Precautions in Cable Fault Testing

1、 Preparation before testing: laying the foundation for precise testing

The core of the preparation phase is to clarify cable information, ensure equipment reliability, and clean up the on-site environment to avoid testing deviations or safety hazards caused by previous omissions

Collect basic information about cables: It is necessary to obtain in advance the cable model (such as YJV22 cross-linked polyethylene cable, VV polyvinyl chloride cable), rated voltage, laying method (direct burial, pipe penetration, bridge), design length, joint position (quantity and distance), historical fault records, etc. - for example, for directly buried cables, the influence of soil temperature on propagation speed should be considered, and for cables with intermediate joints, the joint position should be marked to distinguish between "joint reflection wave" and "fault reflection wave", in order to avoid misjudgment.

Check the status of the testing equipment: Conduct a comprehensive inspection of the megohmmeter (rocking meter), cable fault tester (host+locator), high-voltage generator, testing leads, and other equipment

Confirm that the instrument is within its calibration validity period (e.g. a megohmmeter needs to be calibrated once a year) to avoid inaccurate data due to accuracy failure;

Check whether the insulation layer of the test lead is damaged and whether the terminal is firm (the insulation level of the high-voltage lead needs to match the test voltage, such as 10kV cable testing that requires insulated leads of 10kV or above);

Confirm that the device battery is fully charged (or the external power supply is stable) to prevent data loss caused by sudden power outages during testing.

Clean up the on-site environment:

Exclude flammable and explosive materials (such as gasoline and paint) within the testing area, and maintain a distance of at least 10 meters from open flames during high-pressure testing;

Stay away from strong electromagnetic interference sources (such as high-voltage busbars, frequency converters, and high-power motors). If it is impossible to avoid them, use shielded wires to connect the instrument and reduce the operating power of other equipment in the testing area to minimize the impact of electromagnetic interference on the waveform.

2、 Safety protection: Hold the bottom line of high-voltage operation

Cable fault testing (especially impulse flashover testing for high resistance faults) involves thousands or even tens of thousands of volts of high voltage, and safety protection measures must be strictly implemented:

Execute the process of "power outage - electricity inspection - hanging grounding wire":

Before testing, it is necessary to confirm that the cable has been completely disconnected from both ends of the power supply (including the upper level switch and transformer circuit), and load testing is strictly prohibited;

Use a voltage tester that matches the rated voltage of the cable (such as a 10kV voltage tester for 10kV cables) to test the electricity at the testing end and remote end respectively. The testing time should not be less than 1 minute to confirm that there is no residual charge or risk of incoming electricity;

Install temporary grounding wires (with a cross-sectional area of ≥ 25mm ² copper core wire) that meet the specifications between the test cable core wire and the ground wire, and between the remote cable core wire and the ground wire, respectively, to form a reliable grounding circuit.

Personnel protective equipment in place:

Testers must wear insulated shoes (withstand voltage ≥ 10kV), insulated gloves (selected according to the test voltage, such as 35kV insulated gloves required for 35kV testing), and an additional insulated safety helmet during high-voltage testing;

It is strictly prohibited to touch the test terminals, exposed leads, or output terminals of the high-voltage generator with bare hands. When adjusting the wiring, the high-voltage power supply must be disconnected and discharged first (discharge the cable core wire with a discharge rod for at least 30 seconds).

On site control is not lax:

Set up a 1.2-meter-high hard fence in the testing area, hang a warning sign saying "High voltage danger, no entry", and the fence radius should cover the range of high voltage leads (usually ≥ 5 meters);

During the testing period, unrelated personnel are strictly prohibited from entering the controlled area, and unauthorized removal of grounding wires, changes in wiring, or adjustment of instrument parameters are strictly prohibited.

3、 Testing process: Standardize operations to ensure accurate data

The standardization of testing operations directly determines the accuracy of fault localization, and special attention should be paid to "fault prediction, parameter setting, wiring, and repeated verification":

Predict the type of fault first: Use a megohmmeter to preliminarily determine the nature of the fault and avoid testing method mismatches:

If the insulation resistance is less than 10M Ω (low voltage cable) or less than 100M Ω (high voltage cable), and the core wire is conductive with the ground wire/other core wires, it is a low resistance/short circuit fault and is suitable for testing using low voltage pulse method;

If the insulation resistance is greater than 100M Ω, but insulation breakdown (flashover) occurs after applying high voltage, it is a high resistance/flashover fault and needs to be tested using the impulse flashover method (in conjunction with a high voltage generator).

