Practical Significance
Earthing System or Grounding System is an electrical network work as a safety measure to protect human life as well as equipment. The main objective of the Earth testing is to measure the value of earth resistance and compare and maintain it with recommended value of earth system resistance.
Competency and Practical Skills
This practical is expected to develop the following skills for the industry identified competency: ‘Plan wiring/cabling activities using relevant materials following safe practices’
i. Use earth tester.
ii. Measure earthing resistance.
Practical Outcome
1. Test / measure earthing system resistance of a computer centre.
2. Test / measure earthing system resistance of a building.
Relevant Affective domain related Outcome(s)
i. Practice good housekeeping with safety measures.
ii. Demonstrate working as a leader/ a team member.
iii. Maintain tools and equipment.
iv. Follow ethical practices.
Minimum Theoretical Background
The instrument used for measuring the resistance of the earth is known as earth tester. All the equipment of the power system is connected to the earth through the earth electrode. The earth protects the equipment and personnel from the fault current. The resistance of the earth is very low. The fault current through the earth electrode passes to the earth. Thus, protects the system from damage.
As per Indian standard as well as international standard (IEEE and IEC), earthing resistance should not be more than following recommended value for various installations:
1. Household— 5 Ohm
2. Power station— 0.5 Ohm
3. Major substations— 1 Ohm
4. Minor substations- 2 Ohm
Earth tester is a special type of megger used for measurement of earth resistance having additional constructional features of rotating current reverser and rectifier. It has four terminals P., P2 and C., C2. Two terminals P. and C. are shorted to form a common point to be connected to the earth electrode. The other two terminals P2 and C2 are connected to auxiliary electrodes P and C respectively.
Practical set-up / Circuit diagram / Work Situation
Resources Required
Precautions to be followed
Read carefully the instructions of the manufacturer. There are a number of manufacturers of earth testers. The students are advised to follow the manufacturer’s instructions for the operation and testing. Some manufacturers suggest that the auxiliary electrodes should be placed in an equal-internal triangle instead of in a straight line.
All connection should be tight
Select the proper range of tester
Handle rotation should be as per instruction given on instruments
Procedure
1. Collect the earth tester and connecting cables.
2. Drive the current electrode at a distance of 30 meters from the main electrode.
3. Drive the potential electrode midway between the main and current electrodes.
4. Short the terminal C.P. of the earth tester (if four terminals) and connect the terminals to the main electrode.
5. Connect the terminal PgCg of the earth tester to the potential electrode and current electrodes respectively.
6. Rotate the earth tester at its rated speed (160 rpm).
7. Measure the resistance of the earth electrode directly in the tester and enter the value in observation table
8. Repeat the measurement by shifting the auxiliary electrode position as state in Sr. Nos. 2 to 4 of observation table.
9. Calculate average value of earth resistance.
10. If the value is found more than 5 Ohm, pour the water in funnel of earth electrode and measure the earth resistance.