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The “Other” Pressure Switch
The chart above is from a Carrier heat pump, and it lists some common accessories.
I have heard this common pressure switch (last on the list) called a low-pressure switch (because it opens on fall in pressure) and a high-pressure switch (because it's in the liquid line), but RARELY do I hear it called its true name.
Loss of charge switch
In a heat pump system, the operational suction pressure can vary greatly from the suction you will run on a hot day in cool mode to the suction you will run on a cold winter day in heat mode. That makes a traditional low-pressure switch a challenge.
The loss of charge switch performs much the same role as the low-pressure switch, but since it is in the liquid line in cool mode (before the metering device and after the condenser) and the expansion line in heat mode (after the metering device and before the outdoor coil), it will not be as prone to fluctuations. It will only keep the system off in cases of very low charge or “loss of charge” in cool mode.
In heat mode, it will open during a loss of charge but also if there is a line drier or heat mode metering device restriction.
All in all, it's pretty easy to understand, so long as you know its true identity and purpose.