Large transformers can be monitored for developing failure. 1) sample the oil in the unit. Degrading materials, cellulose if I recall, start to build up as the unit ages. 2) measure and release building pressure in the oil tank. Oil can be turned into gas (maybe by electrolysis), as it degrades. You need to change the oil every now an then, and these devices do wear out over time. They can be rebuilt more easily if you do not allow it to ~~become a bomb~~ rupture, leak, and burn. Also, this may be an old transformer full of PCB. We'll see how big of a mess that is now.