Valve limit switches (ZSO / ZSC)

ISA 5.1-2009

Drawn as

Two small bubbles marked ZSO and ZSC on either side of an actuated valve, joined to the stem by a short line.

Meaning

Open and closed position switches on an actuated valve, wired to the DCS/SIS for valve status and ESD verification. Valve accessories: keep them as notes on the valve, not as separate items.

Variants

Older drawings use ZSH (open) / ZSL (closed)
Sometimes one bubble "ZS" with "O/C" beside it
Transmitters of valve position are ZT