2-34 Cable, Power
Manufacturing Inconsistencies and their Effects on Cable Life
Due to manufacturing inconsistencies, the following conditions can exist:
• PVC insulation material may have a dielectric constant ranging between
4 and 8 depending on the manufacturer. The higher the dielectric
constant, the lower the dielectric strength (and voltage withstand to
transients). A single IGBT drive output may have reflected wave
transient voltage stresses of up to twice (2 per unit) the DC bus voltage
between its own output wires. Multiple drive output wires in a single
conduit or wire tray further increase output wire voltage stress between
multi-drive output wires that are touching. Drive #1 may have a (+) 2 pu
stress while drive #2 may simultaneously have a (–) 2 pu stress. Wires
with dielectric constants (>4) cause the voltage stress to shift to the air
gap between the wires that are barely touching. This electric field may be
high enough to ionize the air surrounding the wire insulation and cause a
partial discharge mechanism (corona) to occur. The electric field
distribution between wires increases the possibility for corona which
further produces ozone. This attacks the PVC insulation and produces
carbon tracking, leading to the susceptibility of insulation breakdown.
• Due to inconsistencies in manufacturing processes or wire pulling, air
voids can also occur in the THHN wire between the nylon jacket and
PVC insulation. Because the dielectric constant of air is much lower than
the dielectric constant of the insulating material, the transient reflected
wave voltage may appear across the small air void capacitance. The
Corona Inception Voltage (CIV) for the air void may be reached which
further produces ozone, which attacks the PVC insulation and produces
carbon tracking, leading to the susceptibility of insulation breakdown as
in the above case.
• Asymmetrical construction of the insulation has also been observed for
some manufacturers of PVC wire. A wire with a 15 mil specification was
observed to have an insulation thickness of 10 mil at some points. The
smaller the insulation thickness, the less voltage the wire can withstand.
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