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I NSULATION TESTER (M EGGER )
Prepared by : Elkhomainy Hamed
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What is megger (insulation tester)?
Megger is a special type of portable instrument used to measure very high resistance of the order of megaohms.
Resistance in the range of 0.1 Megaohm and above are classified as high resistances and to measure this high resistance
we use Megger.
Megger is also called a special type of Ohmmeter used to test the insulation resistance of the cables. It is also known
as an insulation tester.
Why is insulation tester used?
The Megger is used to measure High Insulation Resistance.
What is the principle of insulation test?
Principle of insulation-resistance measurement as implemented
by insulation testers. The insulation resistance Rx of the
measurement target is calculated by measuring the current I that
flows to the target when the voltage V is applied and then
dividing the applied voltage V by the resulting current I.
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Purpose of Insulation Resistance Test
The main objective of use of the megger is to analyze and correctly apply the insulation tester in the evaluation of
the insulation level in equipment and electrical installations according to the standard. Therefore, it is mandatory to
check the insulation resistance before charging of electrical equipment.
The purpose is to check the integrity of both the conductors and the insulators. In this way, it is possible to minimize
the possibility of a short circuit or a ground fault. Otherwise, this may result in a danger of death for users or even an
electrical fire hazard in the installation itself.
What does Insulation Resistance mean?
Insulation resistance is the resistance in ohms of electrical wires, cables, and equipment. Insulation serves to protect
electrical systems such as electric cables from any accidental damage such as electric shock or sudden discharge
caused by leakage of current in the wires. There should be no voltage in the circuits whose insulation resistance to be
measured.
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Insulation resistance ways of measurement
There are two ways by which we can measure the insulation resistance:-
Type 1 measurement: Between power supply conductors connected concerning ground.
Type 2 measurement: Between each pair of active conductors.
Megger Construction
The Megger is useful for measuring a high resistance value. It has the following parts:
❑ DC generator
❑ Two coils. They are coil A and coil B.
❑ Clutch
❑ Crank
❑ Terminal X & Y.
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Working of the Megger
Step (1) Here the resistance to be measured is connected in series with the deflecting coil and across the
generator. When current is provided to both current and the pressure coil then the torque produced by these coils
are in opposite directions.
Step (2) If the resistance to be measured is very high then no current will flow through the deflecting coil. Then
the controlling coil sets itself perpendicular to the magnetic axis and hence sets the pointer at infinity.
Step (3) when the resistance to be measured is of smaller value then a very high current flows through the
deflecting coil and due to the resulting torques the pointer will set to zero or infinity.
Step (4) If the resistance to be measured is of intermediate value, then the pointer is set to infinity or zero
depending on the value of the torques produced.
Hand-driven generator present is of permanent magnet type and it is usually designed to generate a voltage in the
range of 500 to 2500 volts.
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Types of Megger
❑ Electronic or Battery operated Megger.
❑ Hand-driven or Manual type Megger.
Advantages of Electronic Megger
✓ Measurement accuracy is very high.
✓ It is robust and safe to use.
✓ It can be easily operated by a single person.
✓ Digital display is present in it which makes it easy to read the IR value.
✓ Its operation is less time-consuming.
✓ It can work well in a very small space.
✓ It is very compact and easily portable.
✓ It requires less maintenance as compared to other types.
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Disadvantages of Electronic type Megger
✓ An external source of energy such as a dry cell or battery is required to energize the system.
✓ Its initial cost is very high.
Advantages of Manual type Megger
✓ Here to energize the system no external source is required.
✓ For Emergency use, this type is an excellent choice.
✓ This type is cheaper than the electronic type.
Disadvantages of Manual type Megger
✓ Accuracy is lesser than electronic type.
✓ Its operation takes more time means its operation is time-consuming.
✓ An analog result is provided by this type.
✓ In this case, the unstable placement of the tester may affect the result of the IR value.
✓ It requires very high care and safety during its operation.
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Insulation Resistance Test Using Megger
The two methods of Insulation resistance test:-
❑ Conductor to Conductor Test
❑ Conductor to Earth Test
Insulation Resistance Test by Conductor to Conductor Test Method:
By performing this test we will measure the insulation resistance between phase and the neutral conductor.
Before performing this test the below rules must be followed:
✓ The main Switch should be OFF with fuses out.
✓ All switches in the switchboards should be ON.
✓ All lamps, Fans and others loads should be removed.
Now connect the Line terminal(L) of the megger to the Phase conductor and Earth terminal(E) to the Neutral conductor.
The handle of megger is turned on steady speed and measure the resistance.
The value of resistance should not be less than 1 Mega Ohm.
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Conductor to Conductor Test
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Insulation Resistance Test by Conductor to Earth Test:
Before performing this test the below rules must be followed:
✓ The main Switch should be OFF.
✓ All switches in the switchboards should be ON.
✓ All lamps, fans and other loads should be in their position not be removed.
Now phase and neutral connected together and then it connect to the line terminal(L) of the Megger.
The Earth terminal(E) of the Megger connected to the earth conductor.
The handle of megger is turned on steady speed and measure the resistance.
Conductor to Earth Test
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Application of Megger
✓ It is used for testing an Earth or Ground connection.
✓ It is also used for Relay testing.
✓ It is used for the measurement of the insulation resistance of the cable.
✓ Used for circuit breaker testing.
✓ Used for testing winding insulation of motor, generation, and transformer.
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