This article presents a new numerical algorithm suitable for defining recloser reclaim time and blocking automatic reclosing during permanent faults on overhead lines. It is based on terminal voltage input data processing. The decision if it is safe or not to reclose is determined from the voltage signal of faulted and tripped line phase using fundamental and third harmonic calculated by discrete Fourier transform. The algorithm was successfully tested using signals recorded on the real power system. The tests demonstrate the ability of presented algorithm to determine the secondary arc extinction time and to block unsuccessful automatic reclosing of HV lines with permanent fault.