If we do not take any special provisions, axle boxes bearings are mounted in parallel to ERCU’s. Two parallel currents will then be established. One across the ERCU and the other through the bearings of the axle boxes. The current will flow through these 2 circuits inversely to the value of the impedances. When the train is stopped, the bearings are less resistant than the ERCU, they will be crossed by a considerable current coming from auxiliary circuits and in addition, the high starting current. When rotating the bearings become approximately 80 times more resistant compared against the stopped position and it will be the ERCU which will have the lowest impedance, but the remaining current which will pass through the rolling is enough to cause serious damage. To avoid this phenomena, each axel box unequipped of ERCU is provided with an additionnal bonding resistance installed between the car body and the bogie to increase the resistance and make the current flow through the bearing equipped with and Earth Return Current Unit. no current will be diverted through bearings w/o ERCU.