Nondestructive testing method for cable

Abstract

The invention relates to the technical field of electric power, in particular to a cable nondestructive testing method which comprises the following steps: acquiring corresponding calibration parameters from a database according to the model of a to-be-tested cable, and calculating the total length and fault position of the to-be-tested cable at the near end of the to-be-tested cable by adopting a time domain reflection method; at the tail end of the cable to be detected, at the tail end, at least two core wires are short-circuited, a four-wire detection method is adopted to measure the direct-current resistance of a core wire loop at the near end, and standard temperature correction is conducted on the direct-current resistance of a single core wire; the resistivity to be measured is calculated by using the total length of the cable to be measured, the actual cross sectional area of the core wire and the direct current resistance of the single core wire at the standard temperature, and whether the actual core wire material of the cable to be measured is qualified is analyzed according to the resistivity to be measured and a resistivity threshold value set based on the calibrated core wire material. And early warning is carried out based on a core wire material analysis result and a fault position. According to the method, the cable length, the fault position and the core wire material qualification can be quickly, nondestructively and accurately detected on site.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power cable, and in particular to a cable nondestructive testing method. BACKGROUND

[0002] Power cable has been widely used in power, communication, industry, construction and other fields due to its efficient power transmission characteristics, reliability and stability, and the material and quality of the power cable are directly related to the performance, safety and service life of the power cable. In recent years, with the rapid development of industrialization and urbanization, the use amount and applicable specifications of various cables of different materials have significantly increased, and the demand for cable material testing technology has also been growing. The core wire material of the cable on the market is generally copper, but some cable manufacturers or construction parties use aluminum for the core wire material in the middle section of the cable and use copper for the core wire material at both ends of the cable in order to reduce costs. This will increase the loss of electric energy and increase the transportation cost of the power department, thereby causing problems such as virtual increase of engineering quantity.

[0003] Traditional cable material testing methods include physical performance testing and chemical analysis testing. Physical performance testing mainly includes tensile test and impact test methods. The tensile test is to apply uniform tension to both ends of the cable until the cable is deformed or broken, and this method is mainly used to evaluate the mechanical properties of the cable. The impact test is to evaluate the electrical properties of the cable by applying an impact voltage. Chemical analysis testing mainly includes component analysis and aging test methods. The component analysis method is to analyze the cable material by various chemical analysis means, and the aging test is to evaluate the cable performance by using a higher temperature and a longer duration aging experiment. Alternatively, emerging technologies, including X-ray fluorescence spectroscopy, infrared spectroscopy, scanning electron microscopy and other methods, are used to precisely analyze the cable material and accurately determine the element composition in the core wire by using various chemical means.

[0004] Traditional cable material testing methods need to perform destructive tests on power cables when evaluating the mechanical and electrical properties of the cables, which is costly and not conducive to popularization. Emerging technologies are suitable for fine analysis of cable materials, but the detection equipment of emerging technologies is costly, there is a shortage of professionals, and on-site detection is difficult, so there is an urgent need to provide a method suitable for on-site use, nondestructive and low cost for detecting cables. SUMMARY

[0005] The present application aims to provide a cable detection method suitable for on-site use, nondestructive and low cost.

[0006] To achieve the above-mentioned purpose, the present application provides the following technical solutions: A cable nondestructive testing method, comprising: According to the model of the cable under test, the corresponding calibration parameters are obtained from the database, including the calibration core cross-sectional area, calibration core material and calibration wave velocity. The total length and fault location of the cable under test are measured by time domain reflection method at the near end of the cable under test. At least two core wires are shorted at the end of the cable under test to form a core wire loop. The DC resistance of the core wire loop is measured using a four-wire detection method near the end of the cable under test, and the DC resistance of a single core wire is corrected for standard temperature. The resistivity to be tested is calculated using the total length of the cable under test, the actual cross-sectional area of ​​the core wire, and the DC resistance at standard temperature. Based on the resistivity to be tested and the resistivity threshold set based on the calibrated core wire material, the actual core wire material of the cable under test is analyzed to determine whether it is qualified. Based on the core wire material analysis results and the fault location, an early warning is issued.

