Underground Cable Failures Examined from Shipped Samples, Hawaii (2022)

After three underground cable failures in 2021 at an energy site in Hawaii, the operator shipped us failed cable sections, a failed splice and two soil samples. Our letter report followed in 2022. The operator and the site stay confidential.

Working from samples

We did not visit the site or see how the cable was installed. Everything here comes from the material delivered to us, which we examined in Texas, cutting the cables open to see how each had failed. An outside laboratory analyzed the soil samples.

A straight line of damage

The medium voltage cables looked alike. On each one the damage ran in a single straight line along its length. That fits bend radius damage during installation: when a cable is bent tighter than it is rated for, the insulation and ground wires are damaged along one side, and partial discharge at that spot breaks the insulation down over time until it fails. One cable near a failure also had a broken and melted ground wire, another likely result of overbending.

Bend radius damage usually traces back to excessive pulling tension, missing or incorrect pulling calculations, or a cable that is not controlled as it goes into the trench. We suggested a review of the pulling calculations and, where cable is laid into an open trench, reel tensioners so it cannot unspool and fall back in uncontrolled.

Corrosion on the DC cables

The DC cables showed conductor corrosion consistent with impressed current corrosion, mostly on the cable marked negative. Normally it is the positive conductor that corrodes fastest when exposed to soil, and the site later reported that polarity had been reversed in the field. Once water gets inside the jacket of a DC cable, aluminum oxide forms, and because it takes up more volume than the metal, the cable swells, as these samples did. Scrape marks on the jackets pointed to damage during installation and suggested the cables may have been bedded in unsifted material.

Other possibilities

The soil showed no extreme corrosivity. Termites can attack cable jackets, but they would leave damage scattered along a cable rather than in one straight line, so they were unlikely for the medium voltage cables. For the DC cables they remained possible if found on site, and termite resistant jackets, duct banks and conduit reduce that hazard. The splice showed signs of failure at its ground connection, but it was not recovered in full and its installation details were unknown, so its cause was left undetermined. Cable dissections like these are part of electrical failure analysis.