Concrete Encasement of 15KV Cables

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Liberty

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Allentown, PA USA
I am trying to determine if I must concrete encase conduits which will hold type MV-90 15KV cables installed 24" below grade. These are customer cables as part of a primary electric service. 2008 NEC 300.50(A) (1) & (2) confuse me a bit.

(1) indicates non shielded and shielded cables as part of a metal sheathed cable assembly can be direct buried (which I will not do because of safety) or in a raceway identified for the use. (listed)

(2) indicates non shielded cables not covered in (1) must be in a conduit (of types shown in the section) and concrete encased.


My question is (1) really does not indicate the types of non shielded cables specifically so how do I know if MV-90 is required to be concrete encased. Perhaps I do not understand the definition of "non shielded & shielded"
 
Liberty said:
I am trying to determine if I must concrete encase conduits which will hold type MV-90 15KV cables installed 24" below grade. These are customer cables as part of a primary electric service. 2008 NEC 300.50(A) (1) & (2) confuse me a bit.

(1) indicates non shielded and shielded cables as part of a metal sheathed cable assembly can be direct buried (which I will not do because of safety) or in a raceway identified for the use. (listed)

(2) indicates non shielded cables not covered in (1) must be in a conduit (of types shown in the section) and concrete encased.


My question is (1) really does not indicate the types of non shielded cables specifically so how do I know if MV-90 is required to be concrete encased. Perhaps I do not understand the definition of "non shielded & shielded"
Click to expand...

I believe almost all 5kV cables now MUST be shielded cables, so all 15kV cables indeed are shielded cables. Shielding is a conductive tape wire applied over the insulation with a semi-conductive material between those to control and even out electrical stress distribution as not to create a stress point where the insulation would fail.
 
weressl said:
I believe almost all 5kV cables now MUST be shielded cables, so all 15kV cables indeed are shielded cables. Shielding is a conductive tape wire applied over the insulation with a semi-conductive material between those to control and even out electrical stress distribution as not to create a stress point where the insulation would fail.
Click to expand...

Whereas metal sheathed cable provides a mechanical protection along with some incidental shielding, but does not have to be so carefully integrated with the insulation of individual conductors.

In either case, I think that the characteristic that the code section is depending on is that there will be a grounded path to intercept any small fault current before it leaves the immediate area of the cable. And by small, I guess I really mean small enough that it does not trip any OCPD.
 
GoldDigger said:
Whereas metal sheathed cable provides a mechanical protection along with some incidental shielding, but does not have to be so carefully integrated with the insulation of individual conductors.

In either case, I think that the characteristic that the code section is depending on is that there will be a grounded path to intercept any small fault current before it leaves the immediate area of the cable. And by small, I guess I really mean small enough that it does not trip any OCPD.
Click to expand...

Let's not confuse 'sheated' with 'shielding'.

300.40 Insulation Shielding.

Metallic and semiconducting
insulation shielding components of shielded cables shall be
removed for a distance dependent on the circuit voltage and
insulation. Stress reduction means shall be provided at all
terminations of factory-applied shielding.
Metallic shielding components such as tapes, wires, or
braids, or combinations thereof, shall be connected to a
grounding conductor, grounding busbar, or a grounding
electrode.

310.10 (E) Shielding.
Non-shielded, ozone-resistant insulated conductors
with a maximum phase-to-phase voltage of 5000 volts
shall be permitted in Type MC cables in industrial establishments
where the conditions of maintenance and supervision
ensure that only qualified persons service the installation. For
other establishments, solid dielectric insulated conductors operated
above 2000 volts in permanent installations shall have
ozone-resistant insulation and shall be shielded. All metallic
insulation shields shall be connected to a grounding electrode
conductor, a grounding busbar, an equipment grounding conductor,
or a grounding electrode.
Informational Note: The primary purposes of shielding are
to confine the voltage stresses to the insulation, dissipate
insulation leakage current, drain off the capacitive charging
current, and carry ground-fault current to facilitate operation
of ground-fault protective devices in the event of an
electrical cable fault.

 
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