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A harness worn through against the body, a bracket or an edge, until the conductor makes contact with earth. It is the single commonest mechanical cause of intermittent electrical faults, and it is found by looking rather than by measuring.

The fault it produces depends on how far the chafe has gone. Insulation worn thin gives an intermittent short that appears with vibration, temperature or a wet road; worn through gives a hard short to earth that blows a fuse or pins a signal. A fault that changes with road surface or weather is mechanical, and no static test will reproduce it.

The locations are predictable and worth inspecting directly. Where a loom passes through a bulkhead grommet, crosses a suspension or engine mount, runs behind a wheel arch liner, or is clipped against a bracket that has lost its padding. Anywhere the harness moves relative to what it touches is a candidate. Flex each section by hand while monitoring the circuit, and check where a previous repair or accessory installation has rerouted a loom — a harness that no longer follows its original clips will chafe somewhere new.