[0007] Furthermore, the step of measuring the total length and fault location of the cable under test using time-domain reflectometry near its end includes: A low-voltage pulse signal with a calibrated wave velocity is input to the near end of the cable under test. During the transmission of the low-voltage pulse signal along the cable under test, the time difference between the transmission of the signal and the reception of the reflected signal generated by the impedance mismatch point is recorded. The impedance mismatch point includes the fault location and the end location of the cable under test. The fault location of the cable under test is calculated based on the calibrated wave velocity and the time difference between the transmission of the signal and the reception of the reflected signal generated at any fault location near the near end of the cable under test. The total length of the cable under test is calculated based on the calibrated wave velocity and the time difference between the transmission signal and the received reflected signal at the end of the cable.

[0008] Furthermore, the first... i One fault location for:

[0009] Among them, the i One fault location The position relative to the near end of the cable under test. The calibration wave velocity for low-voltage pulse signals, For the cable under test, from the time a low-voltage pulse signal is sent to the time the first pulse is received near the end of the cable, the signal is considered. i The time difference between the reflected signals generated at each fault location.

[0010] Furthermore, the total length of the cable under test L for:

[0011] in, The calibration wave velocity for low-voltage pulse signals, The time difference is the time difference between the transmission of a low-voltage pulse signal at the near end of the cable under test and the reception of the reflected signal generated at the end.

[0012] Furthermore, the DC resistance of the core wire circuit is measured near the end of the cable under test using a constant current source and a high impedance voltmeter. The DC resistance of a single core wire is equal to the DC resistance of the core wire circuit / 2.

[0013] Furthermore, the DC resistance of the single core wire is corrected for standard temperature according to the resistance temperature correction formula; The resistance temperature correction formula is as follows:

[0014] in, R The DC resistance of a single core wire at a standard temperature of 20°C. Current temperature T DC resistance of a single core wire is the temperature coefficient of resistance.

[0015] Furthermore, the resistivity of a single core wire is calculated using the total length L of the cable under test, the actual cross-sectional area A of the core wire, and the DC resistance R of a single core wire at a standard temperature of 20°C. : .

[0016] Further, the step of analyzing whether the actual core wire material of the cable under test is qualified based on the resistivity to be measured and the resistivity threshold set based on the calibrated core wire material includes: Determine whether the resistivity to be measured is greater than a preset resistivity threshold. If not, then the actual core wire material of the cable under test is deemed to be qualified; Otherwise, the actual core wire material of the cable under test is deemed unqualified.

[0017] Furthermore, resistivity threshold for:

[0018] in, , The resistivity of copper and aluminum are respectively. f This refers to the maximum permissible aluminum doping ratio set according to the calibrated core wire material.

[0019] Furthermore, the early warning based on the core wire material analysis results and fault location includes: The fault location of the cable under test is displayed on the display interface, and a first alarm signal is generated. If the calculated DC resistance of the single core wire exceeds the normal resistance range set according to the cable model, a second alarm signal is generated; If it is judged that the actual core wire material of the cable to be measured is unqualified, a third alarm signal is generated.

[0020] Technical effects and advantages of the present application: (1) The cable detection process is only carried out at the exposed end of the cable, without the need to cut off or damage the cable structure, realizing non-destructive detection, and the detection process uses general equipment in the field of electronic measurement, without the need to purchase expensive chemical analysis instruments, greatly reducing equipment costs and maintenance costs; (2) Based on basic circuit theory and electromagnetic wave propagation theory, the measurement process is standardized, and the technical background of the operator is not high, and it is easy to popularize in basic units and construction sites; (3) The temperature correction link is introduced, which eliminates the measurement error caused by the change of the environment temperature, ensures the accuracy of the resistance measurement value and the subsequent resistivity calculation value, and judges the core wire material by calculating the resistivity, which is a material intrinsic parameter, which is more scientific and reliable than simply comparing the resistance value; (4) Different types of cable insulation materials and structures are different, resulting in different calibration wave speeds, the present application accurately measures the wave speed of various types of cables in advance and stores it in the database, ensuring the universality and high precision of the cable length and fault point measurement, avoiding the complexity and inaccuracy of on-site calibration.

