During insulation testing of a long underground cable, a technician observes higher leakage current compared to a shorter cable of the same type. What is the most likely reason?
The larger insulation surface area provides more leakage paths to ground.
When a d.c. test voltage is applied across the insulation of a cable, a small leakage current flows through the imperfect dielectric from the conductor toward earth. Ideally the insulation would be a perfect non-conductor, but in practice it has a very large but finite insulation resistance, and the leakage current is simply the applied voltage divided by that resistance (Ileak = V / Rins).
The behaviour with cable length is the crux of the question. The insulation forms a continuous cylindrical layer wrapped around the conductor for the whole run of the cable, so:
- The insulation surface area is proportional to the cable length — a longer cable presents more dielectric material and a larger surface between conductor and ground.
- These distributed leakage paths act electrically like many resistances in parallel; adding more of them in parallel reduces the overall insulation resistance.
- With the test voltage V held constant, a smaller Rins gives a larger leakage current — exactly what the technician observes on the longer cable.
So the correct explanation is that the larger insulation surface area provides more leakage paths to ground. The claim that increased length reduces the dielectric strength is wrong — dielectric strength is a property of the insulating material (breakdown field per unit thickness), not something changed by making the cable longer. The statement that leakage current is independent of length contradicts the observation and the parallel-path reasoning. And the assertion that a longer cable has higher insulation resistance is the reverse of the truth: insulation resistance decreases with length (unlike ordinary conductor resistance, which increases with length).
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