Vintage Air (www.vintageair.com, 800-862-6658), sells a trinary switch which turns off the compressor if the pressure falls below 30 psi or rises above 406 psi.

These fans are wired in through a trinary switch that controls when the fans start and stop both by water temperature and A/C system pressure. See All 13 Photos 9 Grunt!

Since an air conditioner removes heat from an enclosed area, ... A trinary switch (PN 11076-vus) combines high/low pressure protection with an electric fan operation signal at 254 psi.