WO1996035243A1

WO1996035243A1 - Fault voltage isolator

Info

Publication number: WO1996035243A1
Application number: PCT/AU1996/000262
Authority: WO
Inventors: Peter Birrell; Gavin Kerr
Original assignee: H.P.M. Industries Pty. Ltd.
Priority date: 1995-05-03
Filing date: 1996-05-03
Publication date: 1996-11-07

Abstract

A fault voltage isolator for use in a coaxial cable feeder system. The isolator is connectable in-line between two lengths of coaxial cable and it comprises a central capacitor (19) which is connected between oppositely projecting tubular contact members (20). The central capacitor (19) and the contact members (20) are supported in an insulating material support element (10). Two diametrically disposed outer capacitors (32 and 33) are mounted to a tubular support member (31). The various components are located within a multi-part casing (23) which includes metal end portions (24 and 25). The tubular support member (31) is formed on its outer surface with spaced apart claddings (35 and 36) which are bridged by the outer capacitors (32 and 33) and which engage with an internal wall portion (37) of the metal end portions (24 and 25) of the casing (23).

Description

FAULT VOLTAGE ISOLATOR

FIELD OF THE INVENTION

This invention relates to a fault voltage isolator for use in a coaxial cable feeder system. The isolator is connectable in line between two lengths of coaxial cable, either as a discrete device or as one which forms part of another element such as a wall plate.

BACKGROUND OF THE INVENTION

Fault voltage isolators are used in telecommunication feeder systems, for example, in cable television (CATV) feeder systems, to protect subscribers and electronic equipment from fault voltages. Such isolators take the form of full isolators that protect against fault voltages in both the core and the shielding conductors of coaxial cables and so-called half isolators that are located in circuit with the core conductor only. In each case, the isolator incorporates at least one capacitor that functions as a high pass filter and the or each capacitor is normally mounted to a flat printed circuit board and located within a metal isolator housing. The capacitor(s) should be mounted in a configuration which maintains as best as possible the characteristic impedance of the coaxial cable in order to minimise unwanted reflections; but this frequently is very difficult if not impossible to achieve, principally or at least partially because the geometry-related characteristics of the cable cannot be emulated entirely within the isolator network.

SUMMARY OF THE INVENTION The present invention is directed to a fault voltage isolator which is connectable in circuit with two lengths of coaxial cable and which is configured in a manner to avoid at least some of the problems of prior art housed isolators. To this end, the present invention provides an isolator which incorporates capacitors which are supported and positioned in a manner to emulate the geometry of a coaxial cable, at least as seen by radio frequency signals. Thus, the present invention may be defined broadly as providing a fault voltage isolator which is arranged to interconnect two coaxial cables. The isolator comprises a central capacitor connected between oppositely projecting contact members which are arranged to connect electrically in line with core conductors of the respective coaxial cables, and an insulating material support element supporting the central capacitor and the contact members. At least one capacitor is located radially outwardly of the central capacitor and a casing is provided to contain the support element and the outer capacitor(s) . Also, means are provided for effecting electrical connection between the outer capacitor(s) and shielding conductors of the respective coaxial cables.

PREFERRED FEATURES OF THE INVENTION The or each outer capacitor preferably is mounted to a tubular support member which is formed in part from an insulating material and which is formed on its outer surface with spaced apart conductive material claddings. In such case, an electrical connection will be made between the conductive material claddings and the shielding conductors of respective ones of the coaxial cables, and the outer capacitor(s) will be mounted to the tubular support member in a manner to bridge the spaced apart claddings. The tubular support member preferably is formed as a flexible printed circuit board and is rolled into a hollow cylinder prior to or during assembly of the isolator device.

The casing preferably comprises a multi-part casing having end portions which are arranged to carry end caps which, in use, are employed to connect the shielding conductors of the respective coaxial cables to the end portions of the casing parts. The casing parts preferably are interconnected by an insulating material sleeve, and a locating element preferably is interposed between the casing parts for locating the outer capacitor(s) . The insulating material sleeve most preferably is formed from an elastomeric material of a type which may be activated thermally or chemically to shrink onto and so interconnect the casing parts. The insulating material sleeve preferably is positioned to extend around the outer capacitor(s) .

Capacitors which have been found suitable for use in the fault voltage isolator comprise so-called chip capacitors, and by using such capacitors the isolator may be fabricated as a compact device. A chip capacitor is formed with a ceramic substrate sandwiched between two metallic end pieces. The capacitor is characterised by a small physical size relative to its capacitance value and its specified working voltage. Other capacitors that exhibit these characteristics may be employed in the isolator of the present invention as alternatives to a chip capacitor.

