Resistance to earth

As another example, a fire was said to have been caused by lightning. The report acuTiitted that the grounding was faulty but did not say when it was last checked, if it was scheduled for regular inspection, if there was a specification for the resistance to earth (ground), if employees understood the need for good grounding, and so on. 

Tested by the undersigned Engineer Surveyor Type of earth plate Location Resistance to earth ohms) Date of test... 

Insulation Resistance to Earth of each separate item. (Minimum Requirement 0.5 Megohm)... 

Formulae for calculation of the resistance to earth of each type are as follows ... 

These are anodes that, to reduce costs, use a combination of materials, sometimes coaxially, to extend the life of the primary anode, reduce resistance to earth, improve current distribution, facilitate installation and improve mechanical properties. Often the so-called anode is primarily a means of conducting the current to the more rapidly consumable anode material. These can be classified as follows ... 

The total circuit resistance of a groundbed installation includes cable resistance, resistance of the anode to the carbonaceous backfill plus resistance to earth of the backfill column itself. In the case of sea-water installations, the anode resistance is between the sea-water and the anode surface only. 

Being of low resistivity it has the effect of increasing the anode size with resulting reduction in resistance to earth. 

Resistance to earth of the pipeline at the groundbed location. This resistance to earth depends on the quality of the pipeline coating. The better the coating the higher the resistance. 

The resistance R is formed by the insulation resistance to earth of an object, while capacitance C is the capacitance of a standing person i.e. about 100 pF [30]. A r-value of 0.01 s. or less is required for protection of such a person against discharges [30, 31]. The maximum value of R has to be in this case ... 

DIN 51953 gives, therefore, as a practical rule the resistance to earth per 20 cm2 has to be lower than 1.0E8 Ohm. A too low resistance, however, can also be dangerous for persons in connection with the possibility of shortcircuiting. Hence, a resistance to earth per 20 cm2 between 1.0B5/1.0E6 Ohm and 1.0E8 Ohm is usually considered to be acceptable. 

The resistance to earth R as measured or calculated for a conductor buried in the ground depends upon its shape, volume and orientation in the ground. In favourable conditions the resistance should be less than one ohm. With unfavourable conditions and small sites such as the bases of pylons a value between 1 and 5 ohms should be considered. For simple shapes such as uniform rods, strips and plates, there are formulae available for calculating the resistance. For example a vertical round rod or hollow pipe the resistance is -... 

This equation can also applied to reinforced concrete in which a steel rod is encased. A single rectangular strip of width (a>) buried horizontally has a resistance to earth of -... 

It will be assumed that the source impedance Z feeding the transformers is small enough to be neglected. To illustrate the difficulty in finding a suitably low resistance to earth it will be assumed that the secondary winding of the transformer is solidly earthed and hence the NER resistance Rn is zero, a non-zero value will be recommended at the conclusion of the calculations. However, it will be assumed that the resistance to earth / en at the somce transformer is 1.5 ohms. The resistance to earth at the far end pole is R, which needs to be determined. This requires a suitable grid and rod system to be chosen. The calculation process will be carried out in a series of steps. 

If possible limit the maximnm resistance to earth at the site to 5 ohms. 

Having fonnd K and Ki it is now possible to find the mesh voltage Em as follows. The resistance Rep is substitnted into the fanlt cnrrent eqnations (H.1.1) and (H.1.2), to give the total fault current If. The earth retnm circnit between the pole at point A in Fignre 13.12 and the earthing connection at point B at the sonrce is a parallel circnit of the resistances to earth en and Rgp and the overhead earth retnm line impedance Zgoh- The parallel combination is -... 

Case A. The resistivity of the lower soil was chosen to be a moderate value of 100 ohm-m. Low values of resistance to earth at the pole, R p, were easily obtained. The main criterion is that the corner mesh voltage L must be less than the 50 kg touch voltage iitouchso- Only one case A.6 satisfies this criteria, 343 volts is less than 361 volts. This case requires a relatively large site area of 256 m for a pole and its associated equipment. 

Case B. The resistivities were raised to values typical of dry and arid locations. In all cases the resistance to earth could not be reduced to 5 ohms. Again the mesh-touch criteria could not be achieved. A satisfactory design could not be found. 

If horizontal anodes are used, the resistance to earth is given by a modified Dwight formula [57,87]. 

All tanks must be supported on non-combustible stands with a fire resistance of at least 4h. All metal parts of the storage installation should be bonded together and earthed to avoid the build up of static charges which could cause sparks. The maximum allowable value of resistance to earth is 10 Q. The integrity of the earth must be tested on a regular basis. Weather protection may be appropriate and this can be achieved by the use of a dutch-barn type structure of lightweight non-combustible construction with at least two open sides. 

Dugdale, P. J. (1973). Calculations of the resistance to earth of vertical galvanic anodes. Anticorros. Methods Mater., 20(1), 15-18. 

Earth fault A fault that occurs when a conductor is accidentally grounded/earthed, or when the resistance to earth of an insulator falls below a specified value. 

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