GB2080010A

GB2080010A - High Tension Ignition Cable

Info

Publication number: GB2080010A
Application number: GB8118355A
Authority: GB
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1980-06-19
Filing date: 1981-06-15
Publication date: 1982-01-27
Also published as: GB2080010B

Abstract

A flameproof cable 2 has insulated wires 22, 23 that extend from a gas-tight connector 4, each wire having a braided screening sleeve 26, 27 the sleeves being in electrical contact with one another within the cable. A flexible insulative sleeve 21 insulates the sleeves 26 and 27 from a flexible, conductive outer conduit 20. The cable 2 is included in an ignition system by joining the connector 4 to an ignition unit (3, Fig. 1 not shown) in which the outer conduit 20, the screening sleeves 26 and 27 and one of the wires 23 are connected to a common earth (31). The other end of the cable 2 is joined with an igniter (1), the outer housing 2 of which is connected with the outer conduit 20 of the cable and is earthed. The electrodes (10, 11) of the igniter are connected to respective wires 22 and 23. <IMAGE>

Description

SPECIFICATION Cable Arrangements This invention relates to cable arrangements and to ignition systems including such arrangements.

The invention is more especially concerned with cable arrangements for use in ignition systems or other applications involving relatively high voltage.

Conventional cables for ignition systems, include two insulated conductors extending within an outer flexible metal conduit that is in turn insulated on its inner surface. The two conductors extend from the ignition unit to the igniter electrodes, the outer conduit being earthed.

It has been found that, if the outer conduit becomes disconnected from earth, the full voltage of the system can be induced in the conduit, even though it is electrically isolated from the voltage source in the ignition unit. This induced voltage is caused by the very fast voltage front produced at the moment of igniter discharge, and the high impedance of the conduit at that frequency. With a high tension ignition system the induced voltage can be as high as 1OKV; with a high energy system the induced voltage can be as high as 2KV.

Voltage can also be induced in the conduit even when it is securely connected to earth at both ends. In this case the voltage is highest midway along the cable and, in a long cable, can rise as high as about 1 OOV.

These induced voltages can obviously be a disadvantage, especially where the cable is liabie to be exposed along its length to a flammable or explosive atmosphere, since arcing and ignition of the atmosphere might occur.

It is an object of the present invention to provide a cable arrangement and an ignition system including such an arrangement that may be used to overcome the above-mentioned disadvantage.

According to one aspect of the present invention there is provided a cable arrangement comprising: an electrically-conductive outer conduit; a plurality of insulated conductor elements extending within said conduit along its length; and electrically-conductivescreening sleeves extending about respective conductor elements, said sleeves being in electrical contact with one another along substantially their entire length.

According to another aspect of the present invention there is provided an ignition system comprising an ignition unit, an igniter, and a cable arrangement extending between said ignition unit and said igniter, wherein said cable arrangement includes an electrically-conductive outer conduit; a plurality of insulated conductor elements extending within said conduit from the electrodes of said igniter to said ignition unit; and electrically-conductive screening sleeves extending about respective conductor elements, said sleeves being in electrical contact with one another along substantially their entire length.

The screening sleeves may be electrically insulated along their length from said outer conduit. The screening sleeves may be of braided wire and the outer conduit may be flexible.

The outer conduit may be earthed at said ignition unit and at said igniter. One of said conductor elements may be earthed at said ignition unit and at said igniter. The electricallyconductive screening sleeves may be earthed at said ignition unit at a common point with said outer conduit and one of said conductor elements.

A cable arrangement and an ignition system including such a cable arrangement, both in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic view along the length of the cable arrangement showing the ignition system; Figure 2 is a cross-sectional view, to an enlarged scale, along the line Il-Il of Figure 1 and Figure 3 is a cross-sectional view, to an enlarged scale of a part of Figure 1.

The ignition system includes an igniter 1 that is electrically connected by means of a cable arrangement 2 to an ignition unit 3. The system is of a flameproof construction, that is, it is arranged such that electrical discharge is confined to the region at the tip of the igniter 1, between the igniter electrodes 10 and 11.

The igniter 1 may be of a well-known type such as an air-gap spark igniter (as illustrated) for use with a high-tension ignition unit, or a surfacedischarge igniter for use with a high-energy ignition unit. The tip of the igniter 1 projects from the end of a flameproof metal casing 12 that is sealed with the igniter such as to prevent a flame path to the rear of the igniter. The casing 12, and hence the outer electrode 11 of the igniter 1, are connected to earth.

The cable arrangement 2 has a flexible outer conduit 20 that is connected to the rear of the casing 1 2. The conduit 20 is electricallyconductive being formed, for example, of a braided wire or, a corrugated metal tube. A corrugated insulating sleeve 21 extends within the conduit 20 and may be of any well-known material, such as, for example, PTFE, suitable for the particular environment within which the system is to be used. Within the insulating sleeve 21 extend two electrically-conductive wires 22 and 23 which are connected at their forward ends to the electrodes 10 and 11 respectively of the igniter 1. Both wires 22 and 23 have an insulative coating 24 and 25, such as, for example, of a plastics material, and extend within respective electrically-conductive sleeves 26 and 27.The sleeves 26 and 27 are braided of tinned copper wire and extend rearwardly from a point (not shown) just to the rear of the forward end of the wires 22 and 23 (close to their connection with the electrodes 10 and 11) out of the rear end of the conduit 20. The sleeves 26 and 27 contact, and are therefore in electrical connection with, each other along their length within the conduit 20.