The value of capacitance to be used in any particular application of the fault voltage isolator will be determined by signal frequency transmission requirements of the coaxial cable, the level of filtering required and the likely amplitude of possible fault voltages from which protection may be required. A chip capacitor that has been found suitable for use in CATV applications of the isolator has a capacitance in the order of 1500 pF.

In use, the fault voltage isolator in its various forms may be sited at any desired location, for example in a coaxial cable feeder system at a point prior to entry to a building, at the first point of entry to a building, behind a wall outlet into which a coaxial cable is to be plugged, within such a wall outlet, or in line with a coaxial cable which is employed to connect directly with an appliance to which a radio frequency signal is to be delivered.

The invention will be more fully understood from the following description of a preferred embodiment of fault voltage isolator. The description is provided with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

Figure 1 shows a sectional side elevation view of the isolator;

Figure 2 shows a sectioned end view of the isolator as viewed in the direction of section plan 2-2 shown in Figure 1;

Figure 3 shows a plan view of an insulating material support element which is incorporated within the isolator assembly illustrated in Figure 1;

Figure 4 shows a sectioned end elevation view of the support element as viewed in the direction of section plan 4-4 shown in Figure 3; Figure 5 shows a sectioned end view of the support element as viewed in the direction of section plan 5-5 shown in Figure 3-3;

Figure 6 shows a sectioned end view of the support element as viewed in the direction of section plan 6-6 shown in Figure 3;

Figure 7 shows a side elevation view of a tubular support member which is incorporated within the isolator assembly illustrated in Figure 1;

Figure 8 shows an end view of the support member illustrated in Figure 7;

Figure 9 shows a plan view of a locating element which is incorporated within the isolator assembly illustrated in Figure 1; and

Figure 10 shows an end view of the locating element as seem in the direction of arrow 10 shown in Figure 9. DETAILED DESCRIPTION OF ONE MODE OF THE INVENTION

As illustrated, the fault voltage isolator comprises a support element 10 which is moulded from an insulating material. The support element has a generally cylindrical form, as view in cross-section, and it incorporates a recess 11 which is located mid-way along the length of the support element. Centrally located bores 12 extend in an axial direction through the support element and enter the recess 11 from each end 13 and 14 of the support element. Each of the bores 12 is surrounded by a wall 15 which is in turn surrounded by annular rings 16. The rings are joined by integrally formed longitudinally extending webs 17.

The recess 11 within the support element 10 is formed integrally with moulded portions 18 which are arranged to provide a seat for a centrally positioned chip capacitor 19. Also, oppositely projecting contact members 20 are located within the bores 12 and are connected electrically in abutting contact with end caps of the capacitor 19.

The contact members 20 are formed as tubular elements from metal and they are soldered in situ to the end caps of the capacitor 19. Each contact member 20 has a flared opening 21 and is slit longitudinally along a portion 22 of its length. Each of the contact members is arranged and sized to receive a projecting end of the core conductor of a coaxial cable (not shown) .

Although not so shown in the drawings, the recess 11 within the support element 10 may be filled (ie, potted) with an insulating material after the capacitor 19 has been positioned within the recess 11 and connected with the oppositely projecting contact members 20.

The support element 10 is located within a multi¬ part casing 23 having metal end portions 24 and 25. Each of the end portions is formed with a hexagonal shoulder 26, a forwardly projecting land 27 and a rearwardly projecting male screw-threaded portion 28. The bore 29 of each land 27 has an octagonal form as seen in end view.

The screw-threaded portions 28 of the two casing parts 24 and 25 are arranged to receive end caps 30, one only of which is shown in Figure 1 of the drawings. The end caps 30 are provided to connect shielding conductors of coaxial cable portions (not shown) to the threaded portion 28 of the each of the end portions 24 and 25 of the casing. A tubular support member 31 is located around the insulating material support element 10, and the tubular support member 31 carries two diametrically disposed outer chip capacitors 32 and 33. The outer chip capacitors 32 and 33 are located radially outwardly of the central capacitor 19.

As best seen from Figures 7 and 8 of the drawings, the tubular support member 31 is formed in part from an insulating material 34 which is formed on its outer surface with spaced apart conductive material claddings 35 and 36. An electrical connection is made between the conductive material claddings 35, 36 and an inside wall portion 37 of the metal end portions 24 and 25 respectively of the casing. As a consequence, an electrical connection is made between the conductive material claddings 35 and 36 and the shielding conductors of respective ones of the coaxial cables when the cables are connected to the metal end portions 24 and 25 of the casing.

The two outer chip capacitors 32 and 33 are mounted to the tubular support member 31 in a manner to bridge the spaced apart claddings 35 and 36 and to extend across a peripherally extending gap 38 which is located between the two claddings 35 and 36.

