The vast majority of engineering design decisions are multivariate, and earthing system design is no exception to the rule. Safety targets are but one dimension, supported beneath by layers of decisions. Additionally many other critical decisions are to be made before an installation may be signed off as being ‘fit for purpose’. Designers, constructors and asset owners or principles (utilities, plant/mine owners) all share a duty of care that is not met by simply stating that ‘all precautions should be taken’. This paper examines the various decisions and the implications on the ability to maintain an effective safety critical system. A so-called deterministic design is always a probabilistic design simply by virtue of the inherent nature of the system and process. Significant decisions are made both consciously or unconsciously in any earthing system implementation. Even if not consciously evaluating the probability functions they exist nonetheless and each decision made (or assumed or ignored) will incorporate a probabilistic element. What level of understanding of the values and interdependencies between the various parameters and functions does a designer need to ensure the final result meets and continues to meet their duty of care?