Megger Questions and Answers | Application of Megger

Megger Questions and Answers

1. A Megger is essentially a

  1. Multimeter
  2. Crossed – coil ohmmeter
  3. Shunt type ohmmeter
  4. Series type ohmmeter

Answer.2. Crossed-coil Ohmmeter

Explanation:-

Megger is an insulation testing instrument. It is used to measure very high resistances of the order of megaohms. The instrument is based on the principle of a cross coil ohmmeter. The required deflecting torque is produced by both the system voltage and the current. Because of the interaction between the magnetic fields produced by the voltage and the current, the deflecting torque is produced.

 

2.  A megger is used to measure ____

  1. Medium resistance
  2. Very low value of resistance
  3. High value of resistance
  4. Low value of resistance

Answer.3. High value of resistance

Explanation

The megger is an instrument used for the measurement of high resistance and insulation resistance.

Essentially the megger insulation tester consists of a hand-driven dc generator and a direct reading true ohmmeter. Megger is also called meg-ohmmeter.

An ordinary ohmmeter cannot be used for measuring the resistance of multimillions of ohms, such as conductor insulation.

Megger used to measure high resistance, of the order of one megaohm and above.

The meg-ohmmeters or megger is a portable deflection instrument widely used to check the insulation resistance of electrical cables and equipment.

This instrument measures the insulation resistance of the electric circuits relative to earth and one another.

 

3. The maximum speed of the megger is kept at:

  1. 160 rpm
  2. 100 rpm
  3. 120 rpm
  4. 140 rpm

Answer.1. 160 RPM

Explanation:-

Megger speed is kept as steady as possible because s small change in the speed of the megger would change the charging current and give a false reading of insulation resistance. The speed of megger is kept between 140 -160 RPM.

 

4. Which of the following is used for the measurement of the insulation resistance?

  1. Megger
  2. Wattmeter
  3. Ammeter
  4. Voltmeter

Answer.1. Megger

Explanation:-

The principal purpose of testing the insulation is to verify that there are no inadvertent connections between live conductors and between live and Earth before the installation is energized. Tests are required between live conductors (e.g. between phases and between phase(s) and neutral) and between all live conductors and Earth.

Insulation resistance of the installation depends on many factors such as atmospheric conditions, humidity, dirt, etc. As such its calculation is not possible, but it can be readily measured. Normally the insulation resistance is quite high and can be measured by an instrument called megger usually used for the measurement of high resistance. The main object in performing this test is to ascertain whether the complete wiring is sound enough to avoid leakage current.

Insulation testing megger is a portable instrument used for testing the insulation resistance of a circuit, and for measuring the resistance of the order of megaohms in which the measured value of resistance is directly indicated on a scale.

 

5. High resistances are of the order of ______

  1. 100 KΩ
  2. 10 mῼ
  3.  kῼ
  4. 10 Gῼ

Answer.1. 100 KΩ

Explanation:-

Classification of Resistances

Based upon the value, the resistances can be classified into the following three categories:

1. Low resistance:-  The resistance with values less than or equal to 1Ω are called low resistance. Armatures winding of machines, ammeter shunts, cables, contacts, etc. all have a low resistance value.

2. Medium resistance:- The resistance with values ranging from 1Ω to 100 kΩ is called medium resistance. The resistors employed in electronic circuits usually are of medium resistance type.

3. High resistance:- The resistance with values above 100 kΩ are high resistances. Some of the electrical and electronic circuits do employ resistors with high resistance values.

 

7. Megger works on the principle of _____

  1. Kirchhoff’s current laws
  2. Ohm’s law
  3. Gauss’s law
  4. Electromagnetic induction

Answer.4. Electromagnetic Induction

Explanation:-

We knew that the resistance of the order of 100 kΩ or above is classified as high resistance. This high resistance is measured by a portable instrument known as Megger.

Principle of Operation

It is based on the principle of electromagnetic induction. When a current-carrying conductor is placed in a uniform magnetic field, it experiences a mechanical force whose magnitude depends upon the strength of current and magnetic fields. The direction of the mechanical force depends on the direction of current and magnetic fields.

Construction of Megger

Basically, Megger consists of d.c. Generator and direct reading ohmmeter as shown in Fig.

The construction of megger is consists of five parts as follows:

(a) The Megger consists of a permanent magnet that provides the fields for both the generator and ohmmeter. The moving element of the ohmmeter consists of three coils.

(i) Deflection coil
(ii) Pressure or control coil
(iii) Compensating coil

These Coils are mounted on a central shaft which is free to rotate over a stationary C-shaped iron core.

