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Black Oxidation of Power Distribution Cables

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I'm a power system Electrician and would like to know what makes the copper concentric neutral of power cables turn black in certain areas. In one of our stations where the cables come into the building through the ducts the concentric neutral has all turned black. As cables reach the center of the building it becomes less common but there is still some black coloration. I figure its due to oxidation. The blackest parts of the copper being in areas where water tends to seep into the basement during melts and storms. But why a black coloration? It looks like the cables are overheating turning the copper black but not.

Any insight?

Brian Marsh
Electrical Distribution - Edmonton, Alberta, Canada

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I have seen the same black corrosion of copper wires without knowing exactly what it is.

Sulfides are black, so it could be sulfides. I've seen it on boat wiring, so it could be related to moisture or chlorides. Very finely divided metal particles look black, so it could possibly be some dissolution and redeposition of a few atoms of copper. But unfortunately I can't come up with a unified theory that ties these disparate observations into an answer. Sorry.

Ted Mooney, P.E. finishing.com Brick, New Jersey