EARTH RESISTANCE

Impedance of Earth Electrode Systems

Measurement of the Ohmic value of a buried electrode is carried out for two reasons: -

• To check the value, following installation and prior to connection to the equipment, against the design specification.
• As part of routine maintenance, to confirm that the value has not increased substantially from its design or original measured value.

Important Facts

1. Electricity takes the least path of resistance
2. Earth Resistance Value (Ohmic value) of an earth pit depends solely on soil     resistivity at the location.
3. It is the resistance of soil to the passage of electric current
4. It varies from soil to soil.
5. It depends on the composition of soil, its Moisture content, & electrolyte content, Dissolved salts, grain size and its distribution, seasonal variation, temperature and current magnitude.

EARTH RESISTANCE: DEPENDS ON SEVERAL FACTORS

SOIL RESISTIVITY

1. It is the resistance of soil to the passage of electric current
2. It varies from soil to soil
3. In depends on the composition of soil, Moisture content, Dissolved salts, grain size and its distribution, seasonal variation, current magnitude.

SOIL CONDITION

1. Different soil conditions give different soil resistivity.
2. Most of the soils are very poor conductors of electricity when they are completely dry.
3. Soil resistivity is measured in ohm-meters or ohm-cms
4. Soil plays a significant role in determining the performance of Electrode.
5. Soil with low resistivity is highly corrosive.
6. If soil is dry then soil resistivity value will be very high.
7. If soil resistivity is high, earth resistance of electrode will also be high.         

MOISTURE

1. Moisture significantly influences soil resistivity 
2. Conduction of electricity in soil is through water.
3. Soil resistivity drops significantly in soil with moisture content. 
4. Moisture is the most important element in conductivity.
5. In many locations water table goes down in dry weather conditions. Therefore it is essential to pour water in and around earth pits  to maintain moisture in dry weather conditions.  
   

DISSOLVED SALTS

1. Pure water is poor conductor of electricity.
2. Resistivity of soil depends on resistivity of water which in turn depends on the amount and nature of salts dissolved in it.
3. Small quantity of salts in water reduces soil resistivity by 80%.
4. Common salt is most effective in improving conductivity of soil. But it corrodes metal and hence discouraged. 

GRAIN SIZE & DISTRIBUTION

The grain size, distribution and closeness of packing also contribute to retention of moisture in the soil.

SEASONAL VARIATION

1. Increase or decrease of moisture content determines the increase or decrease of soil resistivity.
2. Thus in dry whether resistivity will be very high and in monsoon months the resistivity will be  low.

CURRENT MAGNITUDE

A current of significant magnitude and duration will cause significant drying condition in soil and thus increase the soil resistivity.

MEASUREMENT OF EARTH RESISTANCE- Through: Earth Tester

For measuring soil resistivity, and earthing device resistance, Earth Tester is used. It is also called the “MEGGER”.

 It has a voltage source, a meter to measure Resistance in ohms, switches to change instrument range, Wires to connect terminal to Earth Electrode and Spikes.  

It is measured by using Four Terminal Earth Tester Instrument. The terminals are connected by wires as in illustration.

P = Potential spike

C = Current spike

The distance between the spikes may be 1M, 2M, 5M, 10M, 35M, 50M.
All depending on space available.

All spikes are equidistant and in straight line to maintain electrical continuity.  Take measurement in different directions.

Soil resistivity ‘P’ may be obtained from the following formula :

P = 2 Л LR

where, R =  Value of Earth resistance in ohm L =  Distance between the spike in cm π  =  3.14 P  =  Earth resistivity ohm-cm

Earth resistance value is directly proportional to Soil resistivity value.
The following illustration describes how resistance is measured Spikes must be in straight line to maintain electrical continuity.
Resistance must be verified by increasing or decreasing the distance between the tester electrode and the spikes by 5 meter. Normally, the length of wires should be 10 and 15 Mtr or in proportion of 62% of ‘D’.  
Suppose, the distance of Current Spike from Earth Electrode D = 60 ft, Then, distance of Potential Spike would be 62 % of D = 0.62D i.e.  0.62 x 60 ft = 37 ft