/images/NEC_button.jpg/images/southwire_100x70.jpgNew NEC Requires Shields over 2,400VNECRequiresShieldsover2400V.htm2005 NEC revisions impact 5kV applications of non-shielded Cables
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New NEC Requires Shields over 2,400V

2005 NEC revisions impact 5kV applications of non-shielded Cables

If you’ve been using non-shielded cable in 4,160V systems, the 2005 National Electrical Code (NEC) may have an impact on your future industrial power cable planning.

“Article 310 of the 2005 NEC requires shielded cables for system voltages above 2,400V,” says David Cooper, Southwire senior product engineer. “Previously, the NEC allowed non-shielded cables up to 8kV.”

The rationale for the 2005 NEC revisions is the potential for damage on the surface of non-shielded cables operating above 2,400V. Without the benefit of an insulation shielding system and jacket, the primary insulation of 5kV non-shielded cables is susceptible to electrical-discharge tracking.

Tracking on the insulation surface causes accelerated aging that can lead to premature cable failure. Tracking damage can also present safety concerns. A deteriorating 5kV non-shielded cable poses an increased risk of electrical shock to personnel entering the enclosure to perform routine maintenance services.

Tracking results from surface contamination

Tracking results from contamination on the surface of polymeric insulations containing carbon atoms in the molecular structure. Contaminants that induce tracking include salt, dust, humidity and air-borne chemicals.

Tracking can take place at the terminations, and also inside conduit. The problem increases in applications prone to contamination, and also where cables are installed close to grounded surfaces such as in electrical switchgear and cabinets.

The photo below shows tracking damage that occurred inside a conduit. The cable sample was a 350kcmil, 5kV non-shielded cable with EPR insulation, operating at 4,160V. The dark spots are composed primarily of carbonized insulation. The immediate area around the dark spots is glossy, while the surface is blackened and hard.

NEC change follows industry recommendations

Even prior to the 2005 NEC requirements, Southwire was proactive in advocating limiting 5kV non-shielded cables to operation at 2,400V or less. When limited to system voltages of 2,400V or less, 5kV non-shielded cables perform with a high margin of safety and reliability.

“Switching to shielded cable for 4,160V systems may create some mechanical space challenges,” Cooper says. “Shielded terminations are larger, so you may have to plan for additional space in cabinets and switchgear enclosures. Some termination manufacturers are already working on solutions for this problem”