Using MV cable concentric neutral for Equipment Ground Conductor (EGC)

[philly]

Can a concentric neutral that is part of an MV cable shield be used to satisfy the equipment grounding conductor (EGC) requirement per NEC 250.122?

For example can a cable with a 1/3 rated concentric neutral be used as an EGC if the current carrying capacity meets the requirement of table 250.122? I believe in this case the upstream overcurrent device rating used for 250.122 would be the CT ratio or ground fault pickup setting of relay?

Can the concentric neutrals of all (3) cables be added to account for EGC size or only one at a time can be considered?

I know the NEC allows use of a tape shield as an EGC as long as it rated to carry fault current based on fault clearing time (typically not applicable on solidly grounded systems) so I was wondering if with a concentric neutral the current carrying capacity of concentric neutral gets applied directly against 250.122 based on CT ratio or GF pickup or if relay clearing time for faults must also be considered?

 

[ron]

It sounds like you have a non-solidly grounded system, since you are considering this, 250.190(C)(2) prohibits using the integral shield. In non-solidly grounded systems, the ground fault current is relatively low (at least for the 1st fault).

I would consider the concentric neutral to be a metallic insulation shield encircling the current-carrying conductors which can be permitted to be used as an equipment grounding conductor. Although not enforceable, the enhanced in NFPA link has a photo that includes a concentric neutral as an example for 250.190(C)(2).

250.190(C)(2) Shielded Cables.
The metallic insulation shield encircling the current-carrying conductors shall be permitted to be used as an equipment grounding conductor, if it is rated for clearing time of ground-fault current protective device operation without damaging the metallic shield. The metallic tape insulation shield and drain wire insulation shield shall not be used as an equipment grounding conductor for solidly grounded systems.