To understand what ground connections or earthing isolated power systems, it is important to know that all electrical power searches for any ground in order to close the circuit.
In this way, energy generally takes the shortest path and the one that represents the least resistance. Sometimes that short path can be through the human being.
Therefore, imagine that a patient is anesthetized and becomes the path to earth, the consequences can be lethal, because he will be unable to perform self-preservation actions or make decisions.