GB1572154A

GB1572154A - Electrical continuity fault location

Info

Publication number: GB1572154A
Application number: GB50917/75A
Authority: GB
Original assignee: Post Office
Current assignee: Post Office
Priority date: 1976-12-13
Filing date: 1976-12-13
Publication date: 1980-07-23

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRICAL CONTINUITY FAULT LOCATION (71) We, THE POST OFFICE, a British corporation established by Statute, of 23 Howland Street, London W1P 6HQ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the isolation of electrical continuity faults in a metallic conducting layer adhered to the inner surface of an outer sheath of a conductor carrying cable. The invention has particular application to the isolation of electrical continuity faults in moisture barriers of conductor carrying cables. Various types of cable are used to interconnect telephone exchanges and repeater stations and an increasing number of these are polyethylene sheathed. Since polyethylene is pervious to water, an aluminium foil is bonded to the inner side of the cable sheath. This foil which is known as the moisture barrier has three functions: (a) It protects the cable core from osmosis which occurs even when the cable is pressurised.

(b) It screens the conductors which prevents noise being induced into them from external sources.

(c) It extends the exchange earth along the cable to pneumatic contactors which are used in the cable pressurisation alarm system.

In order to provide (b) and (c) it is necessary that the moisture barrier should be electrically continuous throughout its entire length and should be earthed at each exchange.

Periodically therefore the moisture barrier continuity should be checked and any electrical discontinuities or high resistance joints should be isolated and repaired. In this specification the term "electrical discontinuity" covers not only breaks in a conductor but also portions of a conductor which are electrically continuous but have high resistance discontinuities.

According to the present invention there is provided a method of locating an electrical discontinuity in a metallic conducting layer adhered to the inner surface of an outer sheath of a conductor carrying cable, said method comprising applying a low frequency signal to the conducting layer, detecting at spaced locations the signal in the conducting layer by means of a detector which is turned to the frequency of the low frequency signal and which is disposed close to the cable sheath, and comparing the de tected signals, the frequency of said signal being sufficiently low that if an electrical discontinuity exists between the spaced loca-tions any coupling of the low frequency signal through the conductors within the cable does not nullify a difference in the compared signals.

The method may include, prior to said detecting step, approximately determining the distance of the discontinuity from the point of application of the low frequency signal by applying the oscillatory signal to said moisture barrier through a series resistance of known value, measuring the voltage drop V across the resistance and approximating the distance from the value V.

The detector may include a capacitive pick-up element which is located on the cable.

The signal may have a frequency of the order of 3Hz.

The step of applying the low frequency signal may be carried out using generating means for generating a low frequency signal and the detecting step may be carried out using a detector for detecting said low frequency signal.

The generator may include an oscillator and an amplifying stage for feeding the oscillatory signals to an output terminal by way of a series resistance. The oscillator may be a phase-shift oscillator.

The output of the generator may be applied to a bridge rectifier and a meter may be connected to said bridge rectifier for indicating the level of the output of the generator.

The signal detector may comprise a pickup element adapted to fit onto the sheath of the cable, an amplifying stage coupled to said pick-up element and turned to the frequency of the signal generator, and indicating means connected to the output of said amplifier to indicate the level of the detected signal. The amplifying stage may have a feedback network comprising a bridge Tfilter.

The invention will be described now by way of example only with particular reference to the accompanying drawings. In the drawings: Figure 1 is a circuit diagram of a low frequency generator and measuring circuit; Figure 2 is a circuit diagram of a low frequency signal detector; Figure 3 is a side elevation of a detector containing the circuit of Figure 2; Figure 4 is an elevation of the detector of Figure 3, and Figure 5 shows a moisture barrier at a cable joint being tested for an electrical discontinuity.

The circuit of Figure 1 has an oscillating stage with an operational amplifier 10 connected as a conventional phase-shift oscillator. The oscillator 10 is arranged to generate a 3Hz oscillating signal. The output of the oscillator 10 is connected to the input of an amplifying stage which includes an operational amplifier 11.

The output from the amplifier 11 is developed across a series resistor 12 which is uesd as a load sensing device, the potential across the resistor 12 increasing as the current increases. Typically the resistor 12 has a value of 100Q. One end of the resistor 12 is connected to a socket 14. By connecting an appropriate connector to the socket 14 the output of the amplifier 11 can be applied to a moisture barrier which is to be tested for discontinuities.

The output of the amplifier 11 is also fed to a rectifying bridge 16 which rectifies the 3Hz signal. The rectifying bridge 16 is connected via a switch 17, having movable contacts 18 and 19, to a meter 20 which is connected in series with a resistor 22. The switch 17 is a push button switch and its contacts are normally in the position shown by the full line in the drawing. In this condition the meter 20 provides an indication of the output voltage fed to the moisture bar rier via the socket 14.

