Got an interesting service call at work today. One lug of a circa 1964 GE 480 3 phase breaker in a feeder panel at our vehicle maintenance facility burned up. Apparently the breaker did not trip right away and caused a single phasing condition and burned up a 10hp 3 phase air compressor motor. After replacing the breaker and checking for additional possible damage, one of the mechanics showed me a burned spot on one of the vehicle lift control panels. The lift did not have power, so I go to the fused disconnect to check the fuse. Well, what I found was that two lifts were wired into this one two pole disconnect, both off the same fuse. One leg was not being used at all. The lift motors are 208-230 volt single phase. Both lifts were connected in what appeared to be a 120 volt circuit. Imagine my surprise when I found 277 volts across the blown fuse!

What apparently happened was that the lifts were replaced within the last two or three weeks. The old lifts were 480 3 phase. The lift installer took one 277V leg from the old disconnect, installed a single fuse in a two pole disconnect, pig tailed both lifts off that one fuse and called it a day. Can you believe these lifts operated as long as they did? Three weeks!

Don’t know if the motors are fried or not yet. I will begin pulling the proper circuits tomorrow and we’ll find out then. I wonder did these 208 volt motors being wired single phase 277 caused the breaker to fail and create the single phasing condition that burned up the 3 phase compressor motor?

You never know what you will find from one day to the next in this trade. Gotta love it!!