Test methods and instrumentation techniques used to measure the ac characteristics of large grounding systems include the following topics: 1) Measurement safety 2) Earth-return mutual errors 3) Low-current measurements 4) Power-system staged faults 5) Communication and control cable transfer impedance 6) Current distribution (current splits) in the grounding system 7) Step, touch, mesh, and proÞle measurements 8) The foot-equivalent electrode earth resistance 9) Instrumentation characteristics and limitations Grounding electrodes consisting of a single ground rod, arrays of ground rods, tower footings, and many grids (if no external grounding is connected) can be measured, interference voltages permitting, with methods outlined in IEEE Std 81-1983 [2] 1 . Even if a large grid has an impedance phase angle of 18 ° the resistance component will be only 5% lower than its impedance. However, for large grounding grids in low-resistive earth (<75 W -m) and for grounding systems that have numerous extended grounding conductors, the impedance could be signiÞcantly greater than the resistive component measured with the conventional test sets of IEEE Std 81-1983