The resistor 12 is connected to an amplifier 24 which comprises three operational amplifiers 25, 26 and 27. The amplifier 24 is arranged to amplify the voltage drop across the resistor 12 and the three operational amplifiers 25 to 27 are required as the voltage drop is quite small. The output of the operational amplifier 27 is connected via a diode 30 to a contact 31 of the switch 17. When the movable contacts of the switch 17 are in their normal position (shown by the full line in Figure 1) the output of the amplifier 24 is not connected to the meter 20. If the push button of the switch 17 is depressed the movable contacts move to the position shown by dotted lines and the output from the amplifier 24 is fed to the meter 20. Thus an indication of the voltage drop across the resistor 12 is given by the meter 20.

Power for circuit is provided by an invertor power supply indicated at 34. The power supply 34 has three terminals indicated E, + 50V, and + 24V. The power supply can be operated from either a 50 volt source or a 24 volt source.

Referring now to Figure 2 a signal deteo tor has a capacitative pick-up element 40 which is arranged to be mounted on the sheath of a cable whose moisture barrier is to be tested. The capacitative pick-up element 40 is connected to the input of a FET operational amplifier 42. The amplifier 42 has a very high input impedance to match the hiph impedance between the element 40 and the moisture barrier. The amplifier 42 has variable feedback provided by the variable resistor 44.

The output of the amplifier 42 is connected via a variable resistor 45 to the input of an operational amplifier 47. The resistor 45 can be used to control the gain of the signal detector. The amplifier 47 has a bridge T-filter feedback network 48 to provide frequency discrimination. The network 48 is tuned to give the amplifier 47 maximum gain at 3Hz. The output of the amplifier 47 is connected via a diode 50, a resistor 51 and a push button switch 52 to a meter 54. Normally the movable contacts of the switch 52 are in the position shown in the drawing. If the push button of the switch 52 is depressed the meter 54 is connected in series with a resistor 55 and a series of batteries 56 which provide power for the circuit. This provides a test of the operation of the batteries 56.

The output signal from the amplifier 47 is half-waved rectified by the diode 50. A capacitor 58 is provided to smooth the rectified signal and the smoothed signal is fed through the meter 54.

The meter and power supply part of the circuit of Figure 2 indicated within the dotted line 60 is contained within a separate housing from the main part of the detector.

The housing for the detector circuit is illustrated in Figures 3 and 4. The pick-up element 40 which is in the form of an inverted V-shaped channel is carried at the base of the housing so that it be placed on a cable to be tested. A plug jack 62 is provided to connect the detector to the housing containing the meter 54 and power supply 56. The plug jack is also shown schematically at 62 in Figure 2.

The detector shown in Figures 3 and 4 is designed so that it is readily portable. The generator of Figure 1 and the meter 54 and power supply 56 of Figure 2 are also contained within housings that are portable.

To isolate a discontinuity, the output terminals of the Hz signal generator at the socket 14 are connected between a cable moisture barrier 65 and earth as shown in Figure 5. The reading of the meter 20 is noted. The push button switch 17 is then depressed so that the meter 20 registers the signal across the resistor 12. From this reading it should be possible to determine the approximate distance of the fault from the point of application of the 3Hz signal, usually an exchange, since the further away the fault is from the application point the greater is the capacitance to earth of the moisture barrier. The load on the generator and the current drawn increases with increasing capacitance so that the further away the fault the greater the potential drop across the resistor 12 and the greater the reading on the meter 20. It should be noted that in many cases the capacitance to earth of the moisture barrier up to the fault position depends upon several factors in addition to the length of cable to the fault, e.g. the environmental conditions existing in the cable duct route. Accordingly the effect of these conditions should be considered in estimating the position of a fault. Furthermore if the discontinuity is merely a high resistance and not a break in the conductive path the approximation cannot be made.

Following the approximate location of a discontinuity to a section of cable (which may consist of several cable lengths) a more positive location is made using the detector of Figures 2, 3 and 4. The detector is placed over the cable at the mid-point of the faulty section so that the pick-up element 40 rests on the surface of the cable. A reading of the meter 54 is made and depending on the level of this signal it can be concluded that the fault lies in one or other of the half lengths of cable between the generator and the end of the faultv section. By successively carrying out similar measurements over successively smaller lengths of cable and comparing the signals the location of the fault can be isolated to a particular length of cable. Invariably the discontinuity is at a cable joint. The continuity at a cable joint is then tested by comparing the signals obtained with the dectector placed on either side of the joint as shown in Figure 5.

When using the detector, fault location is easier if the moisture barrier at the remote exchange is earthed and a number of cable pairs also have an earth potential on them.

This prevents the 3Hz test signal from coupling through the pairs to the other side of fault moisture barrier joint. In addition it is advisable to suspend the cable where it rests on a bearer as the signal detected may be reduced by capacitive coupling to earth.

The generator and detector are designed to operate at 3Hz. Other frequencies can be used but the frequency of the applied signal should be sufficiently low that if an electrical discontinuity exists between any of the detector locations, any coupling of the low frequency signal through the conductors within the cable does not nullify a difference in the compared signals.

