If the asymmetry of the maximum available fault current exceeds 20% asymmetry factor (X/R > 1.8) then Table 2 H-rated temporary protective grounding components should be used.  If the level of asymmetry of your system is unknown, please contact your qualified system engineer.  Typical high X/R situations include locations in and near substations, generation, and high load areas.

https://hubbellcdn.com/literature/BR09107E_H-Rated_Grounding.pdf

http://blog.hubbellpowersystems.com/why-use-h-rated-clamps-for-temporary-grounding