Seems the safest way is to have the limit switch loop as active low i.e the switches are all in series and are normally closed, this way a broken wire will trigger a stop instead of being ignored.
But control inputs - cycle-start buttons, home switches etc - active low or high?
My PLC stuff was always active high - normally open contacts that supply a voltage when and action is needed - this way a short to ground of the supply wire will blow a fuse, short to ground of the return will do nothing, only a supply-return short could be dodgy as it will be seen as a request for action.
Does this all remain true in the world of CNC?