Archives January 2017

Background Southwire’s Medium Voltage Switchgear and Substation Cable is a non-shielded, insulated, finely stranded cable that has no voltage rating. The cable has no UL listing and is not recognized by the National Electrical Code (NEC). The cable’s primarily use is for installation in medium voltage switchgear, motor controllers, and substations. In regard to use inside enclosures and equipment, even though this cable itself is not UL listed, a UL approval can be obtained on the complete assembly by having the system tested and approved.

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Overview While it is acceptable to terminate the phase conductors of a VFD cable as you would any other industrial power cable, special attention needs to be paid to the termination of the cable shield. The shield is an important part of the VFD cable but if it is not terminated properly, most of the benefits that this shield provides are negated. If you don’t properly terminate a VFD cable’s shield you may as well have not spent the extra money on VFD cable to begin with! Proper shield termination allows the shield to become a low impedance path for high frequency common mode current to flow from the motor back to the inverter. Without this controlled path, these currents can travel through motor bearings and building infrastructure and cause problems with other sensitive equipment like PLCs, control, and communication systems located throughout your facility. There are three main types of shield found in VFD cables and Southwire makes VFD cables with each of these shield types. The shield types are: copper braid shield with aluminum foil (Copper Braid); A helically applied copper tape (Copper Tape); and a continuously corrugated welded aluminum used in MC cables (Aluminum Welded Armor). This application note will detail how to terminate each of these shield types.

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