Sheath Voltage Limiter Failure from Improper Bonding of Cable Sheaths by Jon Leman
A sheath voltage limiter (SVL) is an arrester that reduces risk of damage to cable insulation by limiting overvoltage levels on cable sheaths during a short circuit event. Typical sheath voltages are calculated for normal as well as short circuit conditions, given a properly bonded sheath. However, if a cable sheath is not correctly bonded to ground at the end opposite the SVL, a floating potential condition can result. SVLs can then be exposed to steady-state voltages that exceed their rating and allowing low-magnitude currents to flow. The amount of current will depend on the capacitance between cable core and sheath and between cable sheath and external ground planes. Such steady-state current can eventually lead to SVL failure. This presentation summarizes proper application of SVLs, including their relationship to cross bonding of underground cable sheaths, and explains basic electrical specifications. Analysis is then made for the case of an improperly bonded cable sheath. Examples of cable installations are also examined using electrostatic finite element analysis to calculate capacitance between sheath and external ground planes. Sheath voltages are then calculated for floating potential conditions and current magnitudes are estimated for SLVs connected to an unbonded sheath.