Each shuttle orbiter contains about 230 miles of wiring that routes electrical power, computer commands and other signals to all critical systems, including the ship's solid rocket boosters, liquid-fueled main engines and external tank.
The wiring snakes through shuttles in bundles. Some is exposed to damage by workers preparing the orbiters for launch. Some is buried deep within the shuttle and cannot be inspected between flights.
Most of the wiring -- between 140 and 157 miles -- is insulated in Kapton, a lightweight material that has been used in aircraft and spacecraft for decades.
Years of experience, however, have shown that the insulation can split, crack, flake or otherwise physically degrade, leading to a phenomenon known as "arc tracking." Kapton wiring then becomes a conductor that can trigger electrical shorts.
The U.S. military effectively banned its use on new aircraft beginning in 1985.