What's the purpose of a screen in MV cables?

Medium and high voltage power cables in circuits over 2kV usually have a shield layer of copper or aluminium tape. Just as in low voltage cables where metallic wires and tapes are used to prevent electromagnetic interference, with these shields nullifying (or drastically reducing) the field currents around the conductor or core. The induced capacitive and inductive charging currents will be earthed by the metallic screen. These currents occur under normal operating conditions.

But at the point of installation, Medium Voltage cables must have the screen earthed at one end at a minimum. Depending on the system design it may be earthed at both ends. It would be the system designer who will  have to consider the presence of circulating currents when earthed at both ends and design the circuit to mitigate these parameters.

In some system designs where a suitable earth grid is not available (such as trackside in rail installations) the metallic screen can be called upon to act as an earth path under fault conditions (such as short circuit). Where this is the case; the system designer must advise the cable supplier the full fault load and fault duration to enable them to calculate a suitable cross sectional area of the metallic screen. A standard or minimum allowed CWS/CTS CSA will rarely be adequate under these conditions.