CPC 100 V1.417 - 14RGroundA good substation grounding system is crucial to protect people from injury anddamage of equipment. International standards such as DIN VDE 0101/CENELEC HD637S1, IEEE Std 80-2000 or IEEE Std 81-1983 give guidelineshow to measure such impedances.Smaller grounding systems with a diameter of 100 m (300 ft) or smaller can betested with the RGround test card and CPC 100 alone, for large systems theCP CU1 coupling unit and the Sequencer test card is a better choice. Note thatno other grounding system must be close by.The current-voltage method as called in CENELEC HD637S1 or fall of potentialmethod as called in IEEE standards is a good solution to measure the groundimpedance of a substation. The current is fed via a long cable into a remoteground. This ground can be any ground from a simple test probe to another largegrounding system. The distance between this probe and the grounding systemunder test shall be at least 10 times the diameter of the grounding system, 15 to20 times is better. Then measure the voltages with a second test probe atvarious distances around the substation. If possible, choose the measurementpoints in a 90º angle (bird’s-eye view) relative to the current path. In any case,avoid measuring close (< 60º) to the current path.Measurement data at a large distance (typically three times the length of thesubstation) from the substation allow the calculation of the overall substationground impedance Zground as defined in VDE 0101. Measurements at differentdistances are recommended. The measurement points should all show similarresults. If the points are set too close to the substation or close to othergrounding systems or over buried pipes, the results obtained are not stable.Figure 8:Theoretical resistancecharacteristic of anearth electrodelinear range of earth resistanceDistance100200300400500600Earth resistance mΩ0
