Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
2000-07-31
2002-10-01
Karlsen, Ernest (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S551000
Reexamination Certificate
active
06459270
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the testing of electrical cables. More particularly, the present invention relates to a method for testing high voltage breakdown and current leakage of the insulation for the electrical conductors within an electrical cable.
2. Description of the Prior Art
In the communications and similar industries, billions of feet of insulated metallic conductors for use in cables are manufactured each year. These conductors have multiple uses. For example, these metallic conductors can be used to connect distributing frames inside central offices, as inside wiring to provide service to customer station apparatus, and other like applications.
Insulation which covers metallic conductors within a cable must be relatively free of defects if it is to prevent shorting of adjacent conductors and ground when the conductors are placed into service. Accordingly, in the manufacture of insulated conductors for use in a cable, it is customary to test the insulation for defects. Defects may include uninsulated portions of the conductor as well as splints in the insulation or inadequate thickness of the insulation. Depending upon the kinds of defects and the number of each in a given length of cable, the insulated conductors the cable may be rejected for use.
Generally, in the past the testing of electrically cables for inter-conductor current leakage and high voltage breakdown required a considerable amount of time when multiple conductors were being tested.
For example, in the past a cable insulation resistance test was performed using a high voltage insulation resistance meter. One lead of the meter was attached to one conductor of the cable being tested, while the other lead of the meter was attached to a second conductor of the cable being tested. A high voltage is then applied and the insulation resistance between the two conductors is measured by the meter.
When a multi-conductor cable is being tested to verify that the insulation is in good condition, it is generally necessary to perform a high voltage insulation resistance test on every conductor with respect to every other conductor within the cable. This results in the number of combinations of n distinct conductors taken m conductors at a time given by the expression:
⟨
n
m
⟩
=
C
(
n
,
m
)
(
1
)
The following table depicts the number of high voltage insulation resistance test that must be performed to completely test a cable having n conductors within the cable.
TABLE I
Number of high voltage
n
insulation resistance
Number of Conductors
test needed to test the
in the cable
cable
5
10
10
45
15
105
20
190
25
300
30
435
35
595
40
780
45
990
50
1225
As can be seen from Table I, when the number of conductors approaches 15, the number of high voltage insulation resistance test required to thoroughly test a cable for defective insulation becomes significant and therefore highly impractical. For example, when the cable has thirty conductors, the number of test required for testing the cable is 435.
Accordingly, there is a need for a procedure to test the insulation in cables which will not be time consuming and thus costly and which will provide a very efficient and thorough method for testing installation in a multi-conductor cable.
SUMMARY OF THE INVENTION
The present invention overcomes some of the difficulties of the past including those mentioned above in that it comprises a relatively simple yet highly efficient and inexpensive method for testing multi-conductor cables for current leakage and high voltage breakdown of the insulation for the electrical conductors within the cable.
For a multi-conductor cable having a plurality of conductors, the method of the present invention requires a user to connect one end of each conductor to a numbered terminal block. At the opposite end of the cable, the user then measures the insulation resistance between the first and second conductors of the cable using a voltage insulation resistance tester.
After measuring the insulation resistance between conductors one and two, the user shorts conductor one to conductor two at the terminal block using a jumper wire. At the opposite end of the cable, the user then measures the insulation resistance between the first and third conductors of the cable using the voltage insulation resistance tester. Since conductors one and two are shorted at the terminal block, measuring the insulation resistance between conductor one and conductor two also measures the insulation resistance between conductors two and three.
The user continues the above-identified steps until all of the conductors are shorted together except for the last conductor of the cable for which the user has not measured the insulation resistance. The user then measures the insulation resistance between the first and last conductors of the cable using the voltage insulation resistance tester. Measuring the insulation resistance between conductor one and the last conductor also measures the insulation resistance between the last conductor and every other conductor within the cable.
REFERENCES:
patent: 3594635 (1971-07-01), Mitsuo
patent: 3771049 (1973-11-01), Piccione
patent: 4164771 (1979-08-01), Heinrich
patent: 4937519 (1990-06-01), Fields, III
patent: 5528152 (1996-06-01), Hinoshita et al.
Kruchowy Roman
Stewart Ian
Kalmbaugh David
Karlsen Ernest
The United States of America as represented by the Secretary of
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