When a line (Transmission and / or Distribution) short circuit, very large currents can flow for a short time or up until a fuse, breaker or any isolation breaks the circuit. One important aspect of protecting the line from overcurrent and fault is to ensure that the fault arc and fault currents do not cause further, possibly more permanent, damage. The two main considerations are:
From the substation to the fault location, all conductors in the fault current path must withstand the heat generated by the short circuit current. If the relaying of fuse does not clear the fault in time, the conductor anneals and loses strength.
During high currents from faults, conductor can withstand significant temperatures for few seconds without losing strength. For all aluminum conductors (AAC), assuming temperature of 340®C is common. ACSR conductors can withstand even higher temperatures since short duration high temperature does not affect the steel core. You may assume a limit of 645®C melting temperature for Aluminum in ACSR.
Considering the heat inputs and conductor characteristics, the conductor temperature during a fault is related to the current as:
(I/1000A)² t = K log₁₀ [(T2+ƛ)/(T1+ ƛ)]
I = fault current (A)
t = fault duration (sec)
A = cross sectional area of conductor (kcMil)
T2 = conductor temperature from the fault
T1 =conductor temperature before the fault
K = constant
ƛ = inferred temperature of zero resistance
If we set conductors to their maximum temperature and at an ambient temperature of 40® C, these will be their characteristic curves:
I²t = (67.1A)²
I²t = (86.2A)²
Covered conductors have more limited short circuit capability due to its insulation, as they are easily damaged even at relatively lower temperature.
I²t = (43A)²
I²t = (56A)²
Right at the fault location, the hot fault arc can burn the conductor. If a circuit interrupter does not clear the fault in time, the arc will melt the conductor until it breaks apart.
Fault currents can damage overhead conductors. The arc itself generates tremendous heat, and where an arc attaches to a conductor, it can weaken or burn the conductor strands. On a distribution and transmission circuit, two areas stand out:
- Covered Conductors. Covered conductors hold an arc stationary. Arc cannot move, burndowns happen faster than bare. The covering prevents the arc from moving.
- Small Bare Wires. Small bare wires (less than 2/0) are also susceptible to wire burndowns, especially if laterals are not fused.
Conductor damage is a function of the duration of the fault and the current magnitude. Burndown damage occurs more quickly than conductor annealing.