Cable Separation

[timm333]

Need some clarification about NEC 770.47 (B), it says that the direct buried conductive fiber optic cable shall be 12 in (300 mm) away from the power cables. Is this 300 mm separation from the center of the power cable to the center of the fiber optic cable, or is it from the side of the power cable to the adjacent side of the fiber optic cable?

If the power cable is high voltage (like 138 kV), then will the 300 mm separation still be enough?

Also exception 1 of the same rule states that the 300 mm separation would not be required if the power cable is in the raceway, does this raceway have to be metallic raceway, or can it be PVC raceway as well?

Thanks for help!

 

[timm333]

Basically my question is that should the cable separation be measured from the centers of the two cables, or from the sides of the two cables?

 

[jtinge]

Basically my question is that should the cable separation be measured from the centers of the two cables, or from the sides of the two cables?

I would interpret that requirement to mean spacing is measured between the outer jackets of the cable, similar to how I interpret other sections of the code for conductor spacing and how dimensions for cover over a conductor are measured. (i.e. Spacing of individual conductors in cable trays, 392.80(A)(2), Cover, Table 300.50, Gen Note a.)


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[Julius Right]

NESC /2007 Rule 353. Deliberate separations—Equal to or greater than 300 mm (12 in) from underground structures or other cables
If the separation-surface to surface[clearance] is equal or more than 300 mm for all kind of cables in an underground facility.
A. General
2. The radial separation should be adequate to permit access to and maintenance of either facility to limit damage to the other.
If the distance is less than 300 mm it is permissible in a certain conditions. See:
NESC /2007 Rule 354D if the distance is less than 300 mm.
There are a lot of conditions required and among them the following:
Grounded supply systems shall not be operated in excess of 22 000 V to ground
Ungrounded supply systems shall not be operated in excess of 5300 V phase to phase.

 

[Smart $]

...
Also exception 1 of the same rule states that the 300 mm separation would not be required if the power cable is in the raceway, does this raceway have to be metallic raceway, or can it be PVC raceway as well?
...

The exception does not limit the condition to metallic raceways. By default you can use any raceway suitable for burial.

 

[timm333]

[FONT=arial, sans-serif]When 138 kV cable is enclosed in grounded metallic sheath, the maximum voltage that can be induced in the grounded metallic sheath is about 100 V. So if an underground metallic fiber optic cable is laid within 12 inches of this 138 kV cable, will the metallic fiber optic cable still be subject to voltage induction from this 138 kV cable? Thanks.[/FONT]

 

[Smart $]

When 138 kV cable is enclosed in grounded metallic sheath, the maximum voltage that can be induced in the grounded metallic sheath is about 100 V. So if an underground metallic fiber optic cable is laid within 12 inches of this 138 kV cable, will the metallic fiber optic cable still be subject to voltage induction from this 138 kV cable? Thanks.

Where are you getting your information?

 

[timm333]

IEEE 575 Annex C Section C.1.1 states that: most single-conductor underground cable systems with a length of greater than several thousand feet are designed with a maximum sheath voltage of 100 V to 200 V during normal operating conditions. Sheath voltages during emergency operating conditions are generally limited to voltages of less than 275 V.

 

[GoldDigger]

Is that from magnetic induction or capacitive coupling current over the longitudinal resistance of the sheath?

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[timm333]

Magnetic Induction. It is called "induced sheath voltage".

 

[GoldDigger]

Magnetic Induction. It is called "induced sheath voltage".

So if my understanding is correct only a concentric neutral carrying all the return current would decrease that voltage below what the cable geometry dictates, yes?

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[jtinge]

Sheath voltage will be a function of sheath grounding (single point, multi point, cross , etc)


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[Smart $]

IEEE 575 Annex C Section C.1.1 states that: most single-conductor underground cable systems with a length of greater than several thousand feet are designed with a maximum sheath voltage of 100 V to 200 V during normal operating conditions. Sheath voltages during emergency operating conditions are generally limited to voltages of less than 275 V.

Are you planning a run greater than 1000'? ...or anywhere near 1000'?

 

[Julius Right]

IEEE 575 Annex C Section C.1.1 states that: most single-conductor underground cable systems with a length of greater than several thousand feet are designed with a maximum sheath voltage of 100 V to 200 V during normal operating conditions. Sheath voltages during emergency operating conditions are generally limited to voltages of less than 275 V.

You are right, but it is about the built up induced voltage in the single-phase cable shield itself if
only one end is grounded and not in vicinity at 12 inches separation. See-further:
Some examples of current U.S. practices are as follows:
a) A 138 kV XLPE cable system installed by one utility with cross-bonded sheath was designed for a maximum induced sheath voltage of 202 V. The maximum sheath section length for this cable system is 3035 ft. and the rated current is 1400 A.
On the other hand the NEC is limited to 35 kV only [medium voltage not high voltage].
For higher voltages NESC it is the address-in my opinion.

 

[timm333]

Yes the run would be greater than 100 feet. There is no concentric neutral as these are 3 single conductor cables which form a 3-Phase 138 kV supply system. The induced voltage (sheath to ground) on the sheath of each single conductor cable is reduced because one side of the sheath is grounded (single point bonded), in this way the maximum sheath to ground voltage that can be induced on the sheath of each cable would be 100 V and it will be on the end of the cable which is not grounded.

It is correct that NEC would not apply for high voltage, but my question is general. Metallic sheaths are always present for HV cables; but just to clarify the question, let’s assume there are no metallic sheaths on these single conductor cables. Now if the fiber optic cable is run in between phase A and B, the metallic parts of the fiber optic cable would be subject to voltage induction from a supply voltage of 138 kV. My question is that what will happen when cables have sheaths (single point bonded), and the fiber optic cable is run in between phase A and B; will the metallic parts of the FO cable still be subject to voltage induction from a supply voltage of 138 kV?

 

[Julius Right]

Using Eq.D1 and 300 mm separation between fiber optic cable[10 mm diameter] and power cable [60 mm diameter] -single core power cables in flat formation, 600 mm separation, 1000 A an induced voltage of 250 V/km in communication cable shield resulted. However, you may ground this communication cable shield both ends, as no thermal stress will be expected.

 

[timm333]

Thanks. The exception in NESC rule 354-D says that even if the fiber optic cable is completely dielectric (no metal parts), still it has to be 300mm away from the power cable (for maintenance and location purposes) when the cable voltage is more than 5.3 kV. In our case the space is limited and we can only provide a space of 50 mm. What justification can possibly be provided if we want to use a space of 50mm (instead of 300mm) between 138 kV power cable and nonmetallic fiber optic cable?

Also IEEE 575 takes axial spacing (spacing between centers of cables) for the calculation of induced voltage. So maybe for fulfilling the spacing requirements of NESC, we should take the spacing between the center of power cable to the center of fiber optic cable. However, it would make more sense to measure the spacing between the outer jackets of the cables. What do you guys think about that?

 

[timm333]

Basically I want to know that is the spacing (for maintenance) a must when the cable voltage is above 5.3 kV, or is there a way around it?