This is a little out of my area. I don’t have a lot of practical experience with three phase power systems. But isn’t this relay designed to find a phase being overloaded and therefore low voltage on that phase. Pulling the fuse for a single power phase eliminates the the supply for that phase but also eliminates the loading effect for the transformer. Of course two phases of a 3 Phase transformer are very capable of synthesizing the missing phase especially under no critical load conditions, most people don’t realize that transformers are very dynamically self-correcting. The poweri nteraction of the 3 phases are inseparable and proportional to the power being pulled out of them. The resilience of the polyphase system was a major reason for its development. I think perhaps the system under question would respond very quickly to a real overload condition, of a single phase, especially because that will pull power out of the transformer beyond its ability to self-compensate. Opening the supply side of a single phase, shouldn’t present a critical condition especially under no power draw conditions. Having read a lot about Tesla’s development of polyphase power distribution systems this is my take on it, I may be completely wrong, but it is also a little difficult to understand the configuration of the system being described, and the relay being decribed. Of course if one really wants to make sure all supply side phases are present, one would have to measure them separately, no single relay could perform the task, but the testing relay circuit sounds like it would trip under most real world overloading conditions since it measures a missing phase under load.