Use of Earth Testing Megger | Instruments | Electrical Engineering

In this article we will discuss about the use of earth testing megger, explained with the help of an example.

Earth testing megger has either three terminals or four terminals. The three terminals of the instrument are marked as E, P & C. In some instruments the word ‘Earth’ is used in place of E. The main earth electrode under test is connected to E terminal by means of an insulated wire.

Two spikes or iron rods are driven into the ground at a distance of 23 metres (75 feet) and 46 metres (150 feet) away from the main earth electrode. These spikes are driven up to a depth of 0.6 to 1.0 metre. The spike at a distance of 23 metres acts as potential electrode, while that at a distance of 46 metres acts as current electrode.

The potential electrode is connected to P terminal and the current electrode to C terminal of the instrument by means of two insulated wires. Earth testing megger is placed in a horizontal position. It must be firm in position and free from the surrounding magnetic field.

The handle, of the instrument is then rotated and the deflection of the pointer on the scale is noted. At least three readings are taken by placing the two spikes at three different positions keeping their distances from the main earth electrode same as for the first readings. The average of these three readings is the earth resistance.

Earth testing megger shown in fig. 67 has four terminals P₁, P₂, C₁ & C₂. P₁ and C₁ are joined together to form E or ‘Earth’ terminal, while P₂ acts as P terminal and C₂ acts as C terminal. The common terminal of P₁ and C₁ is connected to main earth electrode, the terminal P₂ is connected to the spike at a distance of 23 metres and the terminal C₂ is connected to the spike at a distance of 46 metres away from the main earth electrode by means of insulated wires. The readings are then taken as stated before.

Use of Earth Testing Megger in a Crowded City or Locality:

In a crowded city or locality open space may not be available for driving into the ground spikes or iron rods at distance of 23 metres and 46 metres away from the main earth electrode. In that case the two terminals P and C (or P₂ and C₂) of the instrument are joined together and connected perfectly with a water pipe or any other earth wire by means, of an insulated wire. The main earth electrode is connected to terminal E as usual.

The handle of the instrument is then turned and the deflection of the pointer on the scale is noted. The insulated wire is now disconnected from the water pipe and the short-circuit between the terminals P and C is removed. One end of the insulated wire is connected to terminal P and its other end to terminal C.

The handle of the instrument is again turned and the deflection of the pointer on the scale is noted. This deflection gives the resistance of the insulated wire alone. If this reading of the instrument is subtracted from its previous reading, the result gives the earth resistance.

While conducting the above experiment care must be taken to see that the water pipe or the earth wire (with which the terminals P and C are connected together by means of an insulated wire) is situated at a considerable distance away from the main earth electrode under test. Otherwise the resistance-area of one will overlap on that of the other and the instrument will fail to indicate correct earth resistance.

Example:

Why alternating current is necessary for the measurement of earth resistance of an electrical installation with the help of an earth testing megger? How this alternating current is produced?

Solution:

The hand driven generator of an earth testing megger produces direct current when its handle is rotated. But if direct current is passed through the earth electrode and the earth, electrolytic back e.m.f., stray alternating or direct current etc. may affect the readings of the instrument. For this reason direct current is changed into alternating current before the same is passed through the earth and the electrodes.

In an earth testing megger direct current is changed into alternating current supply by means of a rotary current reverser which is mounted on the same shaft with the hand driven generator. From the positive terminal of the generator direct current comes out and flows through the current toil of the ohmmeter. It then goes to the current reverse from where it flows to the earth electrode following a cycle of changes. Hence, the potential difference between electrodes is alternating.