degradation will be present and detectable in the normal operating environment.
Other flaws may result in serious electromagnetic flux disturbances passing through the cable shield. These flaws are often very difficult to detect and locate without disassembly of the cables from the equipment, and may not be easily detected. Additional flaws may be caused by reinstallation of the cables. Therefore, detection of shield flaws caused by shield degradation should be performed with the cables normally connected.
SUMMARY OF THE INVENTION
The present tester fills a technical void in the areas of system life-cycle survivability, electromagnetic interference control, lightning protection, and nuclear electromagnetic pulse (EMP) hardness maintenance surveillance. The technical advance is an improved ability to inspect the shield systems, detect problems, and to enable repair of degraded cable shields as they are used. The test may be performed on a routine and continuous basis.
The cable shield fault locator of the present invention relies on the inductance test apparatus concept applied to electromagnetic measurement of cable shield flaws. Portions of inductance/resistance test set are shown in U.S. Pat. Nos. 5,189,375 and 5,391,991, issued in the name of the present inventor and assigned to the present assignee. The present invention is composed of four functional elements, which can be separate assemblies: an inductive coupler, a coupler driver test signal source, and an inductively coupled current probe portion that includes a movable sensor array, and a fault detector having an amplifier and threshold detector therein. These elements may optionally be used in combination with a conventional high-quality portable oscilloscope for visual displays indicating defects.
Accordingly, it is an object of the present invention to provide an inductively coupled and sensed measurement method and apparatus for testing the cable shielding for electromagnetic integrity, and that can be used without disconnecting the cable under test.
It is another object of the present invention to provide an inexpensive cable shielding integrity test set that works with a conventional high-quality oscilloscope to provide a continuous display of inductively sensed responses to a pulsed current waveform induced on a cable by the test apparatus.
These and other objects and advantages of the present cable shield fault locator for use with a shielded electrical pathway having an axial dimension extending between first and second locations are achieved by a cable shield fault locator having a coupler driver generating an electrical signal current; an inductive coupler, fixed in place circumjacent the shielded electrical pathway near the first location, adapted to couple the electrical signal current to the shields; an inductive sensor array, circumjacent the shielded electrical pathway and axially movable between the inductive coupler and the second location, adapted to sense an electrical signal current passing along the shield; and means for detecting a disturbance in the electrical signal current passing along the shield.
The method of detecting cable shield faults with a cable shield fault locator according to the present invention includes the steps of generating an electrical signal current in a coupler driver; inductively coupling the electrical signal current to flow axially along an elongated cable shield at a first location and generate a magnetic field about the shield; inductively sensing the presence of the magnetic field about the shield at a second location displaced along the axis of the shield and separated from the first location; detecting a disturbance in the magnetic field at the second location by comparison with the source electrical signal current; and then perceptibly indicating the existence of said magnetic field disturbance. The induced signal current may be pulsed

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