A harness that leaves the factory with one wire in the wrong connector cavity will pass every visual check and still fail on the vehicle or coach. Finding it on the assembly line costs a stoppage; finding it in service costs far more. Harness testing exists to catch that class of defect, along with poor crimps and damaged insulation, before dispatch. The checks fall into three groups.
Crimp quality
Most harness failures in service start at a crimp. A crimp that is too loose raises contact resistance and heats up under load; one that is too tight cuts strands and cracks under vibration. Crimp quality is controlled by measurement rather than by eye.
| Check | What it shows | When |
|---|---|---|
| Crimp height (and width) | Whether the terminal has been compressed to the terminal maker’s specification | At each set-up and at intervals during the run |
| Pull-off force test | Mechanical strength of the conductor crimp against the specified minimum for the wire size | At set-up and on samples |
| Cross-section (micrograph) | Strand compaction, voids, cut strands and symmetry of the crimp barrel | On new tooling, new wire and terminal combinations, and periodically |
| Visual inspection | Bellmouth, conductor brush, insulation grip and no strands outside the barrel | Every crimp or on a defined sample |
Terminal makers publish the crimp height and pull-off values for each terminal and wire size. On Indian Railways work, crimping sockets are specified to EDTS-200, EDTS-201 and ICF drawings, and the harness drawing will name them.
Electrical tests on the finished harness
Continuity testing should be 100%, on every harness, using a fixture that mates with each connector. It confirms that every circuit runs from the right cavity to the right cavity, that no circuit is open and that no two circuits are shorted. Automated testers also catch miswires that a simple buzzer check would miss.
Insulation resistance and high-voltage (dielectric) tests confirm that the insulation between circuits and to ground has not been damaged during assembly. The test voltage and duration come from the harness drawing or the customer specification and should never be set higher than the connectors and wire can take.
Functional checks follow where the drawing calls for them: connector lock and secondary lock engagement, seal presence, polarisation, and the position of branch take-outs against the formboard.
Workmanship standard
IPC/WHMA-A-620 is the widely used acceptance standard for cable and harness workmanship. It defines three product classes, with Class 3 the most demanding for high-reliability equipment. Where a customer specification names it, the class should be stated on the purchase order.
Records a buyer should ask for
Ask for the test report to be traceable to the harness serial number or batch and to the drawing revision, with the crimp set-up records for the batch. First-article inspection, where every dimension and circuit is checked and signed off before series supply, should be agreed at the start of a new part number.
APAR’s locomotive harnesses are assembled to the customer’s schematic, fully terminated and continuity-tested before dispatch. Automotive and EV assemblies are covered under harness for automotives and EV.