25 Pa. Code § 227a.11 – Warning devices.

(a) Label. Warning devices must be labeled so that their purpose is easily identified.
(b) Warning device light. An easily visible warning device light labeled with the words ‘‘X-RAY ON,’’ or words having a similar intent, must be located near every switch that energizes an X-ray tube and must be illuminated only when the tube is energized. This warning light must be of a fail-safe design.

This section cited in 25 Pa. Code § 227a.41 (relating to X-ray on status).