Checking for a loss of phase on a transformer or underground 3 phase lines can get tricky. The phases (s) that are still energized do induce voltage into the un-energized phases. If there is sufficient resistive load connected, the voltage will be small. If there is no or little load, then the un-energized phase can have voltage near the same magnitude of the energized phases.

I've ran into this before, we didn't program a pic to solve the issue, but I'm sure it can be done. We used a programmable power monitor to watch the PHASE ANGLE of the incoming power. Ideally 3 phase will have angles 120 degree's apart. In reality it's not unusual to see 120 +/- 1 degree. I won't get into why in this discussion. So if the phases say get to 120 +/- 20 degrees for more than 30-45 seconds, something is wrong and we send the signal to transfer to the alternate source.

Phase angles can and will swing from 120 degrees when there is a fault on the three phase line you are monitoring. For example, somewhere upstream a bird gets across 2 phases, those two phase's will no longer be 120 degrees apart, both phase shift toward each other and synch until the bird is well done and falls clear, then things return to normal. The time it takes for the phases to return to normal depends on the protective relaying. Generally a few milli-seconds to 5 seconds, reasons that can be discussed later if needed.

So the point being, depending on where you are monitoring and where the "source" of the power is (utility substation, generator, etc.), you may want to modify your code accordingly. The primary trigger for the code should be phase angle, secondary would be voltage magnitude. If you’re protecting a motor, you don't want it to run on imbalanced voltage of more than 5% for more than a couple of minutes.

Shane