GB1602877A

GB1602877A - Process and apparatus for locating a defect in an embedded electric conductor

Info

Publication number: GB1602877A
Application number: GB1676178A
Authority: GB
Original assignee: SOCOTEC SA
Current assignee: SOCOTEC SA
Priority date: 1977-04-28
Filing date: 1978-04-27
Publication date: 1981-11-18
Also published as: FR2389139B1; FR2389139A1

Description

(54) A PROCESS AND APPARATUS FOR LOCATING A DEFECT IN AN EMBEDDED ELECTRIC CONDUCTOR.

(71) We, SocoTEc S.A. of Paris, France, a body corporate organized according to the laws of France; BERNARD BILLARD of Clermont-Ferrand, Puy-de-dome, France; EMILE HURBE of Clermont-Ferrand, Puy-de-dome, France; JEAN-MARC DUFLOT of Limoges, Haute Vienne, France; and JEAN-CLAUDE ARNAUD of Pont du Chatean, Puy-de-dome, France, all French citizens, 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 state ment:- The present invention relates in general to measuring processes and apparatus and more particularly to a process and apparatus for locating defects in electrical heating cables which are embedded in concrete and intended for "integrated" heating.

Heating cables embedded in concrete floorings typically comprise an electrical conductor known as the core, an insulator surrounding the core, an optional conductor screen and an insulating sheath of plastics material such as PVC. Defects arise either when the concrete is laid, or during building works; these defects may result either from loss of insulation between the core and the screen and/or the earth, or from cutting of the core. Various methods can be employed for locating these defects within the concrete mass; for example infra-red cameras, time of travel measurements of an electromagnetic wave in the conductor, or analysis of the magnetic field developed around a conductor when the latter has an electric current flowing through it. The present invention relates particularly to this last method. In a known method, a periodic current produced by a generator is passed between earth and a first end of the defective conductor. A solenoid is used as a magnetic field pickup to detect any change, and more particular ly any diminution of the periodic magnetic field induced by the current flowing through the conductor. The solenoid is connected in an electric circuit which includes a voltmeter. In the neighbourhood of a defect, changes in the deflection of the pointer of the voltmeter are interpreted as an indication of the presence of a fault. This method is relatively imprecise since the deflection of the voltmeter pointer is differently affected depending on the nature of the defect. For example when the core is cut, the reading is differently disturbed according to whether the resistance between the core and the screen becomes larger or smaller. Furthermore, insulating defects, which are more or less straightforward, can only be approached up to a few metres, since, closer than that the flutuations in the indication of the pointer become confused with the indications produced by the defect itself. The detection is completed by a more precise secondary method which consists of "burning" the defect by injecting a high electrical voltage so as to trigger an arc within the concrete. The heating caused by the arc is then located this time more precisely. Unfortunately, this secondary method is not always applicable, for instance where there are pilecarpets or thermal insulators. Sometimes the defect is so straightforward providing a path of such low resistance that an arc cannot be triggered; it is also possible for the injection of a high voltage to trigger an arc and create a defect where one did not previously exist. Finally, this secondary method leads to dielectrical fatigue in the heating cables.

It is an object of the present invention to provide a method and apparatus for locating defects by detection of the magnetic field produced about a conductor, allowing direct and precise location of the defect without recourse to a secondary method such as that discussed hereinabove.

According to a first aspect of the present invention, there is provided a process for locating a defect in an electric conductor embedded in the ground, the process including the steps of: passing a periodic current through a first section of the conductor between an earth-contacting defect and a first end of the said conductor;passing a second periodic current through a second section of the said conductor between the defect and a second end of the said conductor, the said second current differing from the first current in its value of at least one of the following parameters: polarity, duty ratio, amplitude, phase and frequency; and detecting change in a magnetic field induced by the periodic currents.

In this specification, "ground" means earth or concrete flooring.

Preferably, in a first form of the process, the amplitude maxima of the second current.

coincide with the amplitude minima of the first current so that substantially opposite and complementary magnetic signals are induced around the conductor on either side of the defect. The intensity maxima of the first and second currents can be either of the same, or of opposite polarity.

Similarly, the second current may be established either between earth and the second end of the conductor, or between the first and second ends of the conductor.

