GB2258353A

GB2258353A - Excess current protection in coaxial cable

Info

Publication number: GB2258353A
Application number: GB9116569A
Authority: GB
Inventors: Ian Braithwaite
Original assignee: TELEMATIC SYSTEMS Ltd
Current assignee: TELEMATIC SYSTEMS Ltd
Priority date: 1991-08-01
Filing date: 1991-08-01
Publication date: 1993-02-03
Anticipated expiration: 2011-08-01
Also published as: GB2258353B; GB9116569D0

Abstract

An excess current-protected coaxial cable has a circuit-breaking device (e.g. a fuse) and a low-pass filter (L1, L2, C) connected in its shield conductor. Alternatively, the circuit-breaker and filter may be ioined in series with terminals for connection between the shields of two coaxial cables. The inner conductor may also be fused, and a neon lamp may show whether the first fuse has blown. <IMAGE>

Description

Excess Current Protection in Coaxial Cable This invention relates to a circuit-breaking device associated with coaxial cable.

As shown in Figure 3 of the accompanying drawings, conventional fuses in coaxial cable are situated in series with the inner conductor, and protect against excessive current flowing into a device which would otherwise be destroyed by the accidental application of high levels of radio frequency power (e.g. where a radio transceiver is accidentally set to transmit while a signal generator is being used to test it in receive mode).

Systems such as computer networks and video links use coaxial cables for signal transmission. In some cases, the cables connect equipments in separate buildings which may be on different electricity supply earths. Under fault conditions, it is possible for the ends of the cable to be at a considerable potential difference, and in consequence, a heavy fault current may flow down the cable shield (also called the "outer" or "braid" conductor). As a safety measure, it is desirable to be able to break the flow of current, by means of a device such as a fuse or circuit breaker.

Inserting devices into the path of a coaxial cable shield is not straightforward. As figure 1 shows, the inductance of e.g. a fuse renders the system susceptible to external interference. In this figure, E is the emf, and r is the source impedance of an external interfering signal. v is the interfering voltage appearing across a receiving device or load, superimposed on the wanted signal, and z is the impedance of the device placed in series with the shield. For real devices, z can be large enough to upset the system operation, since interfering radio frequency field strengths can be appreciable.

The invention is intended to overcome this problem by providing an excess current-protected coaxial cable whose shield conductor path has both a circuit-breaking device and a low-pass filter. The susceptibility of the circuit-breaking device to external interference is reduced to acceptable levels by the filter, and we have found that the resulting cable is adequate for the successful transmission of computer network signals in normal environments.

Preferably, the low-pass filter comprises a series inductance loop constituted by at least one turn of the cable itself. The filter preferably also has a shunt capacitance connected between the shield of the cable and, for example, a surrounding conductive enclosure.

Advantageously, a further circuit-breaking device is provided in the cable's inner conductor path.

The or each circuit-breaking device can be a fuse-link or a thermally- or magnetically- operated circuit-breaker, for example, but any device responsive to a threshold current level being exceeded would be satisfactory.

Fault conditions could be signalled visually by an indicator connected as a shunt between. the cable shield and, for example, the enclosure where this is provided.

The invention also consists in a current-protection device for in-line connection between coaxial cables, comprising a low-pass filter and circuit-breaking means in series and connected between terminals for connection to the shield conductors of coaxial cables, and a signal path between terminals for connection to the inner conductors of the coaxial cables.

An example of the invention will now be described with reference to Figure 2 of the accompanying drawings, in which: Figures la and lb show schematically a fuse-link inserted in the shield of a coaxial cable, and the -equivalent circuit, respectively; Figures 2a and 2b show schematically a protected coaxial cable embodying the invention, and its equivalent circuit, respectively; and Figure 3 shows a conventional fused coaxial cable.

It is possible to reduce the level of any external interfering signal appearing at the circuit-breaking device by placing a low-pass filter in the braid circuit, as shown in Figure 2.

The filter shown in Figure 2 consists of two inductors and a capacitor. The inductors are formed by winding several turns of the coaxial cable itself around a high permeability core such as a ferrite toroid or rod.

The capacitor should be of a type capable of withstanding fault (e.g. mains) voltages safely (such types are generally known as Y-rated capacitors). The precise filter configuration is not of vital importance, and greater rejection of interfering signals can be obtained by adding further sections of series inductance and shunt capacitance.

Note that the low-pass filter does not affect the wanted signals passing down the coaxial cable since, for these, the inner and outer conductor currents are equal and opposite and hence generate no additional inductance, and also the potential between inner and outer is unaffected by the presence of the capacitor.

Figure 2 shows the essential feature of the device as employed in the prototypes. These are housed in a metal enclosure which affords a degree of screening. The coaxial connector which connects to the part of the system on-the remote earth (and from which the hazard comes), is isolated from the enclosure, whereas the output connector, which feeds the part of the system using the local earth installation, is connected directly to the enclosure. A fuse link and low-pass filter are placed in the shield circuit. Other forms of circuit-breaking device, such as thermally- or magnetically- operated circuit breakers could, in principle, be used. A fuse link is also used in the coaxial inner circuit, although this is not essential.

Claims

1. An excess current-protected coaxial cable whose shield conductor path has both a circuit-breaking device and a low-pass filter.

2. A cable according to Claim 1, in which the low-pass filter comprises a series inductance loop constituted by at least one turn of the cable itself.

3. A cable according to Claim 2, in which the filter also has a shunt capacitance connected between the shield of the cable and an external conductor.

4. A cable according to Claim 1, 2 or 3, in which a further circuit-breaking device is provided in the cable's inner conductor path.

5. A cable according to any preceding Claim, in which the or each circuit-breaking devices is responsive to a threshold current level being exceeded.

6. A cable according to Claim 5, in which the or each circuit-breaking device is a fuse-link or a thermally- or magnetically- operated circuit-breaker.

7. A cable according to any preceding Claim, comprising means for signalling faulty conditions visually by an indicator connected as a shunt between the cable shield and an external conductor.

8. A current protection device for in-line connection between coaxial cables, comprising a low-pass filter and circuit-breaking means in series and connected between terminals for connection to the shield conductors of coaxial cables, and a signal path between terminals for connection to the inner conductors of the coaxial cables.

9. An excess current protected coaxial cable substantially as described herein with reference to Figure 2 of the accompanying drawings.