[0021] Other features and advantages of the present application will be set forth in the following description, and in part will become apparent to those skilled in the art, or will be learned by practice of the present application. The objects and other advantages of the present application can be achieved and obtained by the structures indicated in the specification, claims and drawings. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0023] Figure 1 The flowchart of the cable non-destructive detection method of the embodiment of the present application; Figure 2 The principle diagram of the time domain reflectometry method for measuring the length and fault position of the cable of the embodiment of the present application; Figure 3 The principle diagram of the four-terminal method for measuring resistance of the embodiment of the present application. DETAILED DESCRIPTION

[0024] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the protection scope of the present application.

[0025] The present application discloses a kind of cable nondestructive testing method, as shown in Figure 1 The method comprises: S1, the corresponding calibration parameter is acquired from database according to the model of the cable to be measured, including calibration core cross-sectional area, calibration core material and calibration wave speed, the total length and fault position of the cable to be measured are measured using time domain reflection method at the proximal end of the cable to be measured; S2, at least two core wires are short-circuited to form a core wire loop at the end of the cable to be measured, and the direct current resistance of the core wire loop is measured using four-wire detection method at the proximal end of the cable to be measured, and the direct current resistance of a single core wire is corrected at standard temperature; S3, the total length of the cable to be measured, the actual cross-sectional area of the core wire and the direct current resistance of a single core wire at standard temperature are used to calculate the measured resistivity, whether the actual core wire material of the cable to be measured is qualified is analyzed according to the measured resistivity and the resistivity threshold value set based on the calibration core material, and a warning is given based on the core material analysis result and the fault position.

[0026] In the embodiments of the present application, the cable detection process is only carried out at the end of the cable, without the need to cut off or damage the cable structure, realizing nondestructive testing, and using general equipment in the field of electronic measurement, without the need to purchase expensive chemical analysis instruments, greatly reducing the equipment cost and maintenance cost; based on basic circuit theory and electromagnetic wave propagation theory, the measurement process is standardized, and the technical background requirement of the operator is not high, easy to popularize and popularize in grass-roots units and construction sites; the temperature correction link is introduced, which eliminates the measurement error caused by environmental temperature change, ensures the accuracy of the resistance measurement value and the subsequent resistivity calculation value, and judges the core wire material by calculating the resistivity, a material intrinsic parameter, which has clear physical meaning and is more scientific and reliable than simply comparing resistance values.

[0027] In this invention, a high-precision cable parameter database is pre-established to record calibration parameters for cables of different specifications and models, including the calibration core cross-sectional area, calibration core material (copper or aluminum), and calibration wave velocity obtained through precise measurement. Since differences in insulation materials and structural processes among different cable models can lead to significant variations in signal propagation speed, traditional time-domain reflectometry (TDR) for measuring length does not require high wave velocity accuracy. However, in this invention, the accuracy of cable length measurement directly determines the accuracy of the resistivity calculation, thus affecting the accuracy of core material determination. By improving the accuracy of cable length measurement, the accuracy of resistivity measurement is improved, thereby enhancing the accuracy of core material determination.

[0028] The method for measuring the calibrated wave velocity is as follows: Take a standard cable of known precise length (e.g., 1 meter), input a pulse signal with a preset wave velocity to its head end, measure the time interval from the transmission of the pulse signal to its reflection back to the head end, calculate the cable length, adjust the value of the preset wave velocity, and when the calculated cable length is infinitely close to the known precise length, the preset wave velocity is the calibrated wave velocity of the current model cable.

[0029] In this embodiment of the invention, when testing cables on-site, only the cable model needs to be entered into this database to obtain the accurate calibration wave velocity, avoiding the cumbersome and inaccurate steps of on-site wave velocity calibration, and ensuring the realization of "fast, on-site, and high-precision" non-destructive testing.

[0030] Step S1, which describes measuring the total length and fault location of the cable under test using the time-domain reflectometry method near its end, includes: inputting a low-voltage pulse signal with a calibrated wave velocity to the near end of the cable under test (i.e., the measurement end, corresponding to the end), such as... Figure 2 As shown, during the transmission of the low-voltage pulse signal along the cable under test, reflection will occur when it encounters impedance mismatch points (such as fault points, intermediate joints, ends, etc.). The time difference between the transmission of the signal and the reception of the reflected signal generated by the impedance mismatch point near the end of the cable under test is recorded. The impedance mismatch point includes the fault location and the end location of the cable under test. Based on the calibrated wave velocity and the time difference between the transmission of the signal and the reception of the reflected signal generated by any fault location near the end of the cable under test, the fault location of the cable under test is calculated. Based on the calibrated wave velocity and the time difference between the transmission of the signal and the reception of the reflected signal generated by the end location near the end of the cable under test, the total length of the cable under test is calculated.

