Low resistance medium voltage system feeders EGC not require?

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Juan Sempertegui

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4160v low resistance system, NGR ratings are 100a 10sec w/ 100:5 CT. The feeders schedule does not include an EGC, the feeders conductors are Okonite type MV-105 5kv-133% shielded. I'm thinking the shield of the cable is considered the grounding conductor, but not sure if they are enough for a 100a max fault or just to pull a separate EGC (size?). Thanks for your comments.
 
Juan Sempertegui said:
4160v low resistance system, NGR ratings are 100a 10sec w/ 100:5 CT. The feeders schedule does not include an EGC, the feeders conductors are Okonite type MV-105 5kv-133% shielded. I'm thinking the shield of the cable is considered the grounding conductor, but not sure if they are enough for a 100a max fault or just to pull a separate EGC (size?). Thanks for your comments.
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These kind of questions scare me.
 
This is not a straight forward question.
The answer would be clearly no if it was a solidly grounded system, but since it is an impedance grounded system, you will need to do some calculations and lookups based on the 100A fault current flow max and the shield capability.
See 250.190(C)
Generally if you are not sure, or you plan to only terminate one side of the shield, then pull a separate EGC.
 
Ground continuity is CRITICAL to the safety of a resistance grounded system. Without it the system becomes ungrounded. Especially with MV cable control of transients is very important. In the event of an arcing fault transients can reach 600-800% of nominal voltage. What is the chance a shielded cable will survive that? Overhead line construction and distribution transformers with surge protection and reasonable BIL ratings are one thing but not 5 kV cable rated for surges at 11 kV (twice rated plus 1 kV).

Foil shields are not intended as a current carrying conductor. It is there to establish uniform voltage flux across the insulation. It must be fairly high impedance or it will cause significant losses. So it is like the shields on signal conductors.

That doesn’t mean pulling a separate EGC. The GEMI project is quite clear that properly bonded conduit, cable tray, etc., as well as concentrically wound neutrals are both acceptable grounds.

Keep in mind...NO
Line to neutral loads are allowed on a resistance grounded system. So a neutral is pointless although a ground is even more critical.
 
Juan Sempertegui said:
4160v low resistance system, NGR ratings are 100a 10sec w/ 100:5 CT. The feeders schedule does not include an EGC, the feeders conductors are Okonite type MV-105 5kv-133% shielded. I'm thinking the shield of the cable is considered the grounding conductor, but not sure if they are enough for a 100a max fault or just to pull a separate EGC (size?). Thanks for your comments.
Click to expand...
Typically in a low voltage resistance grounded system you would have a #8AWG coming off the transformer X0 bushing to the NGR system, then a another smaller ground wire from the NGR to the facility ground.

I think what you may be asking is that the shields of the cable need to be grounded to the plant ground or transformer ground point and not the XO bushing?
 
Just to close this post. The final installation of the MV Cable for this job included grounding of both ends of the Metallic Shield and EGC sized according to NEC Art 250 part X Grounding of systems and circuits of over 1000 volts.
 
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