US2439940A

US2439940A - Electrical insulation testing apparatus

Info

Publication number: US2439940A
Application number: US538401A
Authority: US
Inventors: Clay E Lewis
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1944-06-02
Filing date: 1944-06-02
Publication date: 1948-04-20
Anticipated expiration: 1965-04-20

Description

April 20, 1948. c. E. LEWIS ELECTRICAL INSULATION TESTING APPARATUS Filed June 2, 1944 INVEN TOR c. E. LEW/S AT TORNE Y Potented Apr. 20, 1948 ELECTRICAL INSULA TION TESTING A S APPAR TU Clay E. Lewis, Baltimore, Md., mlznor to Westem Electric Companmlncorporated, New York, N. Y., a corporation of New York Application June 2, 1944, Serial No. 538,401

1 This invention relates to electrical insulation testing apparatus, and more particularly to apparatus for determining the presence of faults in insulating covers of insulated conductors, and has for its object the provision of new and improved electrical insulation testing apparatus.

One apparatus embodying the invention includes a source of alternating current potential, means energizable by the potential, means operable to control the energization o: the energizable means, means actuated by a surge of current through the energizable means, and means for preventing the operation of the lastmentioned means by a surge oi current immediately after the switch is closed.

A complete understanding of the invention may be obtained from the following detailed description oi a specific embodiment thereof, when read in conjunction with the appended drawing,

in which the single figure is a schematic view of one embodiment of the invention.

Referring more specifically to the drawing, a conductor it of a power line H, which is connected to a source of alternating current potential (not shown), is connected to a contact l2 oi. a current=responsive latch-in relay 85. The second conductor It of the power line it is connected to one end of a primary winding ll of a transformer it. The conductors Iii and it have manually operable, normally open, single throw switch 30 provided therein.

A conductor Ell serves to connect a coil oi the relay is with the primary winding ill and a duster 2t comiects'the other end'oi the coil to a conductor The conductor 26 is connected to a contact 2i of the relay i5 and to a s chronous electric motor 28. The motor is also connected to the conductor i6 toy a conductor til.

A shaft 3i connected to the armature (not shown) of the motor 28 has a cam 32 secured thereon and is biased in a counterclockwise direction, as viewed in the drawing, by a torsion spring 33 to urge a shoulder 34 formed on the cam 32 against a stop 35. The cam 32 has a dwell 36 and a lobe 31 formed on the periphery thereof. A button 39 of a contactor Ill bears against the dwell 38 of the cam 32 when the cam is in the position shown in the drawing, which is the starting position.

When the contactor 40 is in the starting position, it is in engagement with a pair of contacts 4i and 42. The contact 4| is connected by a. conductor 45 to the conductor 26 and the contact 42 is connected by a conductor 46 to the conductor 2i. When the motor 28 is energized, it serves to Sills 3 Claims. (Cl. -183) ends thereof. When the armature 59 is in the position shown in the drawing, the contactor 5! is in engagement with the contacts i2 and 21. When a current of large magnitude flows through the coil 22, the armature so is forced upwardly, whereby the contactor 52 engages contacts and 5t positioned adjacent thereto. The contact 55 is connected by a conductor all to the conductor it and the contactor E6 is connected by a conductor 58 to the conductor it with an indicating lamp 59 in series therewith, Thus, if the contactor 52 is in engagement with the contacts 55 and. 56 when the switch so is closed, the lamp to will be connected to the conductor 56 by the conductor 58, and to the conductor t through the conductor 58, the contact 56, the contactor 52, the contact 55 and the conductor cl, whereby the lamp 59 will be lighted to indicate that a surge of current is flowing through the coil 22.

A testing circuit to is provided to test the in-- sulation on the individual conductors of a cable The cable includes ductors having termed thorecver. fl mete-E e rounds the c v H l and v the sheath circuit fill includes a secondary winding 69 of the transformer to on end 015. which one oi the conductors st-cc the cable to he tested is connected conductor The sheath as is connected by a conductor it to the upper end of the secondary winding as viewed in the drawing, and the conductors lit -t6 that are not being tested are connected by conductors l l-H to the conductor it.

In the operation of the above described apparatus to determine the presence of a fault in the insulating cover G'l on one of the conductors 68-66, the elements of the cable 63 are connected to the

conductors

64, 10 and 'll|i as described hereinabove. The switch 20 is then closed to supply current to the primary winding ll of the transformer i8. At this time the contactor 40 is in engagement with the contacts 4| and 42 and any initial surge oi magnetizing curacsaeeo rent flowing to the conductors 2| and 26 from the conductor i8 and the primary winding l1, and from the conductor iii, the contact l2, the contactor Bi and the contact 21, respectively, will be shunted by the conductor 25, the contact ll, the

, contactor II). the contact 42 and'the conductor 48. This shunts enough current from the coil 22 of the relay IE to prevent the relay from operating. Thus, the relay is will not be actuated to cause the lamp 59 to be lighted by the initial surge of magnetizing current caused by closing switch 20.

