MV parallel feeders; EGC required per conduit?

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Isaiah

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Do I need to run an EGC for each set of 15kV, 1-C shielded cable, (3 per phase, 500kCMIL) paralleled in underground PVC conduit?


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Isaiah

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Baton Rouge
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Electrical Inspector
electrofelon said:
Tape shield cannot be used as an EGC for solidly grounded systems. I don't see anything in part X of 250 that modifies the typical rule of running an EGC in each parallel conduit.
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This system is not solidly grounded. There’s a NGR in the transformer neutral bushing; 400A, 20 ohm.


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Isaiah

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Baton Rouge
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Electrical Inspector
electrofelon said:
In that case I think you can use the tape shield as the EGC if it's rated for the clearing time other protective device without damaging it. Not sure how exactly you determine that though.
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Exactly my thoughts. Protective relaying will ‘clear’ the fault via the cable shielding before damage to the cable occurs, therefore an additional EGC is not required. These are outdoor, substation type power transformers that feed a very large industrial plant.
Many thanks!
Isaiah


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Isaiah

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Baton Rouge
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Electrical Inspector
electrofelon said:
In that case I think you can use the tape shield as the EGC if it's rated for the clearing time other protective device without damaging it. Not sure how exactly you determine that though.
Click to expand...

The cables emerge from grade and attach directly to an outdoor substation type motorized disconnect switch. Where do I land the shields? Also, I assume only one end needs to be grounded? The opposite cable end terminates at MV switchgear MCB several hundred feet away.


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yesterlectric

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PA
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Electrician
Isaiah said:
The cables emerge from grade and attach directly to an outdoor substation type motorized disconnect switch. Where do I land the shields? Also, I assume only one end needs to be grounded? The opposite cable end terminates at MV switchgear MCB several hundred feet away.


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Click to expand...
This type of an installation is one in which there should be a detailed plan from an engineer that would make this clear for the installer. An installer who didn’t get this in a drawing or specifications package is entitled to receive it.

The NGR and grounding xfmr systems I’ve had on my projects have either concentric neutral or a ground wire. But again, this level of specifics of installation should be provided to the installer.
 
electrofelon

electrofelon

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Location
Cherry Valley NY, Seattle, WA
Isaiah said:
The cables emerge from grade and attach directly to an outdoor substation type motorized disconnect switch. Where do I land the shields? Also, I assume only one end needs to be grounded? The opposite cable end terminates at MV switchgear MCB several hundred feet away.


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Click to expand...
I dont really know those details. My MV knowledge is limited. I think shield currents need to be taken into account and there are times when you only want to ground one end of the shield - but I dont know much more than that. I am more familiar with MGN systems with CN cables.
 
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Isaiah

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Location
Baton Rouge
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Electrical Inspector
yesterlectric said:
This type of an installation is one in which there should be a detailed plan from an engineer that would make this clear for the installer. An installer who didn’t get this in a drawing or specifications package is entitled to receive it.

The NGR and grounding xfmr systems I’ve had on my projects have either concentric neutral or a ground wire. But again, this level of specifics of installation should be provided to the installer.
Click to expand...

But the relay will trip on just a few cycles before cable is damaged using the shield only


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