Handling of Megger.

Experiment No: – 09

Title of the Experiment: –

Handling of Megger.

Apparatus Required: –

Sl. No.	Name	Specification	Quantity
1	Megger	1000V	1 no.
2	Connecting Leads	1100V	2 nos.
3	Single-phase Transformer	5 KVA, 230V/230V	1 no.
4	DC Machine	180V, 5A	1 no.

Circuit Diagram: –

Theory: –

Megger working principle is based on the working principle of moving coil instruments, which states that when a current carrying conductor is placed in a magnetic field, a mechanical force is experienced by it. The magnitude and direction of this force depend upon the strength and direction of the current and magnetic field.

Megger is a portable instrument which is used to measure insulation resistance of the electrical machinery or system. It can be battery operated or mechanically operated (hand crank dc generator) and gives a direct reading in mega-ohms. For this reason, it is also called as mega-ohm meter.

It consists of a hand driven DC generator and a direct reading ohm meter. There are two coils PC and CC which are fixed together at some angle and are free to rotate about a common axis between the poles of a permanent magnet. The coils are connected in the circuit by means of flexible leads (or ligaments) which exerts no restoring torque on the moving system.

A megger has three terminals. The terminal marked E is connected to the ground or to any electrode whose earth resistance is to be found. The other two terminals are connected to two spike as shown in figure The scale is calibrated from 0 to 50 ohms suitable for the measurement. This gives the resistance of the ground directly. When the handle is rotated, current flows through the electrode E and spike through C₁C₂ (current) of the coil. The coil P₁P₂ (pressure coil) is connected to E and spike P. There is no current in Q. Hence there is no drop of voltage at the spike due to contact resistance. Hence only the contact resistance affects the alignment of the pressure coil. The value obtained is not dependent on the earth resistance of the spikes.

Precaution: –

1. When using a megger, the equipment under test should be de-energized.
2. Never connect the megger directly to supply.

Procedures: –

(A) For Insulation Resistance Measurement.

1. Select the Voltages on the Megger according to the rating of the equipment.
2. Isolate the connections.
3. Connect one terminal with the ground.
4. Connect other terminal on the end of the equipment.
5. The voltage required is generally between 300 – 500 – 2500 volts.
6. Note the dial reading.

(B) For Continuity Test.

1. In the continuity test, connect the megger leads to A – AA. If it shows zero reading, there is continuity.
2. Similarly connect the leads to F – FF and Y – YY , it shows continuity.
3. Connect the leads to F – Y , A – Y , Y – AA , FF – AA , A – YY , A – Body, F – Body & Y– Body. Note the corresponding dial readings from megger.
4. Continuity test for transformer is also done in the same manner.

Observation Table: –

(a) Transformer

Sl. No.	Terminals	Megger Readings (MΩ)	Remarks (Continuous/Discontinuous)
1	P1-P2
2	S1-S2
3	S1-P1
4	S2-P2
5	P1 – Body
6	P2 – Body

(b) DC Machine

Sl. No.	Terminals	Megger Readings (MΩ)	Remarks (Continuous/Discontinuous)
1	A-AA
2	F-FF
3	Y-YY
4	F-Y
5	A-Y
6	Y-AA
7	FF-AA
8	A-YY
9	A-Body
10	F- Body
11	Y- Body

Conclusion: –

To be written by Student.