(see also https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=44510)
I found that a 480V motor wire on the secondary side of the Compressor #1 contactor had wiggled loose enough over time so as to be only intermittently in contact with its screw terminal (see attached pict). The resulting phase imbalance when not in contact must have tripped the contactor's integrated circuit breaker. The fix was to re-land this wire and then to reset the breaker. While de-energized I went ahead and tested the other screw terminals and found a few which were also quite loose -> these then were re-torqued.
Unrelated but worth mentioning here is that both Compressor #1 and Compressor #2's 1/4 turn ON/OFF panel switches are mechanically broken such that the panel door can be opened with either or both of these in the "ON" position. At some point in the past, this door must have been forced open with these in the ON position. The design intent of these are to only allow the panel to be opened while in the "OFF" position.
Kyle, that definitely goes to show the value of the annual Thermal Imaging checks that we started doing at LLO on all Motor Starters. We started Thermal Imaging all motor starters after one of the Chillers at LLO tripped due to a pitted contact causing the Contactor to overheat and the overloads to trip . This occurred two years ago, and since, we've implemented an annual Thermal Imaging FAMIS Task to check all starters onsite to identify overheating issues from loose wiring/terminals to pitted contacts or bad components. Not sure if LHO has a similar program, but maybe something to consider if not. I use a FLUKE 279FC Multimeter that is capable of Thermal Imaging as well as performing the usual multimeter functions. The images can then be downloaded and put in an annual report and stored on the DCC (See DCC Document #https://dcc.ligo.org/LIGO-E1700301) which then gives you the capability to compare to previous years and identify any starters that may be trending bad and require maintenance. Repairs can then be scheduled at a convenient time without the hassle of an unexpected outage. Hope this helps for the future.
Paul C.