GB2344239A

GB2344239A - Pulse-echo location of faults in electric cables

Info

Publication number: GB2344239A
Application number: GB9825581A
Authority: GB
Inventors: Alexander Joseph Binnie; Graeme J Mitchell
Original assignee: Edgcumbe Instruments Ltd
Current assignee: Spirent Systems Ltd
Priority date: 1998-11-24
Filing date: 1998-11-24
Publication date: 2000-05-31
Also published as: GB9825581D0

Abstract

The gain of an amplifier (14) is increased progressively with time in order to compensate for the decreasing strength of a pulse reflected from an impedance discontinuity in the transmission line. A pulse is generated (10) and transmitted along a wire, the total time of flight being indicative of the distance to the fault. Reflections from nearby features such as cable joints are subjected to a low level of amplification, whilst distant faults produce more noticeable reflection pulses in the amplifier output. The amplifier may be a voltage controlled hybrid transformer and connected to a balance network and a sawtooth generator.

Description

Location of faults in electric cables The present invention relates to a method and apparatus for locating faults in electric cables, particularly but not solely telecommunication cables.

One apparatus which is used for locating faults in electric cables comprises a time domain reflectometer. In use, a pulse is transmitted along the cable from an end of the cable or from some other convenient access point, and a return pulse (reflected from a fault) is detected: the time interval between the transmission of the pulse and the arrival of the reflected pulse is proportional to the distance to the fault; providing the velocity at which the pulse is propagated along the cable is known, the distance to the fault can be determined. The fault will consist of a discontinuity in the cable, generally either an open circuit or a closed circuit.

There are, however, a number of problems related to the use of time domain reflectometry to locate faults in electric cables. These problems arise because the transmission characteristics of the cable alter with the signal frequency.

In particular, the attenuation and signal delay alter with the signal frequency and with the length over which the signal is transmitted.

In general, the pulse which is transmitted may be rectangular, half-sine or half-sine squared. However, each of these pulses has a frequency spectrum which includes harmonics and sub-harmonics of a principal frequency (corresponding to the period of the transmitted pulse). owing to the harmonics and sub-harmonics, the reflected pulse is distorted because the harmonics and sub-harmonics are attenuated and delayed by different amounts compared to the principal frequency.

In the case of cable faults at a distance of greater than about 2km, the reflected pulse is severely attenuated: in order to offset this, the reflected pulse is amplified.

Generally also, as a compensation, the period of the transmitted pulse is increased with the range, because the attenuation/km is greater the higher the frequency. The amplifier gain required, for locating faults at a far distance, is up to 100dB : such very high gain has the side effect of amplifying reflections from close-in features such as cable joints, which are not faults but which give rise to a minor reflection, this minor reflection becoming visible on the trace displayed by the time domain reflectometer when the amplifier gain is high. In addition, some time domain reflectometers incorporate a balance control which eliminates the transmitted pulse from the displayed trace, so that close-in faults will be detected. The balance control can never provide a perfect balance, so that the residual imbalance becomes visible when the gain is increased and thereby obscures the start of the trace. This problem is overcome if the operator uses the correct procedure.

Thus, the correct procedure for locating cable faults involves initially setting the amplifier gain at a low value and then checking for close-in faults. If none are detected, the amplifier gain is increased and faults further out are checked for. The procedure is repeated, increasing the amplifier gain in successive steps and checking for faults in increasingly far-out ranges. At very high or maximum settings of the amplifier gain, the displayed trace will include a first section of waveform which oscillates between the top and bottom of the display, as shown in Figure 1 in which a reflected fault pulse is indicated at FP.

We have now devised a method and apparatus for locating faults in electric cables, which simplifies the procedure which has just been described.

In accordance with the present invention, there is provided an apparatus for locating any fault in an electric cable, the apparatus comprising means for generating a pulse and transmitting the pulse to the cable for propagation therealong, and an amplifier for receiving return reflections of the transmitted pulse, the amplifier being arranged so that its gain increases progressively over a period of time following transmission of said pulse.

In use of this apparatus, the reflections from near features such as cable joints are subjected to a low level of amplification and appear only to a negligible extent in the output signal of the amplifier. However, the gain of the amplifier is progressively increased, so that faults at progressively greater distances will produce noticeable reflection pulses in the amplifier output.

Preferably the amplifier is a voltage controlled amplifier, the control input of which receives the output of a sawtooth or ramp generator. Preferably a control means is provided, which generates a start signal for application to the pulse generator and sawtooth generator simultaneously.

