Faulted Aluminum

[ptonsparky]

Doing a trouble shoot yesterday on a well service. Three phase corner ground 240V. I expected a ground fault on one of the phases but we had to wait for the meter and primary fuses to be pulled to make sure the problem was not a loose connection in the can. POCO lineman tested to ground with his Fluke amp clamp ohm meter and found nothing. My TPro found nothing but on reversing the leads it picked up the fault. The Lineman rolled his eyes when I explained that aluminum occasional develops a coating that acts like a diode when it faults to earth. It happens often enough...what is the process called? Oxidation? Is there a name for that layer?

 

[GoldDigger]

Oxidization is involved. The formation of an oxide layer, with or without other elements contributing, can act like a diode. Copper oxide was commonly used to make rectifiers before semiconductors like germanium and silicon.

 

[ptonsparky]

Aluminum Oxide would be too easy.

 

[Tony S]

Galvanic action has something to do with it but I can’t remember where I read about it. If I find it I’ll get back to you.