The bubble test is not carried out at full pressure. That's the point: you find the leaks before blowing the engine up because you didn't spot a leak. The tag

The bubble test is not carried out at full pressure. That's the point: you find the leaks before blowing the engine up because you didn't spot a leak. The tag-out system could, for example, use a pressure relief valve. I suspect they even have one already when running the test, but if someone was in the wrong space when the relief valve wasn't in place then I assume they're not using it as a tag point.