With this arrangement, the two outer chip capacitors 32 and 33 are each connected in series with the shielding conductors of the coaxial cables when the cables are connected to the respective ends 24 and 25 of the multi¬ part casing 23. The tubular support member 31 is formed as a flexible printed circuit board, incorporating the insulating material backing 34 and the spaced-apart claddings 35 and 36, and the board is rolled into the hollow cylinder prior to assembly of the isolator device. A locating element 39, which is moulded from a plastics material and which is shown in detail in figures 9 and 10, is positioned within the multi-part casing 23. The locating element 39 performs the dual function of joining the two parts casing portions 24 and 25 of the casing and of positioning the outer capacitors 32 and 33 peripherally within the casing.

The locating element 39 is formed with octagonally shaped end regions 40 which are sized to be a press fit within the bores 29 of the casing end portions 24 and 25. Also, the locating element is formed with a generally cylindrical central region 41 which is sized internally to surround the tubular support member 31. Diametrically disposed slots 42 are formed within the locating element and are sized to accommodate the two outer chip capacitors 32 and 33.

Following interconnection of the two end portions 24 and 25 of the casing, the two casing portions are joined by a sleeve 43 of heat shrink material which is positioned to surround the outer capacitors and bridge the lands 27 of the casing end portions.

When assembling the above described fault voltage isolator to the end of a coaxial cable (not shown) , the end region of the cable is first pared so as to reveal the core conductor and the outer shielding conductor. After slipping the end cap 30 over the coaxial cable, the core conductor of the cable is inserted into the contact socket 20 and the shielding conductor is positioned so as to overlay the screw-threaded portion 28 of the casing end portions 24 or 25. Thereafter, the end cap 30 is secured to the end of the casing and, when screwed into position, the end cap functions to clamp the shielding conductor of the coaxial cable into positive electrical contact with the end portion of the casing. This operation is effected at both ends of the isolator device and, as a consequence, its contained capacitors 19, 32 and 33 are located in line with the core and shielding conductors of the coaxial cable.

Claims

CLAIMS :

l. A fault voltage isolator which is arranged to interconnect two coaxial cables and which comprises: a central capacitor connected between oppositely projecting contact members which are arranged to connect electrically in line with core conductors of the respective coaxial cables, an insulating material support element supporting the central capacitor and the contact members, at least one capacitor located radially outwardly of the central capacitor, a casing containing the support element and the outer capacitor(s) , and means located within the casing for effecting electrical connection between the outer capacitor(s) and shielding conductors of the respective coaxial cables.

2. The fault voltage isolator as claimed in claim 1 wherein the means for effecting electrical connection between the outer capacitor(s) and the shielding conductors includes a tubular support member to which the

(or each) outer capacitor is mounted.

3. The fault voltage isolator as claimed in claim 2 wherein the tubular support member is formed on its outer surface with spaced-apart conductive material claddings and wherein the (or each) outer capacitor is mounted to the tubular support member in a manner to bridge the spaced-apart claddings.

4. The fault voltage isolator as claimed in claim 2 or claim 3 wherein the tubular support member is formed as a flexible printed circuit board which is rolled into a hollow cylinder.

5. The fault voltage isolator as claimed in any one of the preceding claims wherein the casing comprises a multi-part casing and wherein two of the casing parts comprise metal end portions which are arranged to carrying end caps which, in use, are employed to connect the shielding conductors of the respective coaxial cables to the end portions of the casing.

6. The fault voltage isolator as claimed in claim 5 wherein the casing end portions are interconnected by an insulating material sleeve.

7. The fault voltage isolator as claimed in claim 6 wherein the insulating material sleeve is formed from an elastomeric material of a type which can be activated thermally to shrink onto and so interconnect the casing parts.

8. The fault voltage isolator as claimed in any one of the preceding claims wherein a locating element is interposed between the end portions of the casing and wherein the locating element is arranged to preclude relative rotation of the end portions of the casing.

9. The fault voltage isolator as claimed in claim 8 and including two said outer capacitors, and wherein the locating element is employed for separating the outer capacitors in a peripheral direction.

10. The fault voltage isolator as claimed in any one of the preceding claims wherein the contact members comprise tubular sockets.

11. The fault voltage isolator as claimed in any one of the preceding claims wherein the respective contact members are connected directly and electrically to opposite end caps of the central capacitor.

12. The fault voltage isolator as claimed in any one of the preceding claims wherein each capacitor comprises a chip capacitor.

13. The fault voltage isolator as claimed in claim 5 and either of claim 3 or claim 4 wherein the spaced apart conductive material claddings engage with an internal wall portion of respective ones of the end portions of the casing.

14. The fault voltage isolator substantially as shown in the accompanying drawings and substantially as hereinbefore described with reference thereto.