Cross-linked system
Measurement purpose
Check the ability of the cable outer sheath, the outer sheath of the insulation joint and the insulation clamp to withstand the specified voltage: Check the performance of the nonlinear resistance type sheath overvoltage protector to ensure that the metal sheath of the high-voltage cable can withstand the overvoltage induced on the outer sheath when the cable is subjected to overvoltage: Check the contact resistance of the circuit breaker (or connection piece) in the interconnection box.
Scope of application: During commissioning and pre-test.
The instruments (measurement devices) used: One set of DC generator, a 1000V digital megohmmeter, and a loop resistance tester.
Test procedures
DC withstand voltage test for the outer sheath of the cable, the outer sheath of the insulation joint, and the insulation clamp plate;
Non-linear resistance type insulation overvoltage protector;
For silicon carbide resistors:
After disconnecting the connection lines, apply the specified DC voltage of the product to each of the three groups of resistance plates, and then measure the current flowing through the resistance plates.
Compare the measured values with the product specifications.
Regarding zinc oxide resistors:
Disconnect the connecting wire.
Apply a direct current voltage to the product. When the current in the circuit reaches exactly 1mA, record the voltage at this point, as well as the direct current 1mA reference voltage.
The measured U1Ma should comply with the product specifications.
Measure the ground insulation resistance of the nonlinear resistor chip and its leads:
Connect all the leads of the nonlinear resistor in parallel and insulate them from the grounded housing.
Measure the insulation resistance between the lead wire and the casing using a 1000V megohmmeter.
Interconnected Box
Measure the contact resistance of the switch blade (or connection piece).
Check whether the connection position of the switch blade (or contact piece) is correct.
Analysis and judgment of measurement results
During the DC withstand voltage test of the cable outer sheath, the outer sheath of the insulation joint, and the insulation clamp plate, a DC voltage of 5kV is required to be applied for 1 minute. No breakdown should occur. If breakdown does happen, it indicates that there are local defects such as air bubbles or mechanical damage in the cable insulation.
In the leakage current test of silicon carbide resistors, if the test temperature is not 20°C, the measured current value should be multiplied by the correction factor ((120 – t) / 100), where t represents the temperature of the resistor (in °C).
When the insulation resistance between the nonlinear resistor element and its leads and the ground measured with an ohmmeter is less than 10 MQ, it indicates that the resistor element is damp or has aged.
The contact resistance of the switch (or connection piece) in the interconnection box should not exceed 20uΩ. If it exceeds this value, it indicates poor contact and requires handling.
Test Precautions
During the DC withstand voltage test of the outer sheath of the cable, the overvoltage protector of the outer sheath of the insulation joint and the insulation plate must be disconnected during the test. In the interconnection box, the metal sheaths of the three sections of the cable on the other side are all grounded to enable the insulation plates of the insulation joints to be tested together. Then, a DC voltage is applied between the metal shielding or metal sheath of each section of the cable and the ground.
The measurement of the contact resistance of the switch (or contact piece) in the interconnection box should be conducted after all the tests of the insulation overvoltage protector have been completed.
The test for checking the connection position of the switch blade (or connection piece) should be conducted after the cross-interconnection system has passed the test and before the sealing interconnection box is installed.
Post time: Mar-26-2026