WHAT WE CLAIM IS : - 1. A method of locating an electrical discontinuity in a metallic conducting layer adhered to the inner surface of an outer sheath of a conductor carrying cable, said method comprising applying a low frequency signal to the conducting layer, detecting at spaced locations the signal in the conducting layer by means of a detector which is tuned to the frequency of the low frequency signal and which is disposed close to the cable sheath, and comparing the detected signals, the frequency of said signals being sufficiently low that if an electrical discontinuity exists between the spaced locations any coupling of the low frequency signal through the conductors within the cable does not nullify a difference in the compared signals.

2. A method as claimed in claim 1 including, prior to said detecting step, approximately determining the distance of the discontinuity from the point of application of the low frequency signal by applying the oscillatory signal to said moisture barrier through a series resistance of known value, measuring the voltage drop V across the resistance, and approximating the distance from the value V.

3. A method as claimed in claim I or claim 2, wherein said detector includes a capacitive pick-up element which is located on the cable.

4. A method as claimed in any preceding claim, wherein said signal has a frequency of the order of 3Hz.

5. A method as claimed in any preceding claim, wherein said step of applying the low frequency signal is carried out using generating means for generating a low frequency signal and said detecting step is carried out using a detector for detecting said low frequency signal.

6. A method as claimed in claim 5, wherein said generator includes an oscillator, and an amplifying stage for feeding the oscillatory signals to an output terminal by way of a series resistance.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. connect the detector to the housing containing the meter 54 and power supply 56. The plug jack is also shown schematically at 62 in Figure 2. The detector shown in Figures 3 and 4 is designed so that it is readily portable. The generator of Figure 1 and the meter 54 and power supply 56 of Figure 2 are also contained within housings that are portable. To isolate a discontinuity, the output terminals of the Hz signal generator at the socket 14 are connected between a cable moisture barrier 65 and earth as shown in Figure 5. The reading of the meter 20 is noted. The push button switch 17 is then depressed so that the meter 20 registers the signal across the resistor 12. From this reading it should be possible to determine the approximate distance of the fault from the point of application of the 3Hz signal, usually an exchange, since the further away the fault is from the application point the greater is the capacitance to earth of the moisture barrier. The load on the generator and the current drawn increases with increasing capacitance so that the further away the fault the greater the potential drop across the resistor 12 and the greater the reading on the meter 20. It should be noted that in many cases the capacitance to earth of the moisture barrier up to the fault position depends upon several factors in addition to the length of cable to the fault, e.g. the environmental conditions existing in the cable duct route. Accordingly the effect of these conditions should be considered in estimating the position of a fault. Furthermore if the discontinuity is merely a high resistance and not a break in the conductive path the approximation cannot be made. Following the approximate location of a discontinuity to a section of cable (which may consist of several cable lengths) a more positive location is made using the detector of Figures 2, 3 and 4. The detector is placed over the cable at the mid-point of the faulty section so that the pick-up element 40 rests on the surface of the cable. A reading of the meter 54 is made and depending on the level of this signal it can be concluded that the fault lies in one or other of the half lengths of cable between the generator and the end of the faultv section. By successively carrying out similar measurements over successively smaller lengths of cable and comparing the signals the location of the fault can be isolated to a particular length of cable. Invariably the discontinuity is at a cable joint. The continuity at a cable joint is then tested by comparing the signals obtained with the dectector placed on either side of the joint as shown in Figure 5. When using the detector, fault location is easier if the moisture barrier at the remote exchange is earthed and a number of cable pairs also have an earth potential on them. This prevents the 3Hz test signal from coupling through the pairs to the other side of fault moisture barrier joint. In addition it is advisable to suspend the cable where it rests on a bearer as the signal detected may be reduced by capacitive coupling to earth. The generator and detector are designed to operate at 3Hz. Other frequencies can be used but the frequency of the applied signal should be sufficiently low that if an electrical discontinuity exists between any of the detector locations, any coupling of the low frequency signal through the conductors within the cable does not nullify a difference in the compared signals. WHAT WE CLAIM IS : -

1. A method of locating an electrical discontinuity in a metallic conducting layer adhered to the inner surface of an outer sheath of a conductor carrying cable, said method comprising applying a low frequency signal to the conducting layer, detecting at spaced locations the signal in the conducting layer by means of a detector which is tuned to the frequency of the low frequency signal and which is disposed close to the cable sheath, and comparing the detected signals, the frequency of said signals being sufficiently low that if an electrical discontinuity exists between the spaced locations any coupling of the low frequency signal through the conductors within the cable does not nullify a difference in the compared signals.

7. A method as claimed in claim 6,

wherein said oscillator is a phase-shift oscillator.

8. A method as claimed in claim 6 or claim 7, wherein the output of the generator is applied to a bridge rectifier and a meter connected to said bridge rectifier indicates the level of the output of the generator.

9. A method as claimed in any one of claims 5 to 8, wherein said signal detector comprises a pick-up element adapted to fit onto the sheath of the cable, an amplifying stage coupled to said pick-up element and tuned to the frequency of the signal generator, and indicating means connected to the output of said amplifier to indicate the level of the detected signal.

10. A method of locating an electrical discontinuity in a metallic conducting layer adhered to the inner surface of an outer sheath of a conductor cable as claimed in claim 1 and substantially as hereinbefore described.