Question Is it possible to use Malz to calculate the response of a ground system of a lightning strike, or must I use HIFREQ?
You can use MALZ to compute the transient response of a grounding grid as long as:
The lightning signal doesn't contain a lot of energy at frequencies larger than about 1 MHz.
The grounding grid is sufficiently small. For instance, it is probably quite safe to use MALZ for a tower foundation, but maybe not so for a substation grounding grid.
Induction effects are not expected to be important. This is often the most difficult part to estimate. The MALZ computation model doesn't take the mutual induction of conductors into account. Again, if the grounding grid is small, this may not be a very big problem.
When at least one of above the conditions fails, it is preferable to use HIFREQ.