I decided to pressure test the coolant system on am e46 (not the one I had a leak on which was fixed) and pressurised it to 30 psi for a few minutes.

Until I heard water and the expansion tank cracked and started leaking

Always a good thing for it to burst on your drive rather than on the motorway however I just wanted to confirm that I didn't use too much pressure?
As the system vents at 29psi which is around the pressure I used so I assume in the event of the system over pressurised that 30 psi is a real life test?