What is a reverse acting pressure switch?

Reverse acting pressures switches are designed with contacts normally open (N.O.). The contacts open on falling pressure and close on high pressure (rising air or water pressure). 

Reverse acting pressures switches are used in several different applications:

• Designed to ground ignition on gas driven pumps and compressors when desired maximum pressure is reached.
• Can act as a low-pressure alarm.
• Turn off motors in low pressure situations, or for remote switch circuits that "make contact" to turn a motor off to prevent pump operation at low pressure.