Preferably the amplitude of the sawtooth waveform is adjustable, to allow the gain/micro-second to be varied.

Preferably the output of the pulse generator is connected to one node of a hybrid transformer or electrical equivalent, having a second node for connection to a cable in which a fault is to be located, and a third node connected to the amplifier input. Preferably the apparatus also comprises a balance network connected to a fourth node of the hybrid transformer.

Also in accordance with the present invention, there is provided a method of locating any fault in an electric cable, the method comprising generating a pulse and transmitting said pulse into the electric cable for propagation therealong, and passing return reflections of the transmitted pulse to an amplifier, the amplifier being arranged so that its gain increases progressively over a period following transmission of said pulse into the cable.

Am embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: FIGURE 1 is a schematic representation of a typical trace formed on the display of a prior art time domain reflectometer when checking for far-distant faults with very high amplifier gain; FIGURE 2 is a block diagram of a time domain reflectometer in accordance with the present invention; and FIGURE 3 shows a set of waveforms related to the reflectometer of Figure 2.

Referring to Figure 2 of the drawings, an apparatus in accordance with the present invention comprises a pulse generator 10 having its output connected to an input node of a hybrid transformer 12, an adjacent node of which is connected to an electric cable such as a telephone cable, in which a fault is to be detected. An output node of the hybrid transformer 12 is connected via a voltage controlled amplifier 14 to a visual display (not shown). The remaining node of the hybrid transformer 12 is connected to a balance network 16.

A sawtooth or ramp generator 18 is provided and its output is used to control the gain of the amplifier 14. The pulse generator 10 and sawtooth generator 18 have respective inputs to receive a start signal simultaneously.

In use, a control logic of the apparatus is initiated to generate a start signal to the pulse generator 10 and sawtooth generator 18 simultaneously. In Figure 3, the start signal pulse is shown in waveform A, the resulting outputs from the pulse generator 10 and sawtooth generator 18 are shown in waveforms B and C respectively. The ramp signal from the sawtooth generator 18 is applied to the amplifier 14: the logarithmic gain of the amplifier is proportional to its control voltage, such that the amplifier gain in dB increases linearly, as shown by waveform D in Figure 3. The output pulse from the pulse generator 10 is applied to the electric cable via the hybrid transformer and is also passed to the amplifier.

The reflected signals from the cable are also passed via the hybrid transformer to the amplifier. The output of the amplifier is displayed as a trace on a visual display of the apparatus.

It will be appreciated that the apparatus provides low amplification for close-in faults and greater amplification for distant faults. A typical input signal to the amplifier is shown in Figure 3E, whilst the corresponding output signal (which is formed as a trace on the display) is shown in Figure 3F. In particular, the transmitted pulse and minor reflections from features such as cable joints will hardly appear in the displayed trace: the fault itself (indicated at FP) is more easily identified in the relatively uncluttered trace. The waveforms shown at E and F in Figure 3 are idealised but nevertheless illustrate the improvements which are achieved by the apparatus in accordance with the present invention.

Claims

Claims 1) An apparatus for locating a fault in an electric cable, the apparatus comprising means for generating a pulse and transmitting the pulse to the cable for propagation therealong, and an amplifier for receiving return reflections of the transmitted pulse, the amplifier being arranged so that its gain increases progressively over a period of time following transmission of said pulse.
2) An apparatus as claimed in Claim 1, wherein the amplifier comprises a voltage controlled amplifier, the control input of which receives the output of a sawtooth or ramp generator.
3) An apparatus as claimed in Claim 2, comprising a control means which generates a start signal for simultaneous application to the pulse generator and to the sawtooth or ramp generator.
4) An apparatus as claimed in Claim 2, wherein the amplitude of the sawtooth or ramp waveform is adjustable, to allow the gain per micro-second to be varied.
5) An apparatus as claimed in any preceding claim, wherein the output of the pulse generator is connected to one node of a hybrid transformer or electrical equivalent, having a second node for connection to a cable in which a fault is to be located, and a third node connected to the amplifier input.
6) An apparatus as claimed in Claim 5, comprising a balance network connected to a fourth node of the hybrid transformer.
7) An fault locating apparatus substantially as herein described with reference to the accompanying drawings.
8) A method of locating a fault in an electric cable, the method comprising generating a pulse and transmitting said pulse into the electric cable for propagation therealong, and passing return reflections of the transmitted pulse to an amplifier, the amplifier being arranged so that its gain increases progressively over a period following transmission of said pulse into the cable.
9) A fault locating method substantially as herein described with reference to the accompanying drawings.