I'm assuming that your control is an early 0M without memory on the axis position, so when the control is turned on it has no idea where it is. Older Fanucs without axis position memory don't know where they are until you zero-return after a power-up. On those controls, you may power up and find that you can hit a software overtravel even before you perform a 'Zero return' on that axis. These controls can remember where they are even if you turn off the power. Later model Fanucs have either absolute encoders on the motors, or encoders with a battery backup. The other kind of alarm is a SOFTWARE overtravel, which only happens on Fanucs that have 'memory' on their axis position. If one of those switches is actuated, the OT alarm happens and you can only jog that axis in one direction (away from the switch). One is the alarm generated by the OT switches, and there is a switch for each direction in each axis. There are two kinds of overtravel alarms.