Patent Application
20160370415
Technical Field
The invention relates to manufacture of insulated electrical cables. More particularly, the invention relates to apparatus and methods for quality assurance of the cable insulation.
Discussion of Art
A key purpose for insulating an electrical cable is to mitigate hazards of electrical shock or ignition that an energized cable presents to nearby personnel and structures. Another purpose of cable insulation is to avert abrasive damage to the cable. Instead, the insulation will be abraded.
Sometimes, abrasions or imperfections in coating processes can damage the insulation on a cable. Beyond compromising the ability of the insulation to protect the cable, even minor defects (pinholes or thinned spots) can render the insulation ineffectual to protect personnel and structures from electrical current carried by the cable when energized. Larger defects (bare metal or gross bare wire conditions) can present severe risks of electrocution or combustion.
Accordingly, cable manufacturers presently use various modes of cable insulation inspection. One mode of inspection, in long use, which has been approved by the Underwriters Laboratories, is known as “spark testing.” In spark testing, an insulated cable is run lengthwise through an electrode that is charged to a relatively high voltage (on the order of a few thousand to several thousand volts DC or AC). At least one end of the insulated cable is grounded. Voltage at the electrode is monitored. In case the electrode voltage dips below its controlled value, this dip is understood to indicate an insulation defect that is permitting the electrode voltage to at least partially discharge through the insulation and cable to ground. Because the cable is being run lengthwise through the electrode, the voltage dip can be correlated to a location along the cable by rote calculations.
In order to avoid missing an insulation fault, it is important to assure continuous electrical contact from the electrode to the cable. For this purpose, filament-type electrodes (e.g. brush or chain electrodes) are known. Various modes have been considered for inserting the cable through the filament electrodes. Generally, the cable is threaded through the electrode sideways, i.e., transverse the length of the cable. However, sideways insertion often results in an uneven distribution of beads during testing, which can result in a gap in the filaments, adjacent to the surface of the test cable. This gap counters the overall purpose of the filament electrode, in that the gap reduces uniformity and consistency of electrical contact from the filaments to the insulated cable.
An improvement to this type of electrode is to split the electrode, along the vertical axis of the test cable, and to hinge the electrode halves, and to provide a latch for closing the electrode in use. Thus when the cable is inserted into the electrode, and the electrode closed around the cable, it is then centered axially, eliminating gapping adjacent to the test cable.
However, methods currently in use for fixing the filament elements to the hinged electrode troughs allow for small air gaps between the filament sections at the cable centerline. This remains problematic in the testing of small cables, with diameters less than the resultant air gap.
In consideration of the above, embodiments of the invention provide a filament-type electrode in which uniformity and consistency of electrical contact is increased as further discussed below. Aspects of the invention provide for spark testing a moving cable by closing around the cable a split trough type electrode; energizing the electrode; and monitoring a voltage at the electrode, wherein the electrode includes first and second pluralities of filaments that are intermingled with each other. A benefit of this new electrode is that it provides more intimate contact between the filaments and the wire under test, which enables use of lower test voltages for smaller wires. Another benefit is that the new electrode provides for reliable actual contact between the filaments and the wire under test, a novel condition that can support sophisticated insulation fault typing, as disclosed and claimed in co-pending and commonly owned application [Atty Dkt. 0319-0054] “Apparatus and Method for Spark Fault Detection and Typing.”
Thus, embodiments of the invention provide a spark tester electrode article, which includes a first bracket that supports a first plurality of filaments and a second bracket that supports a second plurality of filaments. The first and second brackets are mutually disposed at angles toward each other, such that the first plurality of filaments that are supported from the first bracket intermingle with the second plurality of filaments that are supported from the second bracket.
Other embodiments provide an improved high-voltage spark tester apparatus, which includes an electrical power supply; a voltage monitoring circuit; and an electrode electrically connected with the electrical power supply and with the voltage monitoring circuit. The electrode includes first and second brackets that support respective first and second pluralities of filaments, and the first and second brackets are mutually disposed at angles toward each other, such that the first plurality of filaments that are supported from the first bracket intermingle with the second plurality of filaments that are supported from the second bracket.
The varied exemplary embodiments of the invention, as briefly described above, are illustrated by certain of the following figures.
Referring to
In use of the apparatus 10, the electrical power supply 12 would energize the electrode 16. Typically, the electrical power supply 12 would provide high frequency, high voltage current (e.g., up to 15 kV at a nominal frequency of 3000 Hz) under a current limit of significantly less than 1 A (e.g., less than about 400 mA, or even less than about 4 mA). In case direct current would be used, then a typical embodiment of the electrical power supply 12 could supply current limited voltage within ranges of 0-1 kV, 0-10 kV, 0-20 kV, or 0-50 kV. Generally, the electrical power supply 12 would be designed to connect with AC mains 0-30 kV at either 50 Hz or 60 Hz power line frequency.
In operation, an insulated cable would be passed through the electrode transverse the drawing view, i.e. in a direction into or out of the view plane. The insulated cable would be grounded at least at one end, either directly or through a capacitor, and the electrode voltage would be monitored by the voltage monitoring circuit 14, so that faults in the cable insulation could be detected by the fault detection circuit 100 as dips of the electrode voltage toward ground.
In order to obtain direct electrical contact with the insulated cable, the electrode 16 includes a first mounting bracket 18 and a second mounting bracket 20, each of which supports a respective first or second plurality of filaments 22 for the purpose of establishing substantially continuous electrical contact between the electrode and an insulated cable passing through the electrode. The filaments 22 may be bead chains, wire bristles, or similar flexible and conductive members that may drape or elastically bend against the insulated cable that passes among the filaments.
The first and second brackets 18, 20 are mutually disposed at angles toward each other, such that the first plurality of filaments 22 that are supported from the first bracket intermingle with the second plurality of filaments 22 that are supported from the second bracket. This intermingling of the filaments enhances their mutual electrical contact as well as their electrical contact with the aforesaid insulated cable as the cable moves along its length traversing the filaments. The enhanced electrical contact provides for continuity and consistency of spark fault detection, which is a purpose of the electrode 16.
As shown in
Depending on factors including the stiffness of the filaments 22, the first and second brackets 18, 20 may be mutually angled such that each plurality of filaments protrudes at an angle downward from a horizontal plane and toward the other bracket. Also, the filaments supported by one or the other or both brackets may bend downward away from the other bracket throughout their lengths.
In the embodiments shown in
Alternatively, in another embodiment as shown in
In yet another embodiment as shown in
Referring back to
Although exemplary embodiments of the invention have been described with reference to attached drawings, those skilled in the art nevertheless will apprehend variations in form or detail that are consistent with the scope of the invention as defined by the appended claims.