[0031] In this embodiment of the invention, the location of the fault / abnormal point in the cable and the total length of the cable are calculated by receiving the reflected signals of each impedance mismatch point in real time.

[0032] According to a preferred embodiment, the first... i One fault location for:

[0033] Among them, the i One fault location The position relative to the near end of the cable under test. The calibration wave velocity for low-voltage pulse signals, For the cable under test, from the time a low-voltage pulse signal is sent to the time the first pulse is received near the end of the cable, the signal is considered. i The time difference between the reflected signals generated at each fault location.

[0034] According to another preferred embodiment, the total length of the cable under test L for:

[0035] in, The calibration wave velocity for low-voltage pulse signals, The time difference is the time difference between the transmission of a low-voltage pulse signal at the near end of the cable under test and the reception of the reflected signal generated at the end.

[0036] Figure 3 The diagram shows the principle of resistance measurement using the four-terminal detection method. R s The resistor to be measured Rl A resistor is introduced for wiring. A constant current source provides a stable current to the resistor under test. A voltmeter measures the voltage across the resistor. If the input impedance of the voltage measuring circuit is very high—that is, the input impedance of the voltmeter is much greater than the resistance under test and the wiring resistance—then the measured voltage is the voltage of the cable under test. The wiring resistance... Rl It will not introduce measurement errors, and because it is driven by a constant current source, the wiring resistance is low. Rl It also doesn't affect the current magnitude, therefore the four-wire detection circuit essentially eliminates errors introduced by the measurement circuit, resulting in high accuracy. Based on Ohm's law, the resistance value of the resistor under test can be calculated.

[0037] Preferably, in this embodiment, a constant current source and a high impedance voltmeter are used to measure the DC resistance of the core wire circuit near the end of the cable under test. The DC resistance of a single core wire is equal to the DC resistance of the core wire circuit / 2.

[0038] Specifically, after identifying the cable model to be tested, the testing personnel do not need to cut the cable. They only need to reliably short-circuit any two core wires (e.g., phase A and phase B) at the exposed point at the far end (terminal) of the cable using shorting pliers or a dedicated short-circuit device, forming a complete core wire loop from the near end (measuring end) to the end and back to the near end. When measuring the DC resistance of the core wire loop using the four-wire testing method, at the near end of the cable, the output terminal of a constant current source is connected to one of the two shorted core wires (e.g., phase A), forming a current injection point. The input terminal of a high-impedance voltmeter is directly connected to both ends of the core wire loop, i.e., connected to the phase A and phase B core wires respectively. The constant current source outputs a stable and accurate constant current to the core wire loop. This current flows along the phase A core wire to the short-circuit point at the end, and then flows back through the phase B core wire. At this time, the high-impedance voltmeter measures the voltage drop generated across the entire core wire loop under the action of this constant current. According to Ohm's law, the DC resistance value R_loop of the core wire loop can be accurately calculated.

[0039] Given that the core wires in the same cable are manufactured using the same material and process, and their specifications (cross-sectional area, length) are consistent, it can be reasonably assumed that the resistance values ​​of the core wires in phase A and phase B are equal. Therefore, the DC resistance value R_single of a single core wire can be calculated using the following formula: R_single = R_loop / 2 Since the DC resistance of cable cores is greatly affected by temperature, it is necessary to measure the ambient temperature before measurement and record the DC resistance R of a single core at the current temperature T. T =R_single, by applying temperature correction to the DC resistance value at the current temperature T, obtains the resistance at the standard temperature (usually 20℃). This eliminates the influence of temperature on resistance measurement, resulting in more accurate results and making the measurement results universal and effective.

[0040] In this embodiment of the invention, the DC resistance of the single core wire is corrected for standard temperature according to a resistance temperature correction formula. The resistance temperature correction formula is as follows:

[0041] in, R The DC resistance of a single core wire at a standard temperature of 20°C. Current temperature T DC resistance of a single core wire This refers to the temperature coefficient of resistance. For example, the temperature coefficient of resistance of copper is 0.00393 / ℃, and that of aluminum is 0.00403 / ℃.

