The earth's specific resistivity plays a decisive role in how well an earthing electrode diverts electric currents. It is therefore important to know about the conducting ability to know how extensive the earthing system must be. A system can then be designed that satisfies the requirements for contact voltage and operational reliability
When should resistivity be measured?
When planning and implementing earthing electrodes, tables are often used as a basis for calculating the earthing electrode's transition resistance. The reason that the calculations do not always match actual measurements can be imprecise tables or wrong use of resistivity tables. By measuring the resistivity in the ground before excavation in an area starts, we will be able to find the best formation. Thus the size and scope of the earthing electrode can be designed with the greatest possible precision.
Resistivity measuring has lately received increased attention with respect to verifying earthing systems, for example by using a measuring method to find out if the grid station in an area can be said to be a part of an extensive earthing network (global earthing system), without it necessarily consisting of six or more grid stations in a network. Using resistivity measurements, it can be determined whether fewer grid stations, together with low voltage earthing placed in cable trenches, satisfies the requirements of the definition of a "global earthing system". This can result in great savings for future measurements of electrodes and documentation of earthing systems.
Resistivity measuring also has great bearing on being able to prove that the electrode system satisfies the requirements for contact voltage where measurements of transition resistance to begin with indicate that the requirements for contact voltage are not satisfied.
Why and how do we perform measurements?
Before commissioning an electrical facility, an adequate earthing system shall be documented. This means that the earthing electrode must be adequately designed in relation to the ground in which the electrode shall function A sensible way to map this is to perform resistivity measurements and in this way determine concrete values to be used in designing.
We use the Wenner principle as a method for measuring resistivity. Depending on the measurement set-up's length, each set-up to be carried out will be given a specific point value of the ground's conductivity at a specific depth. The measurements will reveal whether the planned earthing system provides adequate transition resistance for the earthing electrode - before it is placed in the ground.