Rockwell Automation 1336T Wiring and Grounding Guide, (PWM) AC Drives - Manual (Page 74)

74 Rockwell Automation Publication DRIVES-IN001M-EN-P - March 2014
Chapter 5 Reflected Wave
This section provides a summary of our findings:
• 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 can appear across these voids. If the corona inception voltage
(CIV) for the air void is reached, ozone is produced. Ozone attacks the
PVC insulation leading to a breakdown in cable insulation.
• 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.
• THHN jacket material has a relatively brittle nylon that lends itself to
damage (for example, nicks and cuts) when pulled through conduit on
long wire runs. This issue is of even greater concern when the wire is being
pulled through multiple 90
° bends in the conduit. These nicks can be a
starting point for CIV leading to insulation degradation.
• During operation, the conductor heats up and a coldflow condition can
occur with PVC insulation at points where the unsupported weight of the
wire can stretch the insulation. This has been observed at 90
° bends where
wire is dropped down to equipment from an above wireway. This coldflow
condition produces thin spots in the insulation that lowers the voltage
withstand capability of the cable.
• Refer to NEC Article 100 for definitions of damp, dry, and wet locations.
The U.S. NEC permits the use of heat-resistant thermoplastic wire in both
dry and damp applications (Table 310-13). However, PVC insulation
material is more susceptible to absorbing moisture than XLPE insulation
material (XHHN-2) identified for use in wet locations. Because the PVC
insulating material absorbs moisture, the Corona Inception Voltage
insulation capability of the damp or wet THHN was found to be less than
½ of the same wire when dry. For this reason, certain industries where
water is prevalent in the environment do not use THHN wire with IGBT
drives. Rockwell Automation strongly suggests the use of XLPE insulation
for wet areas.
Length Restrictions For
Motor Protection To protect the motor from reflected waves, limit the length of the motor cables
from the drive to the motor. The user manual for each drive lists the lead length
limitations based on drive size and the quality of the insulation system in the
chosen motor.
If the distance between drive and motor must exceed these limits, contact the
local office or factory for analysis and advice. Refer to
Appendix A for complete
tables.