Installing medium voltage power cables underground comes with its own set of challenges. From the engineering standpoint, there are several factors one needs to consider before installing the cable. The most overlooked if not well understood factor is the type of power cable that’s required for the application and when to ground the cable’s neutral wire.
Take a look at Figure 1. This type of cable is used by the utilities for power distribution via underground raceways. It contains either the 1/3rd size neutral (relative to a phase conductor) when it is used for three phase supply or a full size neutral for single phase supply.
When the concentric neutral in this cable is grounded at both ends, there is a possibility of circulating currents in the neutral. This can happen either due to unbalanced currents in the 3-phases, due to magnetic induction from the long run, or due to a short circuit involving a line to ground fault. In any case, this current carrying neutral constitutes the fourth cable. When this cable is installed inside a conduit, derating the cable for this additional wire is necessary (to avoid thermal overload.)
Keep in mind, with three phase conductors with 1/3rd neutral, the equivalent neutral would be 1/3 x 3 = 1 full sized neutral cable in the set of three cables inside a conduit.
Sometimes the concentric neutral conductor on top of the insulation is not required, typically when supplying power to an industrial load (which predominantly contains three phase loads). There are cables like the one shown in Figure 2 that have a tape shield over the insulation in place of a concentric neutral conductor. These cables cost less to manufacture.
Since the tape shields sole purpose is to shield the cable and since it is not rated to carry the unbalanced neutral current or the short circuit current, it is by design required to be grounded at one end of the cable only. However, to afford safety for people working on these cables, it is grounded at both ends. To protect the cable, a dedicated ground conductor is run with the other three phase conductors in the same conduit.
- For utility power distribution application: Use power cable with concentric neutral conductor. Ground neutral at both ends.
- For industrial power distribution application: Use power cable with tape shield. Run a dedicated ground cable. Ground tape shield at both ends.