对于接地故障发生后的暂态信号，其主频率在300Hz到1000Hz之间，这个是远小于配电线路的串联谐振频率，此时线路可以认为只有容抗。对于消弧线圈，接地故障时，其工频运行在接近串联谐振状态（过补偿，会离开谐振点），计算表明，对于100Hz以上的谐波，其感抗比较大，可以忽略感性电流的影响，也就是忽略消弧线圈的补偿，整个网络可以等效为不接地网络。
For the transient signal after the ground fault, its main frequency is between 300Hz and 1000Hz, which is far less than the series resonance frequency of the distribution line. At this time, the line can be considered as only capacitive reactance. For arc suppression coil, when the grounding fault occurs, its power frequency runs close to the series resonance state (over compensation, will leave the resonance point). The calculation shows that for harmonics above 100Hz, its inductance is relatively large, and the influence of inductive current can be ignored, that is, the compensation of arc suppression coil is ignored, and the whole network can be equivalent to ungrounded network.
综上所述，我们对于接地故障暂态信号提取一定的频带，高频选择2000Hz（截止频率高于主频1000Hz，低于配电线路的串联谐振频率即可），低频选择100Hz，此时不管是不接地还是消弧线圈接地的网络，都可以看做纯容性网络,这是暂态电流法和暂态库伦法的基础。
To sum up, we extract a certain frequency band for the transient signal of the ground fault. The high frequency is 2000Hz (the cut-off frequency is higher than 1000Hz, lower than the series resonance frequency of the distribution line), and the low frequency is 100Hz. At this time, no matter the network is not grounded or the arc suppression coil is grounded, it can be regarded as a pure capacitive network, which is the basis of the transient current method and the transient Coulomb method