Back in 80s I was a calibration technician in the US Air Force, working in an Electronic Warfare shop. We had a major expansion that doubled the size of the facility. After the normal new building problems (leaky roof, phones not working, etc.), we had a new, shiny building with plenty of workspace, storage space, a break room, and even a shower.
After about six months, however, some strange electrical problems started showing up. Because this was Florida during the summer, we normally had to shut down for afternoon thunderstorms and lightening advisories to protect the tens of millions of dollars of test stations, avionics, and other systems. It took a while for the patterns to emerge from the normal glitch noise level.
First, my test station started blowing 45A fuses in the disconnect box mounted on the wall. This wasn’t too uncommon, but one of the station power supplies would usually go first. We would replace the fuse and bring the station back up, stage-by-stage, until we found the fault. Sometimes the fuse would just blow -- or so it seemed. The pattern started to emerge when a fuse blew about once a week. We would troubleshoot each time, but couldn't seem to find the problem.
The frequency increased until we were blowing a fuse a day. We then realized that we were only blowing the B phase fuse. At this point, we called in the electrician from base Civil Engineering. His clamp-on ammeter indicated we were only drawing about 17 amps per phase and the phases were fairly well balanced. The CE electrician had run out of 4517A fuses and only had 60A fuses in his kit, so he installed one.
After some lively discussion, we noted the station was on a 50A circuit, and we needed to get back up and running while the electrician ran across base to get the correct fuse. In the meantime, the 60A fuse blew and the 50A breaker didn’t. This time, instead of sending it straight to the trash, I picked up the fuse from the fuse puller to check the rating and burned my fingers.
That was the aha moment. We weren’t blowing the fuses, we were melting them! After shutting the main breaker off, I started probing and prodding the disconnect box, and I found some discoloration on one fuse holder clip and the terminal lug underneath it. The ohmmeter indicated more than 1 ohm on this heavy gauge connection. P= I²R. The 17A draw was enough to cook the fuse until it melted. A closer look revealed water marks inside the disconnect box. The box was mounted on what was the outside wall before the expansion.
The new roof had overlapped the old one, resulting in numerous leaks when we moved in. Water had gotten into the box and corroded the contact. The leaks had been fixed and the damage done before we even moved in.
The electrician returned about an hour later with the new fuses. I explained my findings and indicated that the box needed to be replaced, even after cleaning up the contacts. After three more electricians were called out to view the problem, the senior CE electrician finally agreed and the box was replaced about a week later.