You can pen test OT networks without breaking them 

The report argues that the common belief OT networks are too fragile to test is an oversimplification and presents a staged, risk‑averse methodology for safely performing OT penetration tests. It categorises techniques from zero‑risk (documentation review, passive monitoring) to high‑risk (power cycling, exploit use) and lists no‑go actions (automated vulnerability scanning, fuzzing, firmware changes). The authors emphasize understanding device tolerance, coordinating with site staff, using maintenance windows or lab bench testing for high‑risk components, and the greater risk of not testing at all because untested weaknesses are left for attackers to find.