This paper established a 500kV microcomputer protection model with EFT/B generator. The generator was built based on the mechanism of arc forming and distinguishing when cutting off the no-load transmission line. The according parameters were set by the arc estimating formula and test data. The simulations shows that the traditional disincentive measure is limited for high voltage line and the arithmetic is still sensitive to the disturbance and made the relay set false operation. A comprehensive measure is put forward and tested by the microcomputer protection arithmetic.