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.

 

[philly]

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.

Thanks for the response.

I read 250.190(C)(2) the same way, in that it will allow use of the concentric neutral if its rated for ground fault clearing time without damage to shield. In looking at my particular cable with a 1/3 concentric neutral I don't see this shield withstand information listed anywhere on a datasheet (nor do I recall seeing in in the past) so suspect that the only way to determine this shield withstand info is to request if from manufacturer?

I know most commercial power system analysis tools have ability to plot cable damage curves for evaluation against fault currents and protective device clearing times but as I understand it, this would only be for the cable itself and not specifically for the shielding?

Am I correct in understanding that use of a concentric neutral (or tape shield on non solidly grounded systems) as an EGC (if it has rated capacity discussed above) can only be done if the shield is grounded at both ends? In cases where the shield is only grounded at a single end to prevent circulating currents and reduced ampacity I believe the shield would not be able to serve as the EGC and thus a separate EGC wire would need to be installed. Am I correct in this understanding?

 

[ron]

If the shield / concentric neutral is not terminated at each end, then it cannot provide a path for fault current.

To avoid this, we just install 600V conductor in each duct / raceway to act as the EGC.

 

[philly]

If the shield / concentric neutral is not terminated at each end, then it cannot provide a path for fault current.

To avoid this, we just install 600V conductor in each duct / raceway to act as the EGC.

Thanks again.

So with the use of a separate 600V conductor to serve as an EGC, that conductor is sized in accordance with Table 250.122 based on the CT ratio and ground fault pickup time of the feeder relay (not fault withstand capability as is evaluated with shield). That is my understanding of 250.190(C)(3), is that correct?

 

[ron]

So with the use of a separate 600V conductor to serve as an EGC, that conductor is sized in accordance with Table 250.122 based on the CT ratio and ground fault pickup time of the feeder relay (not fault withstand capability as is evaluated with shield). That is my understanding of 250.190(C)(3), is that correct?

Agreed.

250.190(C)(3) Equipment grounding conductors shall be sized in accordance with Table 250.122 based on the current rating of the fuse or the overcurrent setting of the protective relay.