Pressure Switches
One of the annoying things about pressure switches..
Bean-counter manufacturers build them but chose not to standardize the industry for safety..
You can connect blue wire to blue, and so on.. and get a coil explosion, as easily as a No Fire at all.
So..
Read the switch terminals.. C = "common"
Use that, and the N.O. terminals "normally Open" = NOT on, until Seeing Pressure..
..a good thing.
Those are the terminals to use.
IF you are wiring into 3 conductor wire.. follow to the other end, away from the switch..
..It probably isn't connected,
..unless it is just a ground (chassis / frame,) NON-current-carrying connection.
Note: the context of the above instructions is of engine driven systems.
Electrically drive, is a different set of options..
..and has reason for the 3rd wire other than grounding.
When you receive a pressure switch, rewire it to match the system you own..
It will be easier to Get control, and the rapid elimination of downtime.