Preferably, the first and second currents are signals in the form of square wave pulses.

According to this first form of the process, the defect is located for example with two probes placed perpendicularly to the conductor and close to each other.

The defect is likely to be in a region where a displacement of a probe changes the value of a predetermined parameter of each of the currents, this parameter being for instance dependent on the polarity or phase of the current in the conductor.

Unlike previously-known methods the method of the present invention is not based on detecting a gradual variation of a parameter, but on detecting a sudden change in the value of a parameter and, preferably, on comparision thereof with a corresponding parameter.

According to a second form of the process of the invention, the first and second currents are periodic currents passing simultaneously through the first and second sections of the conductor respectively. The frequencies of the first and second currents differ slightly from each other. In the vicinity of the defect this results in the phenomenon of beats in the magnetic fields produced by the currents. The defect is located by moving a probe which is provided with a pickup along the defective conductor and determining the position at which the beats resulting fron interference of the two magnetic fields of similar period are a maximum. This determination may be made by acoustic transmission of the audible beat of the magnetic field induced by the currents in the conductors.

According to a second aspect of the present invention there is also provided apparatus for locating a defect in an electrical conductor embedded in the ground, said device including two current generators in synchronism with each other, each generator supplying a periodic current signal which is of substantially square wave configuration and having means for controlling one or more of the parameters of the current; a three-channel output permitting connection of two outputs of each generator to two of three elements by a multi-position switch, the three elements being a first end of the conductor, a second end thereof and earth; detection means comprising at least one probe and having at its end at least one magnetic field pickup; a respective "low-pass" filter for selecting the fundamental periodic voltage induced in the or each pickup by the periodic magnetic field; and a three-beam oscilloscope for displaying the current signat of at least one of the generators and at least one respective voltage signal derived from the or a respective said pickup.

In a preferred form, the apparatus comprises a vehicle adapted to travel along a line; said detection means comprising two defect detecting magnetic field detectors mounted one in front of the other towards the rear of said vehicle; two piloting magnetic field detectors mounted on respective sides of the line of travel of the vehicle; means controlled by the piloting detectors for guiding the vehicle along the embedded electrical conductor; means controlled by the defect detecting detectors for stopping the vehicle when it is directly over a defect; and means for tracing the line followed by the vehicle and marking its stopping positions.

In processes or apparatuses embodying the present invention, the magnetic field pickups referred to hereinabove, may be solenoids or Hall probes, the latter having the advantage that they indicate the continuous component of the periodic magnetic field induced.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a heating cable of the "integated" heating type; Figure 2 is a diagrammetic representation of a first preferred form of a circuit for carrying out the process according to the invention; Figure 3 is a second preferred form of a circuit for carrying out the process according to the invention; Figure 4 is an oscillogram corresponding to the first circuit of Figure 2, one of the components having a first polarity; Figure 5 is an oscillgram from the same circuit arrangement, the polarity of one of the components having been inverted; Figure 6 is an oscillgram corresponding to the second citcuit of Figure 3, one of the components having a first polarity; Figure 7 is an oscillgram of the same circuit arrangement, the polarity of one of the components having been inverted; Figure 8 is a block diagram synoptic of a device for carrying out the process of the invention; and Figure 9 is a diagrammatic representation of a device for automatic location of defects in electrical conductors embedded in concrete.

Referring to Figure 1, an electrical conductor C such as a heating cable embedded in a floor has a predetermined number of windings between its two ends A and B.

The said conductor C is divided into two sections 1 and 2 by a defect 3 which may for example be an insulation defect with a leak to earth. Typically, the configuration of the conductor embedded in the ground is unknown, but its two ends A and B and also an earth terminal T are accessible in a box or connecting column. The process of the invention includes the steps of: passing a first current between the end A and an earth terminal T by connecting a first current generator between these two terminals; and passing a second current either between the second end B and earth T, or between the first end A and the second end B, the second current being supplied by a second generator connected for example between the second end B and the first end A of earth T. The currents supplied by the generators are periodic, and differ from each other by one or more parameters.

