Why is a grounded shield required for underground cables rated above 5000 volts?

A grounded shield is required for underground cables rated above 5000 volts primarily to prevent eddy currents and circulating currents. These currents can arise due to the electromagnetic fields produced by the high-voltage lines. When an electrical conductor carries alternating currents, a changing magnetic field is produced, which can induce currents in nearby conductive materials or even within the cable's own metallic shield. The grounded shield serves to safely direct these induced currents away, minimizing losses and preventing potential interference or damage to the cable system.

Additionally, the grounding helps maintain safety by ensuring that any fault currents have a path to ground, reducing the risk of electric shock or fault conditions. This technique is especially important in lowering voltage levels that could otherwise pose a risk to maintenance personnel and mitigate the impact of electrical faults on the system’s performance. Overall, the use of a grounded shield is a critical safety and performance feature for high-voltage underground cables.