In view of the problem of surge protective device（SPDs）that the calculation process of effective protective distance is complicated at present

the variation of SPD protective distance under various load conditions is discussed on the basis of simplifying the lumped parameter circuit model and by the use of circuit principle

and general equation of SPD load voltage and protection distance is given. Then

the correctness of the analysis method is proved through software simulation. Finally

the over voltage of SPD and load under different loads is measured on a certain length of cable and the over voltage of SPD and load under same load is measured on a different length of cable. The result shows that under the condition of resistance load

when resistive load exceeds the characteristic impedance of connection line

the load voltage will occur oscillation and overshoot. The larger the resistive load is

the shorter the effective protection distance of SPD is. In case of reactive power load condition

the protection distance of SPD is greatly affected by the line inductance L. While

in case of capacitive load

the oscillation of the load voltage is the most severe and the effective protection distance is minimum at this load condition.