with the system drained, what pressure should be in the expansion tank? ?

worcester electronic 9.24. when i switch the central heating on, water pressure starts to raise from one bar upto around 2 or sometimes 3 bar. i wondered if this was a problem with the expansion tank, so i would like to know what the pressure in the expansion tank should be then the system is drained. also it has just started switching itself off when put onto central heating after a couple of minutes all else works fine.