The cable arrangement 2 is connected at its rear end to the ignition unit 3 by means of a connector 4. The ignition unit 3 contains an electrical source 30 which may be of the hightension or high-energy kind. One terminal of the source 30 is connected to a common earth point 31 whereas the other terminal is connected to the wire 22 that makes contact with the central electrode 10 of the igniter. The ignition unit 3 has an outer flameproof housing 32 that is also connected to earth at the common earth point 31, together with both the braided sleeves 26 and 27 and the cable wire 23 that makes contact with the outer electrode 11 of the igniter 1.

Both wires 22 and 23, and their braided sleeves 26 and 27 are enclosed, over a short distance at their rear ends, by heat shrunk electrically-insulative sleeves 28 and 29. The insulative sleeves 28 and 29 extend through the connector 4 and serve to prevent the braided sleeves 26 and 27 making electrical contact with conductive components of the connector.

The connector 4 is of standard form such as a Hawke Conduit Stopper Box. The connector 4 has an elastomeric bung 40 with two apertures 41 and 42 through which project the wires 22 and 23. The bung 40 sits within a rear brass housing 43 one end 44 of which is screwed ihto the ignition unit 3. The forward edge of the bung 40 is engaged by the rear end 45 of a forward housing 46. The forward housing 46 and the rear housing 43 are urged together by means of a rotatable nut 47 screw-threaded with the rear housing 43, thereby axially compressing the bung 40. This axial compression causes the bung 40 to expand radially, closing the apertures 41 and 42 tightly about the insulative sleeves 28 and 29, and firmly contacting the rear housing. In this way, a fluidtight seal is formed between the wires 22 and 23, and the ignition unit 3.

By having both wires 22 and 23 individually screened within the outer conduit 20, induced voltages on the outer surface of the conduit 20 can be substantially eliminated.

It will be appreciated that the cable arrangement and ignition system could be modified in various ways. The screening of the wires 22 and 23 might, for example, be accomplished by means of a foil sleeve rather than braiding. The two braided wires 22 and 23 could be twisted about one another along the length of the cable. In this respect, the braided wires 22 and 23 could be twisted together within a solid cylinder of insulative material, this might be done, for example, by extrusion. The igniter 1 and ignition unit 3 could also be of any suitable well-known form.

The cable arrangement 2 could be used in applications other than ignition systems.

Claims

1. A cable arrangement comprising: an electrically-conductive outer conduit; a plurality of insulated conductor elements extending within said conduit along its length; and electricallyconductive screening sleeves extending about respective conductor elements, said sleeves being in electrical contact with one another along substantially their entire length.

2. A cable arrangement according to Claim 1, wherein said screening sleeves are electrically insulated along their length from said outer conduit.

3. A cable arrangement according to Claim 2, wherein said screening sleeves are electrically insulated from said outer conduit by an electrically-insulative sleeve that extends within said conduit around said conductor elements.

4. A cable arrangement according to any one of the preceding claims, wherein said electricallyconductive screening sleeves are of braided wire.

5. A cable arrangement according to any one of the preceding claims, wherein said outer conduit is substantially flexible.

6. An ignition system comprising: an ignition unit, an igniter, and a cable arrangement extending between said ignition unit and said igniter, wherein said cable arrangement includes an electrically-conductive outer conduit; a plurality of insulated conductor elements extending within said conduit from the electrodes of said igniter to said ignition unit; and electrically-conductive screening sleeves extending about respective conductor elements, said sleeves being in electrical contact with one another along substantially their entire length.

7. An ignition system according to Claim 6, wherein said outer conduit is earthed at said ignition unit.

8. An ignition system according to Claim 6 or 7, wherein said outer conduit is earthed at said igniter.

9. An ignition system according to any one of Claims 6 to 8, wherein one of said conductor elements is earthed at said ignition unit and at said igniter.

10. An ignition system according to any one of Claims 6 to 9, wherein said electricallyconductive screening sleeves are earthed at said ignition unit.

11. An ignition system according to any one of Claims 6 to 10, wherein said electricallyconductive screening sleeves are earthed at said igniter.

12. An ignition system according to Claim 6, wherein said outer conduit, said electricallyconductive screening sleeves and one of said conductor elements are earthed at a common point at said ignition unit.

13. A cable arrangement substantially as hereinbefore described with reference to the accompanying drawings.

1 4. An ignition system including a cable arrangement according to any one of Claims 1 to 5 or Claim 13.

1 5. An ignition system substantially as hereinbefore described with reference to the accompanying drawings.