Concentric Neutral vs Copper Tape

[Shoe]

For a 3-phase medium voltage distribution system, is it the case that if a concentric neutral is used in the MV cables, it acts as the ground conductor in the system? With the alternative being using a copper tape shield and a separate 600V rated equipment ground run with the feeder?
Thanks

 

[don_resqcapt19]

If there are no line to neutral loads, the concentric neutral can be used as the required equipment grounding conductor. A cable with a tape shield will always require a separate equipment grounding conductor.

 

[Shoe]

Thanks. That's what my research was telling me, but thanks for the additional confirmation.

 

[paulengr]

Don’t forget the raceway (conduit) can also be the EGC and often is. Most MV loads are delta and since high resistance grounding is so prevalent in MV often EGC requirements are minimal since it is almost an ungrounded system but without the transient and fault issues those suffer from.

 

[dy11]

Don’t forget the raceway (conduit) can also be the EGC and often is.

How do you comply with 250.190(C)(1) using a raceway?

250.190(C)(2) allows the use of the tape shield as the EGC as long as the MV system is resistance grounded and the tape shield can handle the ground current long enough for the system to trip.

 

[paulengr]

A raceway falls under 250.190(A). It is all one contiguous system. Ground cables are only required where 250.190(A) is broken. This section applies to every voltage. For instance in residential voltage the conduit is bonded but if for instance the conduit is PVC or the cable is NM-B, then the ground has to be landed at every metallic enclosure.

As to demonstrating compliance a conduit does have a lower specific resistance than copper or aluminum. However this is outweighed by the vastly larger cross sectional area. The raceway impedance is always lower than conduit fill rules allow. Having a separate ground would be required when using nonmetallic raceways or when trays are not bonded. In metallic raceways that are bonded, the EGC that is present is a supplemental ground. For instance hospital Code requires two separate ground paths so the raceway and an EGC are required. Jumpers are dictated by 250.190(C)(3) subject to the minimum in C(1).

 

[paulengr]

As to demonstrating effective grounding the steel tube institute as part of their GEMI research and using the meme free GEMI software gives length limitations for steel conduit and EMT as well as aluminum and nonmetallic conduits. The longest effective length is with steel conduit. NEC requires effective grounding but does not give specific guidance with regard to lengths. Few people are even aware of the issue and it is largely ignored. At long distances (thousands of feet) the Earth path dominates since Earth impedance varies with the inverse of distance. Hence utilities make use of the fact that at distances of a few miles (remote Earth) substation grounds are effectively shorted together and multi grounded systems provide effective grounding whereas EGCs are completely useless.

This is way off in the weeds and well beyond NEC requirements.