Electric Pressure Switch

This is the compact pressure switch used on small and medium commercial systems, where a KP-size controller is more than the machine needs. The bodies are small, the wiring is by spade terminals, and the unit mounts directly on a 1/4 inch flare or capillary connection at the compressor.

Four high pressure and four low pressure variants cover the usual settings. The H models (H-01 to H-04) are normally closed on pressure rise, opening at 2.5 to 3.0 MPa and reclosing at 2.0 to 2.4 MPa - these are the discharge side cut-outs. The L models (L-01 to L-04) are normally open, closing at 0.02 to 0.3 MPa and opening again at 0.15 to 0.45 MPa, which is the low suction protection and pump-down arrangement.

The two families are deliberately different in contact form: a high pressure cut-out has to open the circuit on fault, and a low pressure control has to close it when pressure is present. Wiring an L unit in place of an H unit - which happens when stock is mixed - gives a compressor that runs with no high pressure protection at all.

Values are given with tolerances because these are non-adjustable switches: the switching point is set at manufacture and repeated to plus or minus 0.06 MPa on the high side. That is fine for protection duty, and it removes the possibility of someone on site winding the setting out of range.

Fit with the connection vertical where possible, keep the capillary away from hot surfaces, and check the spade terminals for tightness at every service - a loose terminal on a pressure switch is indistinguishable from a failed switch, and it is far more common.