Q: I'm not able to get a reading from phase to ground on one of my phases using the Safe Test Point, why is that?

A: The explanation is that the system that the Safe Test Point is being used on is more than likely a corner-grounded delta. Because one of the phases is grounded (typically B phase), it is at the same potential as GND and will therefore show 0V.

You would receive expected values for phase to phase on all ports (IE ~480VAC). Two of the phase to ground readings would read the same voltage (IE ~240VAC). The ground phase to GND reading will be 0V.