9013FSG2J21M4 is a Pumptrol series, standard duty, diaphragm type pressure switch with LPCO (Low Pressure Cut-Off) lever and 30-50 psi factory pressure setting (w/ adjustable cut-out & cut-in pressure and adjustable pressure differential).
This model is a variant of the very popular 9013FSG2J21 switch, but with LPCO functionality, which turns the pump off when the system pressure falls to about ~10 psi below the cut-in pressure (which is 30-10=20 psi for this model). LPCO is essentially a flood protection feature, which senses low pressure in the system (caused by burst pipes and similar) and turns the system off to prevent major flood damage.
This pressure switch is suitable for a wide range of applications, including electric water pumps (deep well submersible, jet well non-submersible, etc.), compressors, agricultural & irrigation water systems, injection moulders & others.
When used with deep well submersible pumps (such as Grundfos SQ/SQE & others), the switch is installed in combination with a well tank (Well-X-Trol or similar), well tank tee and other components - pressure relief valve, pressure gauge, etc. On jet well pumps, the switch is commonly installed directly at the pump.
Note:Due to rebranding at Schneider Electric, Square D is now Telemecanique. The product itself is unchanged.