3.Thunder-proof technical datasheet The insulation resistance between the grounding device and the metal parts of the box is no less than 2´104MΩ/500V (DC); IR≥2´104MΩ/500V. The voltage resistance between the grounding device, and the box and its metal parts is no less than 3000V (DC)/min, no puncture, no flashover; U≥3000V