Power Cable Neutral and System Grounding

Installing medium voltage power cables underground comes with its own set of challenges. From an engineering standpoint, there are several factors one needs to consider before installing the cable. The most overlooked if not a well-understood factor is the type of power cable that’s required for the application and when to ground the cable’s neutral or tape-shield.

Power cable with concentric neutral

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 the 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 wire (current flowing from one end to the other; then into the ground and back into the wire at the starting position). This can happen either due to unbalanced load currents, voltage induction from the stray magnetic field or due to a short-circuit involving a line-to-ground fault. In any case, this current-carrying neutral constitutes the fourth cable (in a 3-phase setup). When this setup is installed inside a conduit, derating the cable ampacity for the 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.

Power cable with tape-shield (no neutral)

Sometimes the concentric neutral conductor on top of the insulation is not required, typically when connecting the power transformer secondary to switchgear nearby or when supplying power to an industrial load (which predominantly contains three-phase loads). For this scenario, a cable without a neutral wire is used.

Figures 3 and 4 highlight a cable that has a tape-shield over the EPR insulation in place of a concentric neutral conductor. The tape is a thin sheet of copper that’s wrapped around the cable – enclosing it completely. This cable costs less (than ones with neutral) to manufacture.

Significance of tape-shield

You may be wondering, what’s the purpose of the tape-shield? The critical function of the tape is to evenly distribute the electric field generated by the voltage on the copper cable. With a damaged tape, the E-field is free to focus on the near-by grounded material. This concentrated field creates stress on the cable insulation. Furthermore, any imperfections in the EPR or XLPE insulation or moisture ingress allows the E-field to pick-apart the insulation, leading to premature cable failure.

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Current carrying capability of tape-shield

Due to the thin gauge of the tape, it is not rated to carry any significant neutral current or short circuit current. Thus, to defeat any current from flowing, the tape shield is grounded at only one point in its entire run. Doing this presents its own issues.

In a long cable run, with the tape grounded at one end only, the voltage starts to build-up on the tape as you move to the other end of the cable. This presents a hazard to personnel working nearby.

Thus, to afford safety for people working near these cables, certain installations have the tape grounded at both ends. In this setup, to protect the tape-shield, a dedicated ground conductor is run with the three-phase conductors in the same conduit.

Summary

1. For utility power distribution application: Use power cable with the concentric neutral conductor. Ground neutral at both ends and in manholes where the cable is spliced.
2. For industrial power distribution application or small runs inside substation: Use power cable with tape shield. Ground tape shield at one end only.