What is Earth tester?
An Earth Tester, also called an Earth Ground Tester or Earth Resistance Test Meter, is used to measure the resistance of the earth connection. This test ensures that the earthing system provides a safe path for fault current to flow into the ground. Earth tester testing helps check the efficiency and safety of electrical installations.
Digital Earth Tester
Analog Earth Tester
Construction
It has a small hand-driven generator that produces DC voltage. Inside the tester, a current reverser is used to change the direction of current flow.
The rectifier changes AC into DC because the moving coil meter needs DC supply. The current coil carries current through the earth, and the potential coil measures voltage drop. Their interaction moves the pointer on the meter, showing the earth resistance in ohms.
The earth tester has four terminals marked C1, P1, P2, and C2. The terminals C1 and P1 are shorted together and connected to the earth electrode. The terminal P2 is connected to the potential electrode (P), and C2 is connected to the current electrode (C).
Protective parts like resistors help to stabilise the reading and protect the instrument.
Working
The electrode under test, i.e., earth electrode E, is connected to the terminals C1 and P1. The terminal P2 is connected to the potential electrode, and C2 is connected to the current electrode. When the handle is rotated, current flows between C2 and C1 through the soil. The potential difference between P1 and P2 is measured.
The potential coil is connected directly to the DC generator and is placed between the north (N) and south (S) poles of the permanent magnet. The pointer attached to the coil moves over a calibrated scale to show the resistance value. The earth resistance is given by the ratio of potential (voltage) to current, that is, R = V / I.
Typical earth resistance values for different places
| Place / Installation | Recommended Earth Resistance |
|---|---|
| Power station / Substation | Less than 1 Ω |
| Transmission tower | Up to 5 Ω |
| Distribution transformer | Less than 2 Ω |
| Domestic (residential) installation | Up to 5 Ω |
| Industrial installation | Between 1 Ω to 2 Ω |
| Lightning arrester | Less than 1 Ω |
| General purpose earthing (ordinary ground) | Up to 10 Ω |
Applications of Earth Tester:
- Electricians use it to measure earth resistance.
- Engineers check the condition of earthing with it.
- People test soil resistance before installing earthing.
- Technicians compare earth resistance at different places.
- Users confirm proper earthing for electrical safety.