MV CABLE SHIELD USED AS EGC?

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TrickleCharge

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I have a scenario where the DP wants to eliminate the EGC. Here is the setup. The DP has eliminated the #8 cu wire that was called out for the 90A feed coming off the 480-4160V step up XFRM. THey are using type mv-105 cable and are trying to qualify the actual copper shield on the MV cable to be used as the EGC. THe circular mil of the tape shield is 8638 and the (3) conductors together would have a Cirular mil area of 25914. A 6awg has a min circular mil area of 25715. Could they in fact use the shield together in lieu of the original #8 CU conductor? The only thing i have seen is that the shield is designed to prevent leakage as well as shock protection. I have not seen anything that states that this copper tape can in fact be used as an effective ground-fault current path. Anyone ran into this? The only reason they are elminating the #8awg is because of constructiblity---basically they dont want it to get wrapped around the other MV cables and they say they dont have to. Any input is appreciated.
 
Using the shield in MV cables as the EGC is a common installation.

Roger
 
is there some type of listed connection that the 3 copper shields terminate to or do they just wrap them together and land them in a lug to bond the equipment? they may also have this going through some underground pullboxes with metal lids that would require bonding as well.

thanks

roger said:
Using the shield in MV cables as the EGC is a common installation.

Roger
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TrickleCharge said:
is there some type of listed connection that the 3 copper shields terminate to or do they just wrap them together and land them in a lug to bond the equipment?
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As much as this sounds so primitive, it is the way it is done and is perfectly fine.

Roger
 
See 250.190(C).
(C) Equipment Grounding Conductor. Equipment grounding
conductors shall comply with 250.190(C)(1) through
(C)(3).
(1) General. Equipment grounding conductors that are not
an integral part of a cable assembly shall not be smaller
than 6 AWG copper or 4 AWG aluminum.
(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.
(3) Sizing. 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.
Click to expand...
 
Smart $ said:
And what about the part of (C)(2) that says, "The metallic tape insulation shield and drain wire insulation shield shall not be used as an equipment grounding conductor for solidly grounded systems"?
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I know nothing about MV cable but it sounds like that is something different then what Don highlighted. NO? One is a metallic tape the other is not.
 
Dennis Alwon said:
I know nothing about MV cable but it sounds like that is something different then what Don highlighted. NO? One is a metallic tape the other is not.
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There are several methods of shielding employed in MV cable construction. You can glean a few from google images:

https://www.google.com/search?q=med...279237b987bda&bpcl=38897761&biw=1008&bih=1121

I don't know for certain if "metallic tape" includes metal foil wrapped shielding or not... but it sounds like it to me. Some have a drain wire run along with the wrap, which definitely sounds like not permitted. If only metal foil, it still has to transition to a drain wire to connect, and practically all transitiions that I've seen and worked with do not make a solid bond IMO.
 
ok thanks for all the input.....im in the 2008 NEC which does not go any further than 250.190----but with this "if it is rated
for clearing time of ground fault current protective device
operation without damaging the metallic shield"

is this something that the manuf. states as part of their specs for the MV cable?
 
TrickleCharge said:
ok thanks for all the input.....im in the 2008 NEC which does not go any further than 250.190----but with this "if it is rated
for clearing time of ground fault current protective device
operation without damaging the metallic shield"

is this something that the manuf. states as part of their specs for the MV cable?
Click to expand...
I've not had to deal with issue before. Just to look, I picked Okonite. It wasn't on the one cable spec sheet that I looked at. Digging a little deeper into their web site, I ran across this page...

http://www.okonite.com/engineering/shielding-currents.html

I'm assuming you have to determine the short circuit current and compare to protective device curves.
 
TrickleCharge said:
ok thanks for all the input.....im in the 2008 NEC which does not go any further than 250.190----but with this "if it is rated
for clearing time of ground fault current protective device
operation without damaging the metallic shield"

is this something that the manuf. states as part of their specs for the MV cable?
Click to expand...
You would have to get the shield "withstand" information from the cable manufacturer. Then you would have to have a short circuit study done and look at the time trip curves for the OCPD to see if the ground fault current would exceed the withstand of the shield conductor.
 
Smart $ said:
There are several methods of shielding employed in MV cable construction. You can glean a few from google images:

https://www.google.com/search?q=med...279237b987bda&bpcl=38897761&biw=1008&bih=1121

I don't know for certain if "metallic tape" includes metal foil wrapped shielding or not... but it sounds like it to me. Some have a drain wire run along with the wrap, which definitely sounds like not permitted. If only metal foil, it still has to transition to a drain wire to connect, and practically all transitiions that I've seen and worked with do not make a solid bond IMO.
Click to expand...

Great sales lead-in for power cable and thanks for the reference. In DC energy cabling, generally switched LV shielded interfaces, the outer metallic tape is bonded by a connector compression grip ring over a metal braid for a 100% shield jacket. These facility specialty hookup configurations probably are more susceptible to spurious conditions and do not fall into the utility intended environs per the OP statement.
 
Smart $ said:
I've not had to deal with issue before. Just to look, I picked Okonite. It wasn't on the one cable spec sheet that I looked at. Digging a little deeper into their web site, I ran across this page...

http://www.okonite.com/engineering/shielding-currents.html

I'm assuming you have to determine the short circuit current and compare to protective device curves.
Click to expand...
And since the shield has a much higher surface area/CM ratio than round conductors, the capacity should be greater than that given by the Onderdonk formula at the bottom of this link
http://home.earthlink.net/~jimlux/hv/fuses.htm
 
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