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This voltage detection relay is located in the top-left of the feeder cabinet.  It checks for phase imbalances that may cause damage to the frequency drives in the cabinet.

======  How it works ======

  * The top, spin wheel (gray) needs to be set to ideal incoming voltage.  In our case, 480V
  * The second, spin wheel needs to be set to the desired amount of delay before the relay opens up and drops out the profibus network power contactor. Fully counter-clockwise is 3/10 of a second, fully clockwise is 30 seconds.  You want this to be a long enough time so that you can check phase-to-phase voltage on top of the relay to see how you need to adjust your settings.
  * The third, spin wheel sets the upper limit for overvoltage.  If, for example, your base voltage from the first spin wheel is set to 480V, and this wheel is set to 6V, then the relay will not trip until phase-to-phase voltage exceeds 486V for the time specified by the second spin wheel.
  * The fourth, spin wheel sets the lower limit for undervoltage  If, for example, your base voltage from the first spin wheel is set to 480V, and this wheel is set to -6V, the the relay will not trip until phase-to-phase voltage drops below 474V for the time specified by the second spin wheel.

===== Why it kills the machine =====

The contact on this relay picks up the coil on the contactor that supplies voltage to the profibus network.  The profibus network talks to all of the drives and servo motors throughout the whole machine.

One of the PLC's checks for communication on this network and if it cannot talk to it, will remotely trip the main cabinet breaker because the machine cannot be safely run without communication/feedback.