I would go with the phase loss monitor (lotsa names for the same thing) as NORCAL suggested. I would also add a Time Delay Operate relay set to at least 5 minutes to prevent short cycling. The PLM looks for phase loss, imbalance, or reversal. A set of contacts from the PLM feed the timer at the HVAC control voltage. This wouldn't be necessary if the controls are already of a smart type that protect compressors. You could drive an indicator from the timer's NC contacts so that building maintenance staff can see that you are waiting for the power to be on long enough before turning on the AC.
Joe