Conveniently, two probes D1, D2 are used as magnetic field detectors to determine the position of the conductor C. The location is determined by comparing the indications supplied by the two probes, similar indications indicating that the probes are situated on the same sections and different indications indicating that the probes are situated on either side of the defect.

Referring to Figure 2, a first current is supplied by a first generator G1 connected between the end A of the conductor and the earth terminal T, a second current being supplied by a second generator G2 connected between the earth terminal T and the end B of the conductor. The second generator may be of the same polarity as the first generator (configuration a) or the opposite polarity (configuration b).

Referring to Figure 3, a first current is supplied by a first generator G1 connected between the first end A of the conductor and the earth terminal T, a second current being supplied by a second generator G2 connected between the first end A of the conductor and its second end B. The second generator may be of the same polarity (configuration c) or the opposite polarity (configuration d).

Figure 4 shows an oscillgram corresponding to configuration a of the circuit of Figure 2 and shows: a first squarewave current 41 emitted by G1 and a second squarewave current 42 emitted by G2 1800 out of phase with current 41. A respective signal is induced in each probe (D1 and D2) by these currents. If the probes D1 and D2 are at different sides of the defect 3, they will detect signals having fun/ damentals 44 and 46 respectively. The detected signals are filtered by circuitry associated with the probes to give continuous components 43 and 45 respectively, differing in polarity when the probes are on different sides of the defect 3. If a probe is moved across the defect, so that both probes are on the same side, polarity inversion occurs, the continuous component changing from 43 to 45 or vice versa.

Thus, the defect is precisely located by moving the probes towards each other on either side of the defect and attempting to maintain the probe signals equal in absolute value (thus indicating that the probes are at roughly constant separations from the conductor) but opposite in polarity.

Figure 5 is an oscillogram corresponding to configuration b of the circuit of Figure 2, and shows: a first current 51 emitted by G1, a second current 52 emitted by G2, the continuous component 53 of the filtered signal and the fundamental harmonic 54 of the signal emitted by D1, the continuous component 55 of the filtered signal and the fundamental harmonic 56 of the signal emmitted by D2, in the case that the probes D1 and D2 are located on either side of the defect 3. In this case, inversion of the fundamental harmonic's phase occurs when one of the probes D1, D2 crosses the defective point, the fundamental harmonic passing from 54 to 56 or vice versa. Precise localisation of the defect is therefore achieved by moving the probes towards each other on either side of the defect and attempting to maintain the probe signals equal in absolute value but opposite in phase.

Figure 6 is an oscillgram corresponding to configuration c of the circuit of Figure 3. The precise location of the defect is where the fundamental harmonic 61 of the signal emitted by D2 appears or disappears, the continuous components 62 and 63 of the signals emitted by D1 and D2 respectively remaining constant. With this configuration, the fundamental harmonic in section 1 of the conductor is zero.

Figure 7 is an oscillgram corresponding to configuration d of the circuit of Fig. 3.

The precise location of the defect is achieved by observing the appearance of a continuous component in the output of probe D1 indicating that it has crossed the defect 3 from section 1 of the conductor (where the component 72 is zero) to section 2. The position of the defect also coincides with changes in amplitude and phase of the fundamental harmonics 73 and 74 in sections 1 and 2 of the conductor respectively.

The periodic currents such as 41 and 42 which are emitted by each of the generators as shown in Figures 4 to 7 are complementary square wave signals. One alternative would be for one of the current signals to have a frequency which was an integral multiple of that of the other, so that a plurality of pulses of the one signal were produced during a half-period of the other signal.

Referring to Figure 8, a device for locating a defect in an electrical conductor embedded in the ground includes a control sub-assembly 81, an oscilloscope 82, and a pair of probes 83 and 84. The control sub-assembly 81 comprises a feed source 85, two current generators 86 and 87, which are in synchronism wilh each othe,r, a multi-position commutator switch 88 which permits connection of the two outputs of each of the generators 86 and 87, such as the outputs 861 and 862 of the generator 86, to any selected two of the folio wing three elements: a first end A of the heating conductor, a second end B thereof and the earth terminal T. Each generator, for example generator 86, has means for controlling the values of current parameters, such as 863 for the duty ratio, 864 for the amplitude, 865 for the phase, 866 for the frequency, and an inverter 867 for the polarity. The signals transmitted by each of the probes 83 and 84 are fed to "low-pass" filters 831 and 841. The filtered signals 832 and 842 are fed to two beams of a three-beam oscilloscope, where they are displayed on the screen 821. By using a switch 881, one of the two current signals emitted by the generators 86 and 87 may be supplied to the third oscilloscope beam in order to serve as phase reference. The two generators 86 and 87 may be replaced by a single generator alternately feeding two separate outputs with currents having differing parameters.