As soon as the switch 20 is closed, current flows to the motor 28 from the conductors 2B and 3t to energize the motor, whereby the cam 32 is rotated at an even speed in a clockwise direction, as viewed in the drawing. The motor 28 is so timed that a few seconds after the switch 29 has been closed, the lobe 31 of the cam 32 is forced into engagement with the button 39 of the contactor 40, whereby the contactor 40 is moved out of engagement with the contacts M and er. By this time the initial surge of magnetizing current caused by the closingof the switch Ell has subsided and, if there are no defects in the cover 67 being tested, the no-load current of the transformer which flows through the coil 22 from the conductors 2i and 26 is not sufficient to actuate the relay it.

This current which flows through the circuit including the primary winding H at this time is small if the conductor being tested is completely insulated from the sheath 68 and the other conductors, since no current flows in the testing circuit tit. However, if a conductive fault is present in the insulating cover 8'7, 9. current induced in the secondary winding Bl of the transformer l8 by the energization of the primary winding l'i will flow through the fault to the insulating cover at from that one of the conductors t66 being tested to the metallic sheath 68 or to the conductors 6666 that are not being tested.

The presence of such a fault in the insulating cover being tested will thus cause a flow of current through the secondary winding an of the transformer it, which flow of current will be reflected as a magnified flow of current through the circuit including the primary winding ll of the transformer it. The magnified flow of current will flow through the coil 22 of the relay l5 and, since the current then flowing is sumcient to actuate the relay, the coil 22 will force the armature 5B upwardly and will bring the contactor 52 into engagement with the

contacts

55 and 55, whereby the light 59 will be illuminated to indicate that a fault is present in the insulating cover 6? being tested.

The switch it is then opened, whereupon the elements of the apparatus return to the positions shown in the drawing. The cable t3 is removed from the testing circuit 60, and the faulty conductor then is tagged to indicate that a fault is present in the cover El thereof. One of the insulating covers 616l of the same or of another cable then may be tested by connecting another one of the conductors 66-66 thereof in the testing circuit inthe manner described.

The above described apparatus serves to determine quickly and accurately the presence of faults in insulating covers of conductors and prevents any false indications of such faults by initial surges of magnetizing current from the power line H.

If it is desired to test for faults in insulating covers of a conductor having no metallic sheath similar to the sheath 88, such a conductor may be inserted in a grounded tank of water or other suitable conductive medium with the ends of the conductor out of contact with the water. The

conductor 38 is connectedto one end of the con-' ductor to be tested and the conductor I0 is grounded. The apparatus then is operated in a manner similar to that described hereinabove. By immersing all of the conductor except the ends thereof in water, the presence of voids or cuts in an insulating cover formed thereon may be determined as well as the presence of conducting faults.

What is claimed is:

1. An apparatus for testing the insulation on an insulatedconductor forming part of an electric cable, which comprises a transformer having an insulated conductor to be tested connected in the secondary circuit thereof, means for energizing the primary side of said transformer to impress a high potential across its secondary side and across the insulation of a conductor connected thereto to break down any faults in the insulation of said conductor, manually operable means for controlling the energization of the transformer, current responsive means connected in series with the primary side of the transformer to automatically deenergize said transformer when an increased current flows therein due to a breakdown in the insulation of the conductor connected thereto, and means including a timing device energized simultaneously with the transformer for shunting the current responsive means for a predetermined period of time thereby preventing the operation of the current responsive means by transient magnetizing inrush currents occurring upon the energization of the transformer.

2. An apparatus for electrically testing an insulated conductor, which comprises a transformer energizable to impress a high potential across an insulated conductor connected in the secondary circuit thereof, means for energizing the transformer, said transformer being subject to transient magnetizing inrush currents when energized, manually operable means for controlling the energization of the transformer, means connected to be responsive to the current flowing through the primary side of said transformer for deenergizing the transformer when a high current flows in the secondary side of the transformer due to faulty insulation in a conductor connected in the secondary circuit, means for indicating the operation of said current responsive means, and means including a timing device energized simultaneously with the transformer for shunting the current responsive means for a predetermined period of time after said transformer is energized to prevent the transient magnetizing inrush currents of said transformer from operating the current responsive means, whereby false interruption of the testing circuit by such tran sient inrush currents is prevented. 3. An electrical testing apparatus which comprises a transformer energizable to impress a high testing potential across an insulated conductor connected in the secondary circuit thereof, said transformer being subject to transient magnetizing inrush currents when energized, manually operable means for controlling the energization of the transformer, a current responsive relay for automatically deenergizing said transformer upon an increased current flow in the primary circuit of said transformer due to a breakdown in the insulation of the conductor con- 5- nected in the secondary circuit, said current responsive relay being unresponsive to the normal magnetizing current of the transformer, means for indicating when said current responsive relay is operated, and means including a timing device connected to be energized upon the energization of said transformer by, said manually operable means for rendering said current responsive relay inoperative for a predetermined period of time to permit the subsidence oi the transient magnetizing inrush currents, whereby energization of said current responsive relay by the transient magnetizing inrush currents is prevented.

CLAY E. LEWIS.

nnrmuons crmn The following references are of record in the file of this patent:

UNITED STATES PATENTS