Alarm and trip annotation
ISA 5.1-2009/2022 (general instrument and function symbols)
Drawn as
Small letters next to a bubble: H, HH, L, LL (or AH, AHH, AL, ALL), sometimes with a setpoint value.
Meaning
The loop has alarms (H/L) or trips (HH/LL) at those points. HH and LL usually drive the cause & effect matrix. Not physical; record them as instrument alarm limits.
Variants
- ISA succeeding letters: A = alarm, S = switch/trip; H, L, M = high, low, middle
- Trips are often in the SIS bubble (e.g. LSHH) instead of annotations
Tags
H · HH · L · LL
Not physical hardware. Read it, then skip it in the 3D model.