(b) The coils are connected to the circuit through flexible leads called ligaments which do not produce a restoring torque on the moving element.

(c) The current coil is connected in series with resistance R1, between one generator terminal and the test terminal T2.

(d) The series resistance R, protects the current coil from the short circuit of the test terminals and it also controls the range of the instrument.

(e) The pressure coil, in series with a compensating coil and protection resistance R, is connected across the generator terminals.

Working

When the current flows from the generator through the pressure coil, the coil tends to set itself at right angles to the field of the permanent magnet.

When the test terminals are open, corresponding to infinite resistance, no current flows through the deflection coil. Thus the pressure coil governs the motion of the moving element making it move to its extreme anticlockwise position. The pointer comes to rest at the infinity end of the scale.

When the test terminals are short-circuited i.e., corresponding to zero resistance, the current from the generator flowing through the current coil is large enough to produce sufficient torque to overcome the counter-clockwise torque of the pressure coil. Due to this, the pointer moves over a scale showing zero resistance.

When the high resistance to be tested is connected between terminals T1, and T2, the opposing torques of the coils balance each other so that pointer attains a stationary position at some intermediate point on the scale. The scale is calibrated in megaohms so that the resistance is directly indicated by the pointer.

The guard ring is provided to eliminate the error due to leakage current. The supply to the meter is usually given by a hand-driven permanent magnet d.c. Generator or sometimes motor-driven generator may also be used.

 

8. The supply to the megger is given by _____

  1. AC motor
  2. AC generator
  3. Permanent magnet dc motor
  4. DC generator

Answer.4. DC generator

Explanation:-

DC supply is provided across the terminals of Megger. A megger is a portable instrument, and hence the dc Supply is made available from a small dc generator that forms a part of the instrument.

DC voltage is generated by rotating the generator by hand with the help of a handle provided for the purpose. R is the resistance of the insulation which is to be measured. The hand-driven dc generator provides a constant dc voltage. When dc supply is available, current flows through the pressure coil.

 

9. The role of the compensating coil in a megger is ____

  1. Reduce current
  2. Increase voltage
  3. Control temperature
  4. Scaling

Answer.4. Scaling

Explanation:-

In megger, the pressure coil, in series with a compensating coil. Compensating coil is provided for better scale proportions and to make the instrument astatic. When the current from the generator flows through the pressure coil, the coil tends to align itself at right angles to the permanent magnet field.

 

10. A purpose of guard ring in a megger is to _____

  1. Protect the circuit
  2. Eliminate error
  3. Reduce current flow
  4. Limit the temperature rise

Answer.2. Eliminate error

Explanation:-

The guard ring in the megaohm meter shunts any leakage currents to the negative side of the generator. This prevents such current from flowing through the coil and affecting the meter reading.

 

11. The role of the permanent magnet in a megger is to ______

  1. Provide field
  2. Provide voltage
  3. Generate power
  4. Balance the circuit

Answer.1. Provide field

Explanation:-

The Megger consist of a permanent magnet that provides the fields for both the generator G and ohmmeter. It Produces a magnetic field to deflect the pointer with the North-South pole magnet.

 

12.  In the given figure the series resistance R2 protect the current coil from

  1. Short circuit
  2. Open circuit
  3. Leakage current
  4. Any of the above

Answer.1. Open circuit

Explanation:-

The current coil is connected in series with terminal T2. The series resistance R2 protects the current coil in case the test terminals are short-circuited and controls the range of the instrument.

 

13. When a megger is used to measure the resistance of an electrical cable, what does a reading of infinity indicate?

  1. The meter is faulty
  2. The cable is shorted
  3. The cable is grounded
  4. The resistance is too large to measure

Answer.4. The resistance is too large to measure

Explanation:-

When megger test terminals are open

1. When the test terminals are open, corresponding to infinite resistance. Then no current flows through the deflection coil.

2. The pressure coil governs the motion of the moving element and makes it move to its extreme anticlockwise position.

3. The pointer comes to rest at the infinity end of the scale. When an extremely high resistance appears across the terminals, Such as in an open circuit, the pointer reads infinity.

 

14. When the test terminal of the Megger is short then the pointer reads

  1. Infinite
  2. Zero
  3. Negative
  4. None of the above

Answer.2. Zero

When test terminals of megger are short

1. When the test terminals are short, corresponding to zero resistance. The current from the generator flowing through the current coil is large to produce sufficient torque to overcome the counter-clockwise torque of the pressure coil.

2. When the resistance of relatively low value appears across the test points, such as when the cable insulation is wet, the current through the series winding causes the pointer to