IS IT WETTER THAN YOU THINK?
Water in conduit can set the stage for 600V failures related to insulation.
- Published in Southwire's Power Cable Update newsletter in December 1997
- Reprint permission granted
Your wet environment may be wetter than you think. And 600V cable that suffers installation damage can fall victim to that wetness. "The NEC definition of wet locations in Article 100 isn't intended to cover circumstances where cable is continually submerged in water," says Dave Mercier, applications engineering manager.
In particular, when you install a cable in conduit in a wet location, the design assumption is usually that the conduit will drain, so the cable is not continually submerged. In reality, conduit in wet locations frequently traps water. At low points in the runs, your conduit may constantly hold enough water to submerge the cable.
"This is really a harsh environment the NEC doesn't address in its definition of wet locations," says Mercier. "It can become a problem if the insulation suffers damage during installation."
Moderate Damage Can Cause Problems
What's the problem? All insulations absorb water. If a cable is continually submerged, there will be more water absorption where there is insulation damage. That can make a path for leakage currents through the weak spot in the insulation. Then the stage is set for a surge in the system to cause a failure in the insulations ground fault that can blow fuses or trip circuit breakers.
"Where wires are submerged, installation damage to the insulation can cause a failure in six months to a year," says Mercier.
One Answer Is Thicker Insulation
"Reports from our Customers indicate that when cable with insulation damage is submerged in water, cables with thick-wall insulation perform better than cables with thin-wall insulation," says Mercier.
Why? With thick-wall insulations, if minor damage occurs during installation, the percentage decrease in insulation thickness is smaller than with thin-wall construction. Utilities specify thick-wall 600V insulation for underground installations.
You can also go to a type TC cable that has a protective PVC outer jacket. If the outer jacket is damaged, there's no increase in electrical stress in the insulation. Or, you can specify an armored construction such as Southwire's ARMOR-X. With ARMOR-X cable, a continuous, seamless aluminum sheath protects the conductor from moisture.
"With any cable installation, careful handling helps prolong cable life," Mercier says. "That's especially true when you're running conduit in wet locations." |
