Medium Voltage Ground Cable Sizing
- Thread starter Mike01
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wirenut1980
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- Plainfield, IN
I believe it has to do with the function of the grounding wires which does not include being a normally current carrying conductor. It would only be carrying any large amount of current during some abnormal event such as a lightning strike and a small portion of neutral return current. This applies for both bare and insulated conductors. It also has to do with the fact that there is not much potential to "contain" with respect to ground, since it is grounded. I'm not sure what standard is used to size grounding conductors on medium voltage systems. Possibly the NEC or NESC depending on who owns the equipment. I haven't thought to look it up before.
72.5kv
Senior Member
Check out this link http://www.usda.gov/rus/electric/bulletins.htm
I assume you mean solidly grounded systems - not impedance grounded. Well, because it is a grounded conductor, the voltage to earth is zero. Under some conditions it doesn't have to be insulated at all
How does one size it? Look at 250, part X, particularly 250.182.A.2 (paraphrased): sufficient for the load imposed, but not less that 33 1/3% of the phase conductors - can go as low as 20% under certain conditions.
I have to admit, I've never seen a single phase line to neutral or Line to Line 4160V, or 13.8kv load. I can't think of a reason to solidly ground either one.
carl
How does one size it? Look at 250, part X, particularly 250.182.A.2 (paraphrased): sufficient for the load imposed, but not less that 33 1/3% of the phase conductors - can go as low as 20% under certain conditions.
I have to admit, I've never seen a single phase line to neutral or Line to Line 4160V, or 13.8kv load. I can't think of a reason to solidly ground either one.
carl
I think the 600 V cables are used because there can be a significant voltage on the ground wires. In paticular when they run long distances like MV cables often do, and during fault conditions. But the voltage wouldn't normally be be up in the kV range.
Are you are asking why a ground wire is ran in addition to the cables shield? If so, I think it has to do with preventing cable damage during a fault. The ground wire takes some of the current that the shield would otherwise carry, and helps trip the circuit faster.
Are you are asking why a ground wire is ran in addition to the cables shield? If so, I think it has to do with preventing cable damage during a fault. The ground wire takes some of the current that the shield would otherwise carry, and helps trip the circuit faster.
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