Essential knowledge for wire and cable inspection - Voltage tests for insulated wire cores and finished cables

I. Inspection Purpose

The basic method of voltage testing is to apply a voltage value that is a certain multiple higher than the working voltage to the insulation of wires and cables and maintain it for a certain period of time. It is required that the sample can withstand this test without breakdown. Therefore, the withstand voltage test is the most fundamental performance test, and the vast majority of withstand voltage tests use power frequency alternating voltage.

The purpose of the withstand voltage test is to assess the reliability of the product's operation under working voltage and to detect serious defects in insulation. Most of the withstand voltage tests for wires and cables are routine tests.

Ii. Scope of Application

It is applicable to the test of wire and cable products withstanding power frequency AC voltage.

Iii. Inspection Basis

Relevant product standards: GB/T 3048.8-2007 and related product standards.

Iv. Test Equipment and Apparatus

High-voltage test benches, flashover breakdown test instruments, etc.

2. Sink (if necessary).

V. Sampling and Sample Preparation (Type Test)

For the finished product voltage test, take a sample of no less than 10.5m. Remove the covering at one end of the sample to expose approximately 20mm of the conductor.

2. For the core voltage test, take a sample of no less than 5.5 meters. Remove the sheath or brbrated layer and any other covering or filling materials. The insulated core must not be damaged. After peeling off one end of the sample, the length of the exposed conductor should be approximately 20mm.

Vi. Test Procedures

When conducting the test in a water tank, the length of the two ends of the sample extending beyond the water surface should be no less than 200mm. It is necessary to ensure that no flashover discharge or internal breakdown occurs under the specified test voltage (the length of the sample in water is 10m for the finished product and 5m for the insulated wire core, with the rest exposed above the water surface). The immersion time in water should be no less than 1 hour. The water temperature during the immersion test should be maintained between 20±5℃ (as stipulated in GB/T 5013 and GB/T 5023).

2. Wiring: Connect the high-voltage terminal to the conductor of the cable, and the ground wire to the remaining conductors, water (if any), or the shielding layer.

3. Connect the power supply and apply voltage as specified in the product standard. The voltage should be applied successively between each conductor and all other conductors and metal layers (if any) or water connected together, and then between all conductors and metal layers or water connected together.

4. When applying voltage, it should be slowly and steadily raised to the specified voltage value and continuously maintained at the voltage value specified in the product standard.

5. If the cable has not been punctured by the specified time, the voltage test is passed.

6. Voltage test for multi-core cables with more than five cores.

The wiring method should ensure that each wire core and its adjacent wire cores undergo at least one power frequency AC voltage test as specified in the product standard. The first time is to apply voltage between the odd-numbered cores (in parallel) and the even-numbered cores (in parallel) in each layer of the wire core. In the second time, voltage is applied between all odd-numbered layers (in parallel) and even-numbered layers (in parallel). If the number of cores in the same layer of the cable is odd, a specified voltage test should be conducted again between adjacent layers or between adjacent cores that have not undergone the voltage test. Then the voltage is applied between all the conductors and metal layers or water that are connected together.

For example: For a coil of 60227 IEC 53 (RVV) 3×1 common polyvinyl chloride sheathed flexible wire with core colors of blue, brown and yellowish green, the voltage test steps using the ZNY-5 flashover breakdown tester are as follows:

Take 10.5m of the finished cable and 5.5m of the insulated core. Strip off the insulation at one end to expose approximately 20mm of the conductor.

(2) Place the sample in the water bath, with the length of both ends of the sample exposed above the water surface being 25mm, and immerse it in water for 1 hour.

(3) Wiring insulation core voltage test: Connect the high-voltage terminal to the blue core, brown core, and yellow-green core conductors of the insulation core respectively, and connect the grounding terminal to a copper rod (water) for the test three times.

① Connect the high-voltage terminal to the blue core, and the grounding terminal to the brown core, yellow-green core and copper rod (water).

② Connect the high-voltage terminal to the brown core, and the grounding terminal to the blue core, the yellow-green core and the copper rod (water).

③ Connect the high-voltage terminal to the yellow-green core, and the grounding terminal to the blue core, brown core and copper rod (water).

④ Connect the high-voltage terminal to the insulated core conductor (with the blue, brown and yellow-green cores twisted together), and connect the grounding terminal to a copper rod (water).

(4) Connect the power supply.

(5) First, turn the switch to the "Auto" position and set the time relay to be powered on for 5 minutes.

(6) Step on the foot switch, press the "Start" button, and slowly and steadily increase the voltage to 1500V of the insulated wire core or 2000V of the finished product. If it does not break down within 5 minutes, it is qualified.

(7) If breakdown occurs, the "Breakdown" light will be on.

(8) Turn off the power supply and the test is completed.

Vii. Evaluation of Test Results

If there is no breakdown phenomenon in the sample within the specified test voltage and duration, it can be considered that the sample has passed the voltage test.

The sample length, immersion time, water temperature, applied voltage and the duration of voltage application for various insulated wire cores and finished cables are different. Relevant product standards should be referred to for implementation.