Case Study: On-Site Troubleshooting of Poor Harness Insulation

1. Case Background

We received an urgent request from a long-standing customer in Wuxi. The customer processes harness terminals, and a batch of finished harnesses shipped to an appliance customer in Guangdong was returned as a whole batch because insulation performance did not meet requirements. The customer initially suspected that the recycled plastic used for terminal housings had an insulation defect. After discussion, we took the AT6820 insulation resistance tester to the site to verify and locate the cause.

2. On-Site Inspection Process

1. Testing the Returned Harnesses

With the AT6820 set to a test voltage of 500V, the returned non-conforming harnesses were tested directly: many samples showed insulation resistance of only tens of MΩ, well below the customer's outgoing acceptance standard of ≥100MΩ, and there was no visible damage or cracking.

2. Comparison with New Original Terminals

Unopened, unassembled new terminals from the warehouse were taken and tested separately; their insulation resistance all reached the GΩ level, far above the acceptance standard, which ruled out the initial assumption that the recycled material itself has poor insulation.

3. Humidity Verification Test

Considering Guangdong's rainy, highly humid environment and the tendency of harnesses to absorb moisture when stored after processing, the terminal areas of the returned harnesses were dried with a hair dryer for several minutes; insulation resistance then returned entirely to the standard range and the tests passed.

3. Determination of the Root Cause

The batch insulation failure was not caused by the quality of the terminal plastic raw material. The real causes were:

1. The terminal plastic parts readily absorb moisture, and the air in the production workshop, warehouse and transport environment is very humid, so water vapour is absorbed inside the terminals;

2. After harness assembly, the cavity sealing is poor and water vapour remains on the contact surfaces of the harness terminals;

3. Guangdong has high temperature and humidity all year round, so water vapour cannot evaporate during transport of finished products; leakage current increases during the 500V insulation test and insulation resistance drops sharply.

4. Improvement Recommendations for the Customer

1. Moisture Control in Production

Injection-moulded and incoming terminals should be assembled immediately after opening, and unused terminals should be stored in sealed moisture-proof bags; dehumidifiers should be installed in the workshop to keep ambient humidity within a reasonable range.