What is a possible cause for a seized-up first stage compressor in a multi-zone, two-stage roof top A/C unit?

A faulty crankcase heater can lead to a seized-up first stage compressor in a multi-zone, two-stage rooftop air conditioning unit because its primary function is to prevent refrigerant from pooling in the compressor's crankcase during periods of inactivity. When the compressor is not running, refrigerant can migrate back to the compressor and, if the crankcase heater is not functioning properly, this refrigerant can condense into a liquid state. If a sufficient amount of liquid refrigerant accumulates, it may create hydraulic lock or slugging conditions when starting the compressor, potentially leading to it seizing up due to oil being displaced or inadequate lubrication during the start cycle.

The other options present causes that may affect the compressor's operation but do not directly contribute to a seizure. For instance, a plugged equalization line or oil level regulator could create pressure issues but might not lead directly to seizing. Low voltage to the first stage motor might cause the motor to underperform or fail to start, but it would not necessarily cause a mechanical seizure. A faulty first stage cooling relay could lead to operational issues or cycling problems, but again, it doesn’t inherently cause the compressor to seize. Thus, understanding the role of the crankcase heater clarifies how its failure can lead