I have a situation where a customer reported an extremely hot 600 Amp disconnect which feeds a chiller. Opening the disconnect, the wire insulation was burned around 8" up the cables on C Phase. The heat coming from the area around the 2 sets of 300 MCM CU feeding this phase felt like it was coming from a burner. An infra-red spot temp gauge measured 260-280 deg F at the wire termination which was screw lugs, well above the wire rating and ambient in the room. The other phases were close to ambient.
Measuring current and voltage, each phase measured close to 400 Amp, and PH to PH voltages were 208V to 210V. The chiller has electronic controls and had never gone off on over-current that we were aware of.
Upon close inspection, the switch knife blade appeared to be pitted and contact with the switch jaws was in an extremely small area. The only repair part available for this disconnect was the switch jaw.
The switch was disassembled on a shut-down as the switch is fed directly from the utility. New lugs were installed, all current-carrying parts were cleaned and reassembled, and the cables were cut back to reach unburned insulation.
Weeks after the repair, the switch is still overheating on the same phase. The owner believes it may be a utility problem, but I can think of no condition of utility power such as voltage fluctuations or surges that would continuously affect the one phase. I don't think 5th harmonics could be involved either as it is a motor load, therefore I never checked the current with a 5th harmonic Amprobe.
I did not reccommend to the owner that a new disconnect be installed as I don't feel at this point it is a problem with the switch. I may put a recording ammeter on the disconnect for a week just to make sure there isn't any intermittent problems we did not catch the several times we measured the current.
Sorry for the long post, but does anyone have any suggestions? :-?
Measuring current and voltage, each phase measured close to 400 Amp, and PH to PH voltages were 208V to 210V. The chiller has electronic controls and had never gone off on over-current that we were aware of.
Upon close inspection, the switch knife blade appeared to be pitted and contact with the switch jaws was in an extremely small area. The only repair part available for this disconnect was the switch jaw.
The switch was disassembled on a shut-down as the switch is fed directly from the utility. New lugs were installed, all current-carrying parts were cleaned and reassembled, and the cables were cut back to reach unburned insulation.
Weeks after the repair, the switch is still overheating on the same phase. The owner believes it may be a utility problem, but I can think of no condition of utility power such as voltage fluctuations or surges that would continuously affect the one phase. I don't think 5th harmonics could be involved either as it is a motor load, therefore I never checked the current with a 5th harmonic Amprobe.
I did not reccommend to the owner that a new disconnect be installed as I don't feel at this point it is a problem with the switch. I may put a recording ammeter on the disconnect for a week just to make sure there isn't any intermittent problems we did not catch the several times we measured the current.
Sorry for the long post, but does anyone have any suggestions? :-?