Referring to Figure 9, a vehicle 91 has driving wheels 92 and a guiding wheel 93.

The vehicle has a first pair of piloting magnetic field detectors 94 mounted laterally and on either side of a line of travel of the vehicle. The detectors 94 pilot the vehicle along an embedded conductor through which a current is passing, by acting on the guide wheel 93 via cybernetic control means (not shown). The control means may comprise a bridge circuit wherein an axial orientation of the guide wheel corresponds to the equilibrium position. The vehicle also has two pickups 95 and 96 (known as the defect-detecting means), mounted one in front of the other towards the rear of the vehicle. These two pickups 95 and 96 are analogous to the probes 83 and 84 of the device shown in Figure 8 and via control means they can stop the vehicle directly over a defect when they receive different signals. The path travelled by the vehicle and its stopping positions are recorded by tracing means such as the leakage of powdered chalk. Display means, such as the oscilloscope in the device shown in Figure 8 may be incorporated in the vehicle of Figure 9.

In apparatus embodying the present invention the magnetic field pickup incorporated in a probe may be a solenoid or a Hall probe.

The process of the present invention can equally well be used to search for rupture points, or to search for ruptures of electrically insulating protection systems of conduits or lines capable of passing an electric current and which are embedded in the ground at a small depth.

WHAT WE CLAIM IS: 1. A process for locating a defect in an electric conductor embedded in the ground, the process including the steps of; passing a periodic current through a first section of the conductor between an earth-contacting defect and a first end of the said conductor; passing a second periodic current through a second section of the said conductor between the defect and a second end of the said conductor, the said second current differing from the first current in its value of at least one of the following parameters: polarity, duty ratio, amplitude, phase and frequency; and detecting a change in a magnetic field induced by said periodic currents.

2. A process according to Claim 1, in which the amplitude maxima of the second current coincide with the amplitude minima of the first current, substantially opposite and complementary magnetic signals thus being induced about the conductor on either side of the defect.

3. A process according to claim 1 or claim 2, in which said maxima of the first and second currents are of the same poparity.

