**6. Conclusions**

For the DC-IM circuit of EVs, the traditional unbalanced electric bridge method switches the positive and negative bridge resistances and calculates the insulation resistance value by sampling the positive and negative bridge voltages. However, when the DC positive and negative poles have GC, the bridge voltages must be sampled after the capacitor is charged completely; thus, the measurement time is very long. This study proposes a novel method of DC-IM using a three-point climbing algorithm. The insulation resistance can be calculated by sampling the voltage of the positive and negative bridges three times and keeping the sampling interval equal. Moreover, the method filters and automatically corrects the three sampling voltages, which can improve the accuracy of the calculation results. Combined with experimental data, the following conclusions can be drawn: (1) The advantage of proposed method can perform a faster time and maintain a constant monitoring period compared with the traditional method. (2) The restriction of proposed method only apply in larger GC situation. If GC is small, the traditional method could be used. (3) The characteristics of proposed method: Increasing the sampling interval, increasing the difference between *R<sup>a</sup>* and *R<sup>b</sup>* , increasing DC voltage *v*dc, all make the results more accurate. Overall, the proposed method can be applied to some practical industrial applications. The future work is to study how to set the constant *C<sup>a</sup>* to determine which method to use or find a different rule to generally judge the value of GC, and study a method to reduce error when *v*dc is constant changing.

**Author Contributions:** J.D. and T.Q.Z. provided the method and solution; Y.Y. and H.Z. performed the experiments; J.D. and Y.Y. analyzed the data; J.D. and Y.Z. wrote the paper; T.Q.Z. and H.L. checked paper.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
