Even with a hydrostatic test there is still potential for injury - particularly for example if a failure occurs that results in a missile - possibly a stud bolt, or a flange, or a cap or similar. Hence the 30m rule is most likely precautionary.

There is a British HSE publication entitled "Safety in Pressure Testing" that you may find useful. Guidance Note GS4 (Third Edition). Refer www.hsebooks.com

This note "gives advice to employers, managers and supervisors of pressure testing teams/operators and self-employed persons. It provides guidance on how pressure testing can be safely carried out by means of risk assessment, a safe system of work and suitable precautions. Its technical appendix gives advice on the design of protective barriers if there is a risk of pressure equipment rupture or detachment of a component while under test".