4. A process according to claim 1 or claim 2, in which said maxima of the first and second currents are of opposite polarity.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Figure 7 is an oscillgram corresponding to configuration d of the circuit of Fig. 3. The precise location of the defect is achieved by observing the appearance of a continuous component in the output of probe D1 indicating that it has crossed the defect 3 from section 1 of the conductor (where the component 72 is zero) to section 2. The position of the defect also coincides with changes in amplitude and phase of the fundamental harmonics 73 and 74 in sections 1 and 2 of the conductor respectively. The periodic currents such as 41 and 42 which are emitted by each of the generators as shown in Figures 4 to 7 are complementary square wave signals. One alternative would be for one of the current signals to have a frequency which was an integral multiple of that of the other, so that a plurality of pulses of the one signal were produced during a half-period of the other signal. Referring to Figure 8, a device for locating a defect in an electrical conductor embedded in the ground includes a control sub-assembly 81, an oscilloscope 82, and a pair of probes 83 and 84. The control sub-assembly 81 comprises a feed source 85, two current generators 86 and 87, which are in synchronism wilh each othe,r, a multi-position commutator switch 88 which permits connection of the two outputs of each of the generators 86 and 87, such as the outputs 861 and 862 of the generator 86, to any selected two of the folio wing three elements: a first end A of the heating conductor, a second end B thereof and the earth terminal T. Each generator, for example generator 86, has means for controlling the values of current parameters, such as 863 for the duty ratio, 864 for the amplitude, 865 for the phase, 866 for the frequency, and an inverter 867 for the polarity. The signals transmitted by each of the probes 83 and 84 are fed to "low-pass" filters 831 and 841. The filtered signals 832 and 842 are fed to two beams of a three-beam oscilloscope, where they are displayed on the screen 821. By using a switch 881, one of the two current signals emitted by the generators 86 and 87 may be supplied to the third oscilloscope beam in order to serve as phase reference. The two generators 86 and 87 may be replaced by a single generator alternately feeding two separate outputs with currents having differing parameters. Referring to Figure 9, a vehicle 91 has driving wheels 92 and a guiding wheel 93. The vehicle has a first pair of piloting magnetic field detectors 94 mounted laterally and on either side of a line of travel of the vehicle. The detectors 94 pilot the vehicle along an embedded conductor through which a current is passing, by acting on the guide wheel 93 via cybernetic control means (not shown). The control means may comprise a bridge circuit wherein an axial orientation of the guide wheel corresponds to the equilibrium position. The vehicle also has two pickups 95 and 96 (known as the defect-detecting means), mounted one in front of the other towards the rear of the vehicle. These two pickups 95 and 96 are analogous to the probes 83 and 84 of the device shown in Figure 8 and via control means they can stop the vehicle directly over a defect when they receive different signals. The path travelled by the vehicle and its stopping positions are recorded by tracing means such as the leakage of powdered chalk. Display means, such as the oscilloscope in the device shown in Figure 8 may be incorporated in the vehicle of Figure 9. In apparatus embodying the present invention the magnetic field pickup incorporated in a probe may be a solenoid or a Hall probe. The process of the present invention can equally well be used to search for rupture points, or to search for ruptures of electrically insulating protection systems of conduits or lines capable of passing an electric current and which are embedded in the ground at a small depth. WHAT WE CLAIM IS:

1. A process for locating a defect in an electric conductor embedded in the ground, the process including the steps of; passing a periodic current through a first section of the conductor between an earth-contacting defect and a first end of the said conductor; passing a second periodic current through a second section of the said conductor between the defect and a second end of the said conductor, the said second current differing from the first current in its value of at least one of the following parameters: polarity, duty ratio, amplitude, phase and frequency; and detecting a change in a magnetic field induced by said periodic currents.

5. A process according to any one of

the preceding claims in which the second current passes along the conductor only between earth and the second end of the conductor.

6. A process according to any one of claims 1 to 4 in which the second current passes along the whole length of the conductor between the first and second ends.

7. A process according to any one of the preceding claims, in which the first and second currents are square wave pulse signals.

8. A process according to any of the preceding claims, in which the first and second currents flow simultaneously in the first and second sections respectively of the conductor, the frequencies of the first and second currents differing slightly from each other.

9. Apparatus for locating a defect in an electrical conductor embedded in the ground, said device including two current generators in synchronism with each other, each generator supplying a periodic current signal which is of substantially square wave configuration and having means for controlling one or more of the parameters of the current a three-channel output permitting connection of two outputs of each generator to two of three elements by a multi-position switch, the three elements being a first end of the conductor, a second end thereof and earth; detection means comprising at least one probe having at its end at least one magnetic field pickup; a respective "low-pass" filter for selecting the fundamental periodic voltage induced in the or each pickup by the periodic magnetic field; and a three-beam oscilloscope for displaying the current signal of at least one of the generators and at least one respective voltage signal derived from the or a respective said pickup.

10. Apparatus according to claim 9, wherein the magnetic field pickup is a solenoid.

11. Apparatus according to claim 9, wherein the magnetic field pickup is a Hall probe.

12. Apparatus according to any one of claims 9 to 11 comprising a vehicle adapted to travel along a line; said detection means comprising two defect detecting magnetic field detectors mounted one in front of the other towards the rear of said vehicle; two piloting magnetic field detectors mounted on respective sides of the line of travel of the vehicle; means controlled by the piloting detectors for guiding the vehicle along the embedded electrical conductor; means controlled by the defect detecting detectors for stopping the vehicle when it is directly over a defect; and means for tracing the line followed by the vehicle and marking its stopping positions.

13. Apparatus for locating a defect in an electrical conductor embedded in the ground substantially as hereinbefore described with reference to and as illustrated in Figures 8 and 9 of the accompanying drawings.