(a) Maximum earth fault current.
(b) Resistivity of the soil at the sub-station site.
(c) Fault clearing time.
(d) Areas covered by the sub-station.
1. Earth Fault Current :
The maximum earth fault current constitutes a major factor in the earthing system design this determined the cross-section of the grounding conductor and the total rise of the earthing system potential. The potential gradients are also a function of this current
It is common to consider the symmetrical value of the maximum single line to ground fault.
When the fault duration is less than 0.5 sec, the RMS value of the earth fault current is increased by a factor as given below to allow for D.C. offset and A.C. and D.C. decrement.
Fault Duration VS Decrement Factor
0.08 ------ 1.65
0.10 ------- 1.25
0.25 -------- 1.10
0.50 or more ---- 1.0
To allow for the future growth of the system and the consequent increase in fault current a multiplication factor of 1.2 to 1.5 depending on the state of growth of the system is also adopted.
The current diverted away from the earthing system by the earthing wires connected to the station earthing is neglected.
2. Soil Resistivity :
The resistivity of the earth varies within extremely wide limits; between 1 and 10,000 ohm-meters.de
To design the most economical earthing system for a sub-station it is necessary to obtain accurate data on the soil resistivity and its variation at the station site.
Even at the same site, the resistivity of the soil may vary considerably at different times of the year. It is, therefore, necessary that the measurements are made a number of times during different parts of the year.
Where time does not permit, measurements should be made in the dry season to set the maximum value of the soil resistivity.
A number of test locations should be chosen at each site to cover a wide area of the soil. When 4 to 5 test sites are chosen and measurements spread over a period of full one year, it may be quite realistic to adopt for earth resistivity the arithmetic average of various readings.
3. Fault Clearing Time :
There are many considerations that influence the choice of fault clearing time such as the system stability, the type of switchgear and relaying used.
The Russian practice is to adopt 0.2 sec. as the time of adopting 4 sec. i.e. the same duration as is used for short time rating of switchgear.
The short time rating of switchgear as per Indian practice is based on 3 seconds. Therefore, 3 sec. time may be adopted as the fault clearing time for Earthing Calculations.
However, the system design engineer has to use his judgment to adopt any lower value consistent with the operating times of circuit breakers are relays used on the system (0.5 to 1.0 sec.)
4. Area Covered by Sub-Station :
The area covered by the sub-station is governed by the number and type of equipment.
After the layout has been decided and the sub-station plan finalized it is a simple matter to calculate the area covered by the sub-station.
The next step in the design procedure is the
(a) Selection of earthing material.
(b) Determination of the size of earthing conductor.
(c) Preliminary arrangement of earthing conductors.
(d) Determination of conductor length required for gradient control.
(e) Calculation of resistance of the earthing system.
(f) Checking of design for various voltages.