This type of control is used in heavy commercial and industrial refrigeration. They have two lines with flare nut connections: one is connected to the compressor oil line (usually located directly on the compressor), and the other one connects to the suction line.
Because the discharge line pressure causes some amount of oil to leave the compressor and circulate with the refrigerant, this oil must be returned to the compressor from the compressor suction line to maintain proper lubrication. If the low-side pressure goes higher than the oil pressure, the oil will not be circulated back to the compressor.
If, for any reason, the oil pressure drops below the low-side pressure, the pressure switch in the control will shut off the power to the compressor.
This type of control operates the same way as the low-pressure control in figure 94. There are two bellows (or diaphragms) within the pressure control which are set to sense a safe pressure difference between the suction-line pressure and the oil pressure. If this pressure difference drops below a predetermined point, the mechanism will open the circuit to the compressor.