06. What happens to cables during a short circuit?
Since alternating current follows a repetitive oscillating (sinusoidal) pattern, short circuit current produces alternating attractive and repulsive forces between conductors. These dynamic electromechanical forces are constantly changing in magnitude and direction and can exceed 40 kN/m (2,741 lbf/ft). Even when the short circuit current is quickly interrupted, it is important to recognize the maximum electromechanical forces typically occur within the first half cycle, which is long before a circuit breaker can open. During a short circuit, the high current flowing through the conductor’s resistance rapidly heats the cable, resulting in axial (longitudinal) and lateral (radial) thermal cable expansion. The combination of thermal expansion and electromechanical forces causes cable deflection (bowing/ bending), producing an axial strain on the cable terminations. If a cable termination fails before the short circuit is interrupted (a typical failure mode on under-restrained cables), the cable can whip uncontrollably until the current is extinguished, risking further electrical and mechanical damage, as well as unleashing other safety concerns.
Optimum cable protection is provided when the electromechanical and thermal expansion forces are controlled and do not result in cable restraint deformation, stressed cable terminations, damaged cable tray, etc. Cables that are exposed to potential short circuit current should be restrained by Talon® cable cleats. Talon® cable cleats protect high voltage, medium voltage and low voltage cables from mechanical damage resulting from short circuits.