What are the testing items for electrical wires and cables? What are the testing methods?

What are the testing items for electrical wires and cables? What are the testing methods?

一、Finished Product Testing of Electrical Wires and Cables

The finished product testing of electrical wires and cables is mainly divided into three types of testing items:

1. Appearance, Dimensions, and Structure Inspection

2. Electrical Performance Testing

3. Mechanical Performance Testing

Appearance, Dimensions, and Structure Inspection of Electrical Wires and Cables

The appearance, dimensions, and structure inspection is the first step in quality control for electrical wires and cables. It is divided into three main aspects: appearance inspection, dimension inspection, and structure inspection.

1. Appearance Inspection

The purpose of appearance inspection is to ensure that the surface of the wire or cable is smooth and free of burrs, cracks, spots, oil stains, or any other defects and impurities that may affect normal operation. Additionally, it ensures that the level of corrosion and oxidation meets the required standards.

2. Dimension Inspection

Dimension inspection includes measurements of outer diameter, thickness, eccentricity, sector height, pitch, cross-sectional area, and density. Samples are taken to measure insulation thickness and conductor diameter. The sampling method requires selecting three sections of wire at 1-meter intervals, and measurements are taken using a micrometer. The insulation thickness of various types of wires and cables should not be less than the specified standard, and the diameter deviation of different wire sizes should not exceed 1% of the nominal diameter.

3. Structure Inspection

Structure inspection includes cross-sectional inspection, sheath inspection, cable core structure inspection, and insulated core inspection. This final inspection combines both appearance and dimension checks, ensuring that the cable has no visible defects while also verifying that its dimensions are within acceptable tolerances.

Testing Items and Methods for Electrical Wires and Cables

1. Electrical Performance Testing

Electrical performance is a critical factor for wires and cables, directly affecting their usability. The main tests include DC resistance testing, voltage tests, and insulation resistance testing.

1.1 DC Resistance Testing

DC resistance testing is conducted based on the resistance law:

where ρ is the resistivity, L is the conductor length, and S is the conductor cross-sectional area.

Testing Method: Uses single-arm or double-arm DC bridges

Measurement Range:

o Double-arm bridge: ≤1Ω

o Single-arm or double-arm bridge: 1Ω – 99.9Ω

o Single-arm bridge: <100Ω

Standardized Resistance Conversion at 20°C:

R20=1000Rx1+α(t−20)LR_{20} = \frac{1000R_x}{1 + \alpha (t-20)L}R20​=1+α(t−20)L1000Rx​​

where Rₓ is the measured resistance, α is the temperature coefficient, and L is the test sample length.

Key Considerations:

o Temperature variation during testing should not exceed ±1°C.

o The thermometer should be placed at least 1m above the ground and within 1m of the sample.

o Current density limits:

§ Aluminum core: ≤0.5 A/mm²

§ Copper core: ≤1 A/mm²

1.2 Voltage Testing

The insulation strength of a wire or cable depends on its insulation structure and material, which determine its ability to withstand electric fields without breakdown. Voltage tests ensure the safety of electrical wires and cables.

· Test Method: Apply a specified voltage under certain conditions for a set duration and observe whether breakdown occurs.

· Precautions:

o The test area should have metal grounding fences and emergency protective measures.

o The grounding electrode resistance should be ≤4Ω.

1.3 Insulation Resistance Testing

Insulation resistance is a key indicator of the insulation properties of electrical cables, reflecting their ability to withstand electrical and thermal breakdown as well as insulation material durability.

2.1 Mechanical Strength Testing

Mechanical strength refers to a cable’s ability to resist external forces without breaking.

· Key Parameters:

o Tensile Strength (σ): σ=FmS\sigma = \frac{F_m}{S}σ=SFm​​

o Elongation at Break (γ): γ=(L1−L0)L0×100%\gamma = \frac{(L_1 - L_0)}{L_0} \times 100\%γ=L0​(L1​−L0​)​×100%

o Common tests include:

§ Strength and elongation tests for copper and aluminum wires.

§ Strength and elongation tests for insulation and sheath materials before and after aging.

2.2 Bending Performance Testing

Bending stress during production and usage affects cable quality and lifespan.

· Test Method: The sample is bent repeatedly on a bending test machine until breakage occurs. The total bending cycles before failure determine its bending performance.

2.3 Torsional Performance Testing

· Purpose: Evaluates the plastic deformation ability and detects internal structural defects.

· Test Method: Measures the number of twists before breakage to assess compliance with standards.

2.4 Winding Performance Testing

· Purpose: Ensures good winding properties of metallic wire materials.

· Test Method: The sample is wound around a specified diameter mandrel for a set number of turns, and surface changes are observed.

3. Process Inspection

Process inspection is crucial for ensuring final product quality. Although the manufacturing process of electrical wires and cables is not overly complex, it involves numerous steps, including:

· Wire drawing process

· Annealing process

· Conductor stranding process

· Insulation extrusion process

To ensure quality, each step should undergo scientific and comprehensive inspections, which typically include:

1. Self-inspection during work

2. Mutual inspection between processes

3. Routine inspections

4. Final inspections

5. First inspection at the beginning of shifts