Telecommunication standards define a "zone of influence" around a substation, inside of which, equipment and circuits must be protected from the effect of ground potential rise. In North American practice, the zone of influence is considered to be bounded by the "300 volt point", which is the point along a telecommunications circuit at which the GPR reaches 300 volts with respect to distant earth. The 300 volt point defining a zone of influence around a sub station is dependent on the ground resistivity and the amount of fault current. It will define a boundary a certain distance from the ground grid of the sub station. Each sub station has its own zone of influence since the variables explained above are different for each location.