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 singlephase supply.

Concentric Neutral Cable
Figure 1: Power Cable with Concentric Neutral

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 fullsized neutral cable in the set of three cables inside a conduit.

Power Cable Neutral and System Grounding 1
Figure 2: Power cable with concentric neutral used for feeds going out of substation to load centers.

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.

Power Cable with Tape Shield
Figure 3: Power Cable with Tape Shield
Power cable with tapeshield
Figure 4: Power cable with tape-shield.

You may be wondering; what’s the purpose of the tape-shield? The critical function of the tape is to contain the electric field (among others).

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 it 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.

Power Cable Neutral and System Grounding 2
Figure 5: For small runs especially between transformer and switchgear, a power conductor with a tape shield can be used. The neutral wire is run separately in this case – either to switchgear neutral bus or to neutral reactor or resistor.

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.

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Tim Wieand

What is the current in the shield as a 1/3 neutral. A CT on the cable will read zero.

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Prem

Please tell me , in a 33 kV OHL distribution system , if single core copper U/G cable are used for crossings with 1 no cu ground wire for earthing continuity, Should the cable end terminated at the crossing ends bonded on both sides in case 1. Cu Tape shield 2 . Copper concentric neutral and . Please explain the reasons , advantage and disadvantage.

Hilton Lawson Jr

Both ends are grounded for safety reasons. There is always a separate neutral for current carrying purposes.

Jay

I think it was deliberate (not a misunderstanding). There are two feeders that come out of a substation (24.9kV) in concrete encased ducts. The cables are 750MCM CN and multi-point grounded – in every vault that they go through. At the substation end, the cable neutrals are bonded to the substation ground bus (and of-course the grounds in every vault). The last vault is about 2000 ft away and the the feeders go aerial after that. This separate 4/0 conductor is also terminated at the substation to the ground bus and to the ground inside the last vault (so I… Read more »

Jay

Is there any reason to have a separate neutral conductor when your feeder is made of concentric neutral cables? I am looking at a feeder that is designed and constructed just like that.

Jim Klessig

re your comments on tape shields “Since its 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 grounded at one end of the cable only. ”
That is not the case. Typical practice is to bond the shiled at both ends.

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