A multiple-point gounded system is one that has grounds at more than one point. It is frequently called a closed or short-circuit shield system. Each of the arrangements has its particular advantages and disadvantages for selection. Knowledge of the total system should be taken into account when making these decisions. An ampacity improvement may be achieved if the primary shield is grounded at only one point.
Any improvement is also dependent upon conductor size, cable spacing, and shield resistivity.
Current is induced in the shield by the electromagnetic field produced by the load current in the conductor. Putting an open in a loop by grounding the shield at only one point, stops this circulating current flow. Single-point grounding will result in an increasing voltage along the shield. The value of this build-up of voltage is influenced by the electromagnetic field created by the load current in the power conductor and the length of the shield. To keep the voltage at the undergrounded end of the shield to the recommended level of 25 volts it may be necessary to ground the shield at the midsection of the cable route.