It's 2:00 AM, and the replacement transformer has been installed and is ready to go. All you have to do now is verify that the transformer's secondary bus voltage is in-phase with the system voltage. You energize the transformer, and across the racked out secondary breaker stabs you check for zero voltages that will verify that phasing is OK. But the voltages are not zero, and the systems are not in-phase. So what went wrong? What do you do now?