The purpose of ths report is to shed light on the problems surrounding error detection in returncables, specifically in connection with tunnels. In Sweden, among other things, has chosen to usea BT system with a suction transformer and a dedicated return conductor to reduce interferencesuch as occurs on telecommunications and signal cables along the track. A fault that occurs in thereturn conductor does not necessarily need to be noticed in other parts of the system or gives anyoperational impact. What happens is that the local error may be undetected underneath a lowertime which can cause cable fires or other disruptions to train traffic.This thesis has a model over a tunnel system simulated to study how currents in the circuit areaffected by various faults in the conductor. Saturation of the transformer has examined in the wrongtime as it is important for the possibilities around detection in the local windows that are simulated.The result based on the simulation can clearly show that a local measurement of currents in thecurrent BT window, near the exposed tunnel best results. Measurement of the current derivativein the rail could also demonstrate a measurement possibility, provided that the earth lines can bedistinguished for each current line.