100andthirty
Member
The power supply on the LU sections is known as a floating system. The figures quoted.....+500V, -250V are nominal values which would be measured using a volt meter connected between one or other of the power rails and an earthed running rail (but please don't try to do this!). These values are determined by high value resistors connected between a current rail to earth. The resistance between positive and earth has twice the value of the resistance in the negative leg. These resistors help the system to detect earth faults. For example, if there was a negative earth fault the voltage relative to earth would be approx -700V, the system would notice and the control operators would be notified. All the train sees is 750V (or 600V) nominal between the current rails. And remember, these values are nominal. The standards governing all this allows voltage swings of +/- 25%. + 25% is the margin that enables regenerative braking, and for legacy reasons, LU expects trains to continue to operate if the voltage falls to 450V.