Auxiliary limit switches have two purposes:
• To shut down or operate associated equipment when the EBV either starts to move or has completed its movement.
• To send the valve position indication signals and alarms to remote locations.
The application hardware for installing limit switches on piston actuators for quarter-turn valves has improved significantly. Historically, proximity switches have proven to be highly reliable and are now packaged inside NEMA 4 enclosures which can be stacked on top of the EBV actuator or on the stem of the bypass valve.
SPDT contacts are standard. If the proximity switches are U.L. Listed for Class I, Div 2 areas, no arcing contacts are present; and, therefore, they do not require explosion-proof enclosures and conduit seals. On P&IDs and control drawings, these limit switches should be labeled ZSO and ZSC for position switch open and position switch closed respectively.
For electric and pneumatic motors, auxiliary limit switches normally are microswitches with open or arcing contacts. Typically, four or more adjustable switches are available for open, closed, and intermediate positions.