IEEE 1617-2007 pdf download IEEE Guide for Detection, Mitigation, and Control of Concentric Neutral Corrosion in Medium-Voltage Underground Cables
4.2 Path for flow of fault currents
The presence of a metallic conductor in the ground circuit necessarily results in this conductor beingsubjected to fault current requirements as well as to the charging currents. The conductor must be largeenough not to be damaged or result in cable damage due to the fault current flow. In fact, it is veryimportant that this conductor remain intact as its impedance is used to determine the maximum andprobable minimum fault current that will flow for different types of faults. This information is used to sizeprotective devices such as fuses, reclosers. and breakers protecting the circuit.
4.3 Reduce step and touch potential
The metallic shield conductor also provides a degree of personnel safety in the event of a dig into the cableby limiting touch potential on the object touching (digging into) the cable and the step potentialexperienced by nearby workers. The impedance of this conductor is a large determining factor in the dig-inpotential that will be developed.
4.4 Provide a system neutral
In the case of electric utility, multigrounded wye circuits,the ground fault currents are commonly highenough to require a significant amount of metal in the shield conductor, Thus. it was convenient to providea system neutral as well in the Underground Residential Distribution/Underground Distribution (URD/UDcable design. Of course, a separate wire could be used to provide for the neutral, and this is commonly donein industrial cables where fault-current requirements tend to be lower. The point is that the charging currentand fault current requirements ust be integral to the cable, whereas the neutral or load return requirementscan optionally be a part of the metallic shield or as a separate conductor external to the cable.
4.5 Types of concentric wires
The most commonneutral in URD/UD cables has been composed of several copper wiresconcentricconcentrically applied as a helix over the non-metallic-insulation stress-relief layer (shield). However, flatcopper straps have also been used. In the past, it was common to use coated (lead-tin alloy) wires and strapsto facilitate solder connections and for what was then believed benefits to reduce corrosion. Limited(experimental) attempts were also made to use iron-coated copper (for corrosion) and aluminum (for cost).
5.Consequences of significant neutral corrosion
5.1 Cable failures caused by loss of metallic shield component
The loss of the defined path of low impedance for the flow of charging currents results in the insulationshield capacitive “plate-voltage” driving currents in “unintended” paths. This result includes the surface olthe non-metallic shield layer with significant heating and erosion (tracking) of the layer, ultimatelyexposing the insulation. Small air gaps between the non-metallic shield surface and a conducting pathmight result in arcing with pitting and burning of the non-metallic shield. The result is concentrated stressand localized heating resulting in cable failure.
IEEE 1617-2007 pdf download
PS:Thank you for your support!
