The test box, built of wood protected by plastic laminate, is filled with damp soil. The box is several feet long, about a foot deep and about a foot wide. A copper screen running the length of the bottom of the box provides a ground plane. Cable samples about a foot long pass through holes in the side of the box and are covered with soil. Corrosion site are controlled Cofer Technology Center engineers make identical insulation defects in the cable samples with a falling weight heavy enough to contact the aluminum conductor.
Marginal damage gives widely varying results.
In earlier tests, samples with apparently identical small amounts of damage behaved very differently. One sample showed severe corrosion overnight, with high leakage currents, hydrogen gas evolution, heating of the soil and arcing damage to the insulation. A similar sample showed very little leakage after months of testing. These differences may help explain why some cables last for long periods and then suddenly fail due to corrosion. Later tests have increased the amount of damage to the cable to ensure that corrosion occurs in a reasonable amount of time.