Accurately set instrument parameters:

Propagation speed: Set according to the cable type (such as cross-linked polyethylene cable with a propagation speed of about 172m/μ s at room temperature and polyvinyl chloride cable with a propagation speed of about 160m/μ s). If the ambient temperature deviates from 20 ℃, it needs to be corrected (the propagation speed decreases by about 1% for every 10 ℃ increase in temperature);

Sampling frequency: It is necessary to meet the requirement of "sampling frequency ≥ 2 times the frequency of the reflected wave corresponding to the maximum length of the cable". For example, for a 1000 meter cable, the round-trip time of the reflected wave is about 11.6 μ s (calculated at 172m/μ s), and the sampling frequency should be ≥ 200MHz to avoid waveform distortion.

Standardized wiring and waveform verification:

The test lead should be firmly connected to the cable core and ground wire, and the joint should be sanded with sandpaper to remove the oxide layer, ensuring that the contact resistance is less than 0.5 Ω to prevent the introduction of additional reflected waves;

The same fault point needs to be tested 3-5 times to compare waveform consistency: if the waveform is stable (the position of the fault reflection wave is fixed and the amplitude is consistent), it indicates that the data is reliable; If the waveform fluctuates greatly, it is necessary to check the wiring or eliminate electromagnetic interference.

4、 Data processing: precise analysis to eliminate interference

The analysis of test data needs to distinguish between "effective waveforms and interference waves" and make corrections based on actual situations

Accurately identify waveforms:

In the low-voltage pulse method, the position of the "end reflection wave" of a normal cable matches the design length of the cable (calculated based on propagation speed), and the amplitude is opposite to the incident wave; The reflected wave at the fault point will appear in advance, and the amplitude is related to the type of fault (negative reflection for short circuit faults and positive reflection for open circuit faults);

It is necessary to eliminate the interference of "joint reflection wave": the amplitude of joint reflection wave is usually small (about 10% -20% of the incident wave), and the position is consistent with the known joint distance, which cannot be misjudged as the fault point.

Correcting environmental and installation impacts:

If buried cables pass through areas with high soil moisture or temperature differences, the propagation speed needs to be corrected in sections (insulation dielectric loss increases when humidity is high, and propagation speed decreases by about 0.5%);

If the cables laid through pipes or cable trays have bends (bending radius<10 times the outer diameter of the cable), the length calculation should be adjusted according to the actual laying path (not directly using the designed straight-line distance) to avoid positioning deviation.

Complete data recording: The recording content should include: testing time, environmental temperature/humidity, cable model/length/laying method, testing method (low-voltage pulse/impulse flashover), instrument parameters (propagation speed, sampling frequency), waveform screenshots, fault location distance, for subsequent traceability or re evaluation.

5、 Closing verification: avoid incomplete repair or misoperation

After fault location, it is necessary to ensure no omissions through "secondary confirmation, safe excavation, and repair testing":

Secondary confirmation of fault location: Based on the test distance and cable laying drawings, use an acoustic magnetic locator to scan near the estimated fault point (buried cables need to be tested with a probe), confirm the deviation between the fault point and the test data (allowable deviation ≤ 1 meter), and avoid accidental excavation caused by cable offset or joint misalignment.

Safe excavation and repair:

When excavating, priority should be given to manual excavation (direct excavation of soil above the cable by machinery is prohibited). After excavating to the surface of the cable (about 50cm depth), stop mechanical operation to prevent damage to the cable insulation;

Fault repair (such as replacing joints and repairing insulation) must comply with cable construction specifications, for example, cross-linked polyethylene cable joints must use heat shrink or cold shrink processes to ensure insulation strength meets standards.

Testing and power transmission after repair:

After repair, first use a megohmmeter to measure the insulation resistance (the insulation resistance of high-voltage cables should be ≥ 100M Ω, and low-voltage cables should be ≥ 10M Ω);

Then use the low-voltage pulse method to test and confirm that there are no residual fault reflection waves;

Before dismantling the temporary grounding wire, it is necessary to conduct another electrical test. When restoring power supply, a no-load test run (test run time ≥ 5 minutes) should be carried out first. After no abnormalities are found, normal operation can be resumed.