The essential parameters need to design a grounding resistor are: 

Nominal voltage

Fault current

Fault duration (10s is customary) 

Other relevant parameters are: 

Protection degree of enclosure: from IP00-i.e. no enclosure to IP55, standard solution IP23.

Enclosure finish: our standard is mild galvanized, but different stainless steel grades are available;

Painting in the desired RAL  color is also an option

Continuous current rating: it may affect significantly the performance of the resistor, especially when high IP degrees are required

Environment and elevation: we design resistors for the harshest industrial or natural settings

Auxiliary components: during our many years of operation we have selected a number of trusted supplies for a wide choice of components, such as current transformer, switches, disconnectors, etc.