Earth Resistance Tester: Construction & Working

As the name suggests, the earth resistance tester is an instrument used for measuring earth resistance. The device was first invented by Evershed & Vignoles Meggers in the 1950s. They made the first insulation and earth resistance testers. Since the 21st century, many manufacturing companies have produced a range of digital earth resistance tester. There are different types of resistant testers that come in varieties of sizes and ranges for measuring resistance.

To be precise, the main function of earth resistance testers is to determine the adequacy of the grounding of an electrical system. These instruments tell the resistance value of the earth, and we call it a ground resistance tester when the resistance value is too low. Besides, the resistance range of these devices varies from 20 ohms to 200 ohms. Now that you know the main purpose of this device, let’s discuss how it works.

Keep reading to know the construction of the Earth resistance tester and it’s working.

Construction

Before earthing an instrument or an expanse, it is necessary to limit the resistance of the earth to the concentrated area from where the earth cavity is dug. It lets the fault current pass to the earth easily. Thus, experts use earth resistance testers to test the earth’s resistance.

Talking about the construction of the device, it uses a hand-driven generator. The two principal components of this instrument are the rectifier and the rotational current reverser. In addition, the function of the rectifier is to convert AC (Alternating Current) into DC (Direct Current). Moreover, the resistance tester has two commutators installed at the same time with the rectifier and the current reverser. Each commutator comes with four fixed brushes. If you are wondering what it is, a commutator is a device that experts use for switching the direction of flow of the current.

The brushes transfer the power from stationary components to the moving portion of the instrument. These brushes and the commutator are connected for the continuous flow of current. Therefore, an earth resistance tester has the following components: two pressure coils, two current coils, a DC generator, a current reverser, a potential coil, a rectifier, and an analog resistance indicator.

Working

Now that you know the construction of this earth resistance testing device, let’s dive into its working. Both pressure coils and current coils consist of two -two terminals each. A synchronized pair of current and pressure coils are short-circuited. Also, it is connected to the ancillary electrode while the other pressure coil is connected to the rectifier at one end and an earthing electrode to another end. The potential coil, placed amidst the permanent magnet, is united to the DC generator. Moreover, this coil is connected to the pointer while the pointer is calibrated to the scale. And this resistance pointer indicates the earth’s resistance. Furthermore, the deflection of the pointer relies on the quotient of the current of the current coil to the voltage of the pressure coil. The short circuit passes the current to the soil, and thus, experts measure the resistance using Ohm’s Law.

According to Ohm’s law, the voltage is equal to the product of current and resistance. When we talk about the digital earth resistance tester, it is realized by digital electronic blocks such as voltage regulators, timers, and a digital display.

Difference Between Earth Resistance Testers & Meggers

Some people think that Meggers works the same as the earth resistance tester as both devices give a direct reading of resistance. However, here are the major differences between these two instruments.

The Earth resistance tester measures low resistance, and you cannot use it to test the quality of insulation. The Megger device measures high resistance, and you can use it to test the quality of insulation. The resistance tester cannot work above its specified voltage, whereas meggers can function at or above its specified voltage.

The measurements of resistance testers are in Ohms, while the measurements of meggers are in Mega Ohms.

Conclusion

Earth Resistance Tester is one of the most useful tools that help experts like engineers to save time and skip the hard work by telling them accurate resistance value. Hopefully, now you know the construction and working of these devices. The areas of application of earth testers include Pad and Pole mounted transformers, street lights and Street cabinets, cell towers, telephone pedestals, and lightning protection. Just like other instruments like an AC DC clamp meter, you can easily purchase earth resistance testers from online stores.