EGC for Medium Voltage Runs

[charlie b]

The attached photo shows a handhole containing medium voltage cables and load break junctions. This is downstream of a fused disconnect within the service switchboard. I don't see an EGC being routed along with the MV cables entering the handhole. I don't think 250.190(C)(2) would allow the cable's shield to serve as an EGC. But looking at the photo, that is what I think I am seeing. I think the copper wire is a ground ring within the handhole, and the green flat thing is the cable's shield. Can anyone tell me whether I need to tell the owner that a wire-type EGC needs to be installed from the service switchgear to the ground ring within this handhole?

 

[Smart $]

Are we looking at the supply or load end of the cables?

You have to disqualify cables per the first sentence of 250.190(C)(2) before you can say the shields cannot serve as the EGC.

 

[charlie b]

Are we looking at the supply or load end of the cables?

Both. All three cables on the right side of the photo are Phase A. Incoming power from the switchboard lands on one junction point. Outgoing power to two existing buildings lands on two other junction points. I plan to use the existing spare junction point to provide power to a new building.

You have to disqualify cables per the first sentence of 250.190(C)(2) before you can say the shields cannot serve as the EGC.

Help me with this. My boss and I disagree on something. The first sentence of that article includes the three word phrase, "metallic insulation shield." The next sentence includes the pair of four word phrases, "metallic tape insulation shield" and "drain wire insulation shield." Are we talking about three different things, the first of which can be used as an EGC (under specified conditions) and the later two cannot (at least not for a solidly grounded system)?

 

[GoldDigger]

I think that is exactly what it is saying.

A metallic insulation shield (such as a heavy wire braid for flexibility) will be on the outside of a semiconductor equalization layer and is capable of carrying significant current.
A tape shield would be much thinner and could not be depended upon to carry fault current without melting.
A drain wire associated just with the semiconductor layer or with a thin foil shield will be even less capable of carrying fault current.

And for a solidly grounded system, unlike an HRG or LRG system, the fault current from a ground fault can be very large, at least until the OCPD trips.

 

[Smart $]

What he said. Also, check if the cable has a concentric neutral. Hard to tell after it is terminated.

 

[electrofelon]

What he said. Also, check if the cable has a concentric neutral. Hard to tell after it is terminated.

I have been curious about this as well. So are the features described in 250.190(C)(2) different and separate from the concentric neutral? Can a cable have both? Say I had a three phase feeder that had a neutral. Could I get the three phases, neutral, and EGC with just three MV cables?

 

[Smart $]

I have been curious about this as well. So are the features described in 250.190(C)(2) different and separate from the concentric neutral? Can a cable have both? Say I had a three phase feeder that had a neutral. Could I get the three phases, neutral, and EGC with just three MV cables?

Not an expert here but I believe you'd have a better chance of getting it in one cable than three.

I didn't see an exception but would one need to run an EGC if it's a multigrounded neutral system?

 

[mivey]

The attached photo shows a handhole containing medium voltage cables and load break junctions. This is downstream of a fused disconnect within the service switchboard. I don't see an EGC being routed along with the MV cables entering the handhole. I don't think 250.190(C)(2) would allow the cable's shield to serve as an EGC. But looking at the photo, that is what I think I am seeing. I think the copper wire is a ground ring within the handhole, and the green flat thing is the cable's shield. Can anyone tell me whether I need to tell the owner that a wire-type EGC needs to be installed from the service switchgear to the ground ring within this handhole?

I believe the green thing is not the shield but rather the connection to it. This type is usually a braided cable (or multiple depending on cable size) with a tinned end that is secured against the shield (by compression spring or similar means).

With concentrics we strip it and fold it out.

Check the cable specs to see if it can handle the ground fault and clearing time. It is not abnormal for that to be the case.

 

[mivey]

I think that is exactly what it is saying.

A metallic insulation shield (such as a heavy wire braid for flexibility) will be on the outside of a semiconductor equalization layer and is capable of carrying significant current.
A tape shield would be much thinner and could not be depended upon to carry fault current without melting.
A drain wire associated just with the semiconductor layer or with a thin foil shield will be even less capable of carrying fault current.

And for a solidly grounded system, unlike an HRG or LRG system, the fault current from a ground fault can be very large, at least until the OCPD trips.

Tape shields can indeed adequately carry fault current. It just depends on the cable design. Charlie b just needs to check the cable specs.

 

[Smart $]

Tape shields can indeed adequately carry fault current. It just depends on the cable design. Charlie b just needs to check the cable specs.

I agree.

But a major problem is the distinction of metallic insulation shield from metallic tape insulation shield. The Code section clearly states a metallic tape shield cannot be used as an EGC.... and several manufacturer illustrations label a foil shield as "tape shield".

 

[GoldDigger]

Nice picture.
In that example there may not be particularly good connectivity between the edges of the successive wraps of tape/foil/whatever, and the identified bonding wire is there to make up for that.
The bonding wire by itself is possibly too small to safely carry fault current.

The whole situation is very similar to using the jacket of MC versus AC and the problem of early steel jacketed BC forcing current to follow the spiral path instead of crossing from turn to turn of the spiral jacket.
It really does come down to the details of cable construction and whether or not the spec allows use of the shield system as an EGC.

 

[electrofelon]

Not an expert here but I believe you'd have a better chance of getting it in one cable than three.

I didn't see an exception but would one need to run an EGC if it's a multigrounded neutral system?

I am not a MV guy, so nearly all of what I know is from the utility side of things. The way they would run a three phase "feeder" is run three individual concentric cables and tie the concentric neutrals all together for the neutral and that would also be the bonding conductor. For the NEC, we would need a separate EGC, unless we are using 250.30(A)(1) exception #2. I will also note that the few MV installs I see that fall under the NEC are not done correctly and are installed "utility style."

So in the diagram you posted, what is used for the Neutral, and what is used for the EGC - or does that cable not have provisions for a neutral...?

 

[Smart $]

I am not a MV guy, so nearly all of what I know is from the utility side of things. The way they would run a three phase "feeder" is run three individual concentric cables and tie the concentric neutrals all together for the neutral and that would also be the bonding conductor. For the NEC, we would need a separate EGC, unless we are using 250.30(A)(1) exception #2. I will also note that the few MV installs I see that fall under the NEC are not done correctly and are installed "utility style."

IME most MV is run as a multigrounded neutral system [250.184(C)]. Running and connecting an EGC would create a parallel path for neutral current so the EGC is omitted under 250.6(B).

So in the diagram you posted, what is used for the Neutral, and what is used for the EGC - or does that cable not have provisions for a neutral...?

IMO that cable does not have a neutral. IME the drain and shields are used as an EGC, contrary to 250.190.

 

[electrofelon]

IME most MV is run as a multigrounded neutral system [250.184(C)]. Running and connecting an EGC would create a parallel path for neutral current so the EGC is omitted under 250.6(B).

Interesting, I was not aware of that section - I rarely read the over 1KV sections. It seems to me that 250.184 is kind of equivalent to how we used to be able to "rebond" the neutral at a separate structure, and the provision that still exists in 250.30(A)(1) exception 2.

 

[charlie b]

Many thanks for all the information. I can now say that the issue has gone away. The EC working on the project visually confirmed that there is a wire-type EGC, #4 copper, routed with the MV cables.

 

[mivey]

Thanks for the follow up. Nothing like a good mystery solved!