Earthing

Why Earthing?

                Earthing system is a key element of electrical system. It is critical to ensure personnel safety as well as to provide protection for equipment and to minimise interruption of service.

Standard

                Earthing is carried out based on IEEE-80, IS:3043 and also by finite element method.

Input required?

–        Fault current & Duration

–        Ground current and fault clearing time

–        Soil resistivity

–        Area of the switchyard

Soil Resistivity Measurement

•          Resistivity test should be made to determine any important variation of resistivity  with the depth of soil.

•          Resistivity test should be made at number of places  within the site. The wenner’s four pin methods  is  the most commonly used technique. Four probes are driven in to the earth along the straight line, at equal distance (A) apart, driven to the depth (B). The voltage between the two inner electrodes is then measured and divided by the current between the two outer electrodes to give a value of mutual resistance R. Then,

Where,

r= Resistivity of soil in W-m

R= resistance in ohms resulting from dividing the voltage between the potential probes by the current flowing between the current electrodes.

A=Distance between adjacent electrodes.

B=Depth of electrodes in meter.

•          If B is small compared to A, as is the case of probes penetrating the ground a short distance only, the above equation can be reduced to

•          The current tends to flow near the surface for small probe spacing, whereas more of the current penetrates deeper soils for large spacing. Thus the resistivity measured for a given probe spacing A represents the apparent resistivity of a soil to a depth of A.

•          The above derivation for the soil measurement is based on the assumption that the soil resistivity is uniform. Uniform soil resistivity means, soil resistivity remains constant both laterally and with depth to infinity.

Variation of soil resistivity with moisure content

—* Marginal moisure value is 20%

—* The moisure will value decrease 35% to 10% at summer season

Earth Rod length

Earthing Design

•          Following aspects are checked while designing earthing system

–        Adequacy of earthing conductor cross section

–        Grid size of the earthing conductor

–        Step potential and Touch potential

               

                Cross section of earthing conductor is to be sufficient to withstand the short circuit current. Also while selecting the size corrosion allowance is taken into consideration.

                In the switchyard earthing conductors are laid forming a mesh. Usually the grid spacing is uniform. But non-uniform grid spacing also can be adopted by calculation earthing through finite element method.

Step Potential     

Step Potential is the voltage difference between a person's feet cause by the dissipation gradient of a fault entering the earth. Typically, one meter from a fault entry point voltage reduced by 50%. (e.g. a 1000A fault entering through ground impedance of 5 ohms. will create voltage of 5000V. At a distance of less than one meter away a fatal potential of 2500V will exist). Earthmat installed in that area will make the voltage gradient uniform and less than the safety limit.

Touch Potential                  

Touch Potential is similar to step potential except that the part of the fault current passes through the person's hand and body if he/she touches any structure which is carrying fault current. Touch potential is eliminated by connecting structure to the Earthmat so that least resistance path for fault current is established.

Critical issues for earthing design

Earthing design becomes critical when…

•          Soil resistivity is high in rocky areas ( > 150 ohm-m )

•          High value of fault current and fault duration

•          Switchyard area is too small

•          Tolerable values of touch potential & step potential

Earth resitance Measurement