Answer With concentric cables, the .F27 file may contain inappropriate negative resistive values representing the impedances to earth of inner conductors or the impedances between conductors, one of which is within another one. These represent an unsuccessful attempt by your computer to represent infinity. Until we provide a patch for this, you should manually set the impedances to ground of any conductors which are inside other concentric conductors to a very large value (e.g., if a pipe enclosure is present, then the impedances to ground of all cores and sheaths should be infinity). Similarly, the interconnection impedance between a core and any other conductor except its own sheath should be very large. A suitable large value might be 1 Megohm. This should be done directly in the .F27 file, which can subsequently be imported by the SPLITS module of the Input Toolbox.