A pushbutton switch can get stuck and fail in either states closed or open, I have come across both cases.
That procedure is there to make sure the system is isolated, a defective system has to be isolated the legal way regardless of the condition of its pushbutton, you never know that faulty p/b in the open state at any time can fail in the closed again causing the system to switch on, therefore proper isolation is essential.
That alternate procedure is there to avoid any unprecedented events as such (push button reverting to closed state putting power back).
Last edited by flame_bringer; 23rd May 2013 at 20:47.