Have you ever wondered whta's inside of a feeder transformer?
This one was humming loudly on a gig, so a technician was called out to investigate. Transformers can hum for different reasons. It could be caused by high harmonic loads, loose fittings, aging insulation, or some other issue. In this case, the tech turned off power to the transformer, locked it out, removed the front panel, and started looking for clues. First, he used a multimeter to check for continuity between the neutral and the chassis of the transformer. There was continuity. Next, he checked for loose fittings. In the process he discovered that the iron core was extremely hot, even though the transformer was lightly loaded. He concluded that the age of the transformer (about 15 years) was contributing to it's failure and that it should be replaced. I took the opportunity to take some pictures.