The inertia switch is intended to shut off the fuel supply in case of an accident. If an accident occurs the inertia switch is activated and shuts off the power to the fuel pump.
Sorry, I have no idea where the wiring for the fuel pump is.
Looking at a schematic for a Zetec SE 1999 Focus, the only component between the fuse box and the pump is an inertia switch. I have no idea what it does, but it might be worth having a look at that. Although it looks like a simple switch, so I'd be surprised if that was the issue. Is it Fuse F12 that keeps blowing?
A defective inertia switch results in an open connection of the fuel pump wiring. If this happens the car will not start. The inertia switch has nothing to do with the blown fuel pump fuse.