High Voltage Ground Advice

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SMichael

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I have an installation where we pulled in 3- 15kV #4/0 cables in a duct bank. The cables are between a step up transformer (480V x 12.5kV) and HV switch feeding to a step down transformer 12.5kV x 480V.
We have ground grids installed at both ends of the feeder located at both transformers (bonded through the HV switch also).
The HV cable is single conductor, insulation level 133%, 105deg, full copper tape shield. The copper shield is connected internally to a grounding strap in the load breaks on each end of the conductor. The grounding straps are then bonded to the ground grid at both ends.
We are being instructed by the electrical engr. to install a #4/0 ground in the duct bank to clear any faults that may occur. My thoughts are the shields which are bonded to the ground grids at both ends shoud suffice and we would not be required to pull the #4/0 ground. Am I correct in my thinking or is this not acceptable. My foreman, who has many years of experience, does not agree with having a separate ground and the local inspector does not see the need but I need code and theory to back myself.
 

Nick

Senior Member
The shields in the cable are for shielding the cable, not grounding and bonding. Yes, they provide a return path back to the source but if you check the cable manufacturers literature you will find they are not rated like a copper conductor. The fact that the circuit is over 600V does not relieve you of the requirements of article 250. You need to pull an equipment ground.
 

Nick

Senior Member
My take on it is that the following section
250.180 General.
Where high-voltage systems are grounded, they shall comply with all applicable provisions of the preceding sections of this article and with 250.182 through 250.190, which supplement and modify the preceding sections.
Requires we follow 250.118 when installing medium voltage circuits. I don't see copper tape shield listed as an approved grounding conductor.
Also, here is a little engineering background on damage that can be caused to the cable during a fault.
http://ecmweb.com/mag/electric_groundfault_coordination_include/

It looks like your protection is fuses not sophisticated relays. A fault could cook that cable for a long time.
 

jghrist

Senior Member
Nick said:
My take on it is that the following section
250.180 General.
Where high-voltage systems are grounded, they shall comply with all applicable provisions of the preceding sections of this article and with 250.182 through 250.190, which supplement and modify the preceding sections.
It's possible that the 12.5 kV system is not grounded. If the step-up and step-down 12.5 kV-480 volt transformers are delta on the 12.5 kV side and wye on the 480 volt side, then the 12.5 kV system will be ungrounded.

In this situation, the tape shield may be adequate. The elbow connectors may be inadequate, however, if they are rated for a grounded system only. 15 kV class elbows are only rated 8.3 kV phase-to-ground.
 

jrannis

Senior Member
SMichael said:
I have an installation where we pulled in 3- 15kV #4/0 cables in a duct bank. The cables are between a step up transformer (480V x 12.5kV) and HV switch feeding to a step down transformer 12.5kV x 480V.
We have ground grids installed at both ends of the feeder located at both transformers (bonded through the HV switch also).
The HV cable is single conductor, insulation level 133%, 105deg, full copper tape shield. The copper shield is connected internally to a grounding strap in the load breaks on each end of the conductor. The grounding straps are then bonded to the ground grid at both ends.
We are being instructed by the electrical engr. to install a #4/0 ground in the duct bank to clear any faults that may occur. My thoughts are the shields which are bonded to the ground grids at both ends shoud suffice and we would not be required to pull the #4/0 ground. Am I correct in my thinking or is this not acceptable. My foreman, who has many years of experience, does not agree with having a separate ground and the local inspector does not see the need but I need code and theory to back myself.

Its very common for us to see a bare 4/0 buried with the ductbank.
 

coulter

Senior Member
SM -
As jg alluded, it matters if the 12.5 is ungrounded, solidly grounded, or impedance grounded.

As nick said, there is some guidance (but not a lot) in the high end of 250.

My understanding of the tape shield is to even out the voltage stress across the dielectric insulation. Even in an impedance grounded system, say limited to 50A, with GR relaying set for 1sec, could easily destroy the tape.

carl
 

SMichael

Member
We have a #4/0 ground conductor installed in the duct bank( seperate conduit). However, the engr. is stating that the NEC requires it to be located in the same conduit as the phase conductors. Can anyone please help state the code that would allow it to be located not in the same conduit?
 
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