[0042] Furthermore, the resistivity to be measured is calculated using the total length L of the cable under test, the actual cross-sectional area A of the core wire, and the DC resistance R of a single core wire at standard temperature. :

[0043] resistivity to be measured , which is the resistivity of a single core wire of the cable under test at a standard temperature of 20℃.

[0044] In this embodiment of the invention, analyzing whether the actual core wire material of the cable under test is qualified based on the resistivity to be measured and the resistivity threshold set based on the calibrated core wire material includes: determining whether the resistivity to be measured is greater than the preset resistivity threshold; if not, determining that the actual core wire material of the cable under test is qualified; otherwise, determining that the actual core wire material of the cable under test is unqualified.

[0045] resistivity of copper Approximately 1.72 × 10 -8 Ω·m, resistivity of aluminum Approximately 2.82 × 10 -8 Ω·m, by comparison, if the resistivity to be measured and Same or similar (e.g., less than the tolerable error) If the resistivity of the cable under test is 0, then the actual core material of the cable under test is determined to be pure copper; if the resistivity of the cable under test is 0, then the actual core material of the cable under test is determined to be pure copper. and If the resistivity is the same or similar, then the actual core material of the cable under test is determined to be pure aluminum; if the resistivity is... Between and If the actual core material of the cable under test is determined to be a copper-aluminum mixture, a resistivity threshold is set based on the calibrated core material. If the resistivity to be measured Greater than If the actual core material of the cable under test is unqualified, then the resistivity of the cable under test is determined to be unqualified. Less than or equal to If the actual core material of the cable under test is qualified, then it can be determined that the cable under test is qualified.

[0046] According to a preferred embodiment, a set resistivity threshold is established. for:

[0047] in, , The resistivity of copper and aluminum are respectively. f This refers to the maximum permissible aluminum doping ratio set according to the calibrated core wire material.

[0048] In this embodiment of the invention, early warning is provided based on the core wire material analysis results and fault location, including: The display interface shows the location of the fault in the cable under test and generates a first alarm signal. The first alarm signal is used to indicate that there is a mechanical fault or abnormal connection point in the cable (such as loose joints, insulation damage, deformation under pressure, etc.) and guide maintenance personnel to the designated location for repair. If the calculated DC resistance of a single core wire exceeds the normal resistance range specified according to the cable type (e.g.) If the cable exhibits overall electrical performance degradation (such as severe conductor oxidation, overall moisture absorption, or changes in conductor microstructure due to long-term overload), a second alarm signal will be generated. This indicates that a full-line evaluation or replacement of the cable is required. If the actual core material of the cable under test is found to be unqualified based on the resistivity to be measured and the preset resistivity threshold, a third alarm signal is generated. The third alarm signal is used to indicate that the cable has a material fraud of "using aluminum instead of copper", indicating that the project quality acceptance is unqualified, the relevant responsible party needs to be investigated and the entire cable needs to be replaced.

[0049] In this embodiment of the invention, the cable length measured by time-domain reflectometry and the DC resistance value of the core wire measured by four-terminal detection method are used together, after standard temperature correction, to calculate the resistivity of the core wire. Without damaging the cable structure, this method can quickly and accurately diagnose whether the cable core wire material is qualified on site, and also has fault location capability. It realizes rapid, non-destructive, and accurate detection of cable length, fault location, and core wire material qualification. It has the advantages of simple equipment, low cost, and easy promotion, and is particularly suitable for rapid on-site assessment of cable quality in the power, communication and other fields.

[0050] Based on the same inventive concept, the present invention also provides a system and product apparatus for implementing the above-mentioned cable non-destructive testing method, wherein the specific manner in which each unit module performs its operation has been described in detail in the embodiments of the relevant method, and will not be elaborated here.

[0051] It should be noted that, for the sake of simplicity, the foregoing method embodiments are all described as a series of actions. However, those skilled in the art should understand that the present invention is not limited to the described order of actions, because according to the present invention, some steps can be performed in other orders or simultaneously. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to the present invention.

