When looking for resistance between a grounding electrode and the earth, we are considering Ground Testing, where we insert the electrode into the ground. Since the earth has comparatively low resistivity, a voltage drop occurs near the electrode from which the current used to make the measurement flows. Soil resistivity is the critical factor that determines a grounding electrode's resistance and to what depth it must be driven to get low ground resistance. The resistivity of the soil usually changes seasonally.
Properly measuring ground resistance of poor grounds is essential as it prevents costly downtime as they withhold service interruptions.
Here are some most common methods of ground resistance tester:
2-point (dead earth) method
The two-point method can be used where driving ground rods may be unworkable. 2-point measures the resistance of two electrodes in a series by connecting 2 terminals to the ground electrode subject to the test.
It is a simple method commonly used to get results for ground resistance reading. Though the accuracy is not 100% compared to other methods, the technicians only use it to quickly test the connections and conductors between points.
3-point (Fall-of-potential) method
The three-point method is the most complete and reliable test method; used for calculating the resistance to earth of an installed grounding electrode. The curves depicting resistance vs. distance are used for measurements.
There are three basic types of the fall-of-potential method:
Full fall-of-potential
Simplified fall-of-potential
4-point method
Usually, the 4-point method is used for measuring soil resistivity. To frame or design electrical grounding systems, this method uses four small-sized electrodes into the earth at the same depth and equal distance apart - in a straight line - and is accordingly measured. The moisture and salt content of soil majorly affects its resistivity. Also, nearby grounded electrodes affect soil resistivity measurements. Sowed conductive objects, when connected with the soil, can invalidate readings if they are close enough to change the test current flow pattern, which primarily works for large or long objects.
Clamp-on method
The clamp-on method can calculate resistance without unfastening the ground system as it is rapid and effortless and includes the bond to the ground and grounding connection resistances in its measurement. Measurements are noted by "clamping" the tester around the grounding electrode under test.
However, the tests have some limitations. Those are:
Works when multiple grounds are in parallel.
Remote grounds are not the ones applicable for the clamp-on method; they are not appropriate for installation checks or commissioning of new sites.
Not applicable if an alternate lower resistance return exists not involving the soil.
OMNI CONTROLS INC.: https://www.omnicontrols.com/
Commerce Park Professional Center 5309 Technology Dr. Tampa, FL 33647
P: 813.971.5001
Email: sales@omnicontrols.com
Comments
Post a Comment