Reliability fire pressure switch

A fire pressure switch comprising:

• a case having a first down-hole end and a second up-hole end and an axial passage therethrough
• the axial passage having at least two sections having different diameters;
• an electrically conductive piston having a down-hole end and an up-hole end, the second end having a recessed portion;
• a portion of the up-hole portion of the piston surrounded by an injection molded sheath;
• the piston having one or more circumferential grooves which are filled with insulating material comprising portions of the sheath;
• the sheath frictionally engaging a portion of the axial passage of the case;
• a space between the up-hole end of the piston and the down-hole end of an insert;
• a lubricant in the space comprising approximately 0.0065 ounces of lubricant;
• a conductive rigid dart having a conical down-hole head and a tapered up-hole tail;
• a portion of the tapered tail surrounded by a spring;
• the spring maintaining the dart in electrical connection with the piston and permitting movement of the dart between a position which is electrically insulated from an insert and a position which is electrically connected to the insert;
• the dart having an insulating sleeve surrounding a portion of the tail;
• an insert having an electrically conductive inner portion and an electrically insulated sheath and having an axial passage having a diameter larger than the diameter of the dart;
• the electrically insulating sheath having a uniform outer diameter which is larger than the interior diameter of a corresponding portion of the case by between 0.001 and 0.01 inches.