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

Medium voltage and high voltage power cables (for circuits over 2kV) usually have a screening layer of copper or aluminium wire and/or tape. It’s similar to where metallic wires and tapes are used as a shield in low voltage cables to prevent electromagnetic interference.

In MV & HV, these metallic screens nullify (or drastically reduce) the field currents around the conductor or core, with the induced capacitive and inductive charging currents being earthed by the metallic screen. These currents occur under normal operating conditions, and the minimum cross-sectional area size of the screen as determined by the cable standard will be sufficient to provide this protection. 

At the point of installation, Medium Voltage cables must have the screen earthed at one end at a minimum and, depending on the system design, may be earthed at both ends. The system designer needs 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. In such instances, 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.  

Note that a standard or minimum allowed CWS/CTS cross-sectional area size will rarely be adequate under these conditions.