Not sure if this belongs in electrical or plumbing but I will take a chance here. Woke this morning to no hot water heater, water heater was cold. 3 year old AO Smith, 40 gallon electric. Power to heater OK, red reset button on upper t-stat tripped. Pushed in and got power to the upper element. Went to the hardware store and bought a new upper and lower t-stat. (Figured I do them both as a precaution.) Changed upper t-stat, turned power back on and tested that I was getting power to the upper element. Turned power off and went after lower t-stat. I noticed that the connection on the lower element was black and there was some melting on the fiber glass insulation. The connection was loose and apparently been arcing. Cleaned up the connection, replaced the terminal screw and re-terminated the wire. Closed it up and turned the power back on. After an hour I checked and the upper t-stat had been satisfied and the lower element was firing. All seems well.
Question is, could the loose, arcing connection on the lower element have caused the reset on the upper element to trip or is this just a coincidence?
Yes it can. The upper t-stat supplies electricity to the lower one when the top of the tank is hot. The wiring goes to the lower t-stat and if the terminal is loose and arcing it will cause a draw high enough to trip the upper t-stat.