[0052] In the above embodiments, the descriptions of each embodiment have their own emphasis. Parts not described in detail in a particular embodiment can be found in the relevant descriptions of other embodiments. Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of non-destructive testing of a cable, characterized in that, The method comprises: According to the type of the cable to be measured, corresponding calibration parameters are obtained from a database, including calibration core cross-sectional area, calibration core material and calibration wave speed, the total length and fault position of the cable to be measured are measured at the proximal end of the cable to be measured by using time domain reflection method; At least two cores are short-circuited to form a core loop at the distal end of the cable to be measured, and the DC resistance of the core loop is measured at the proximal end of the cable to be measured by using four-wire detection method, and the DC resistance of a single core is corrected at standard temperature; The actual resistivity of the cable to be measured is calculated by using the total length of the cable to be measured, the actual cross-sectional area of the core and the DC resistance of a single core at standard temperature, whether the actual core material of the cable to be measured is qualified is analyzed according to the actual resistivity and the resistivity threshold value set based on the calibration core material, and early warning is performed based on the core material analysis result and the fault position.

2. The method of claim 1, wherein, The total length and fault position of the cable to be measured are measured at the proximal end of the cable to be measured by using time domain reflection method, which comprises: A low-voltage pulse signal of the calibration wave speed is input to the proximal end of the cable to be measured, and the time difference of the reflected signal generated from the sending signal to the receiving of the impedance mismatch point at the proximal end of the cable to be measured is recorded during the transmission of the low-voltage pulse signal along the cable to be measured, the impedance mismatch point including the fault position and the distal end position of the cable to be measured; The fault position of the cable to be measured is calculated according to the calibration wave speed and the time difference of the reflected signal generated from the sending signal to the receiving of any fault position at the proximal end of the cable to be measured; The total length of the cable to be measured is calculated according to the calibration wave speed and the time difference of the reflected signal generated from the sending signal to the receiving of the distal end position at the proximal end of the cable to be measured.

3. The method of claim 2, wherein, The first fault position of the to-be-tested cable is: i :​ wherein the first fault location i is a location relative to a near end of the cable under test, is a nominal wave speed of the low voltage pulse signal, is a time difference between a transmission of the low voltage pulse signal from the near end of the cable under test and a reception of a reflected signal resulting from the first fault location. i is a location relative to a near end of the cable under test,​ 4. The method of claim 2, wherein, the total length of the cable to be tested L is: wherein, is the nominal wave velocity of the low voltage pulse signal, is the time difference of the reflected signal generated by the near end of the cable under test from sending the low voltage pulse signal to receiving the end of the line position.

5. The method of claim 4, wherein, The DC resistance of the core loop is measured at the proximal end of the cable to be measured by using a constant current source and a high-impedance voltmeter, and the DC resistance of a single core = the DC resistance of the core loop / 2.

6. The method of claim 5, wherein, The DC resistance of a single core is corrected at standard temperature according to the resistance temperature correction formula; The resistance temperature correction formula is as follows: wherein R R0 is the DC resistance of the single core wire at standard temperature 20°C, R is the DC resistance of the single core wire at current temperature T R0 is the DC resistance of the single core wire at standard temperature 20°C, R is the DC resistance of the single core wire at current temperature 7. The method of claim 6, wherein, The total length L of the cable to be measured, the actual cross-sectional area A of the core wire, and the DC resistance R of a single core wire at a standard temperature of 20°C are used to calculate the resistivity of a single core wire : 。 8. The method of claim 7, wherein, Whether the actual core material of the cable to be measured is qualified is analyzed according to the actual resistivity and the resistivity threshold value set based on the calibration core material, which comprises: It is judged whether the actual resistivity is greater than the pre-set resistivity threshold value; If not, it is judged that the actual core material of the cable to be measured is qualified; Otherwise, it is judged that the actual core material of the cable to be measured is unqualified.

9. The method of claim 8, wherein, resistivity threshold is: wherein, , are the resistivities of copper and aluminum, respectively, f is the maximum allowed aluminum doping ratio set according to the gauge of the core wire.

10. The method of claim 1, wherein, Early warning is performed based on the core material analysis result and the fault position, which comprises: The fault position of the cable to be measured is displayed by using a display interface, and a first alarm signal is generated; If the calculated DC resistance of a single core exceeds the normal resistance range set according to the type of the cable, a second alarm signal is generated; If it is judged that the actual core material of the cable to be measured is unqualified, a third alarm signal is generated.

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