Resistivity measurements for Design : MEASUREMENT OF SOIL RESISTIVITY.

The soil resistivity is measured primarily to find the depth and thickness of the rock in geophysical studies and to find the optimal points to locate the land network of a substation, electronics, power plant or RF transmitter. It may also be used to indicate the degree of corrosion of underground pipelines.

In general, places with low resistivity tend to increase corrosion.

At this point it is necessary to clarify that the measurement of soil resistivity is not a requirement to make a mesh ground. While designing a land of great size, it is advisable to find the area of lower resistivity to achieve efficient fixtures.

The profile of the soil resistivity determines the resistance value of the land and the depth of our ground system.

To measure soil resistivity requires a terrómetro (called in other countries: telurómetro) or land Megger four terminals.

The most commonly used devices, according to its principle of operation, may be of 2 types: the kind of compensation balance to zero and the direct reading.

The terrómetros should inject a current frequency of 60 Hz is not to prevent voltages and currents are measured that are not due to the device but to electrical noise. For example, if we are near a substation or line in service, and we to make measurements of resistivity and ground resistance, a unit of 60 Hz, these systems will induce currents on the ground due to 60 Hz electromagnetic fields and give an incorrect reading.

Similarly happens when the test electrodes are poorly connected or have false contacts, give false signals of current and voltage. If currents are different from those sent by the device, it will read other voltage and current signals are not adequate.

These devices also suddenly have oscillations in their readings and you can not read them.

A smart device, has been shielded conductors, coaxial cable, has filtering systems, analysis and measures what it is, but this information is analyzed, filtered and then the clear. For example, a measurement to a signal of 100 Hz and measured, then sends another signal of 150 Hz and re-measure and can continue sending other high frequencies until values will be similar, forms and obtain a statistical average.

The terrómetros are analog or digital and should contain 4 rolls of 14 AWG cable normally. For fast winding is recommended to build a winder system to help reduce the time of measurement. They also bring 4 electrode material hard enough to be driven into the ground with mallet. They are of an approximate length of 60 cm and a diameter of 16 mm. In addition to this it is necessary to have a non-metallic tape 50 m away.

The terrómetros have four current terminals 2 (C1, C2) and 2 potential (P1, P2) and are numbered in the apparatus P1 P2 C1 C2. The terrómetros must be certified and tested in the field with a strength before making measurements.

As the measurement is obtained by a timely terrómetro, measurements must be made in a sense, in another 90 degrees to the first, and in the direction of the diagonals. In the measurement of ground resistivity is common to find very different values, caused by the geology of the area, so it is common practice for a table with readings, eliminating the values that are 50% above or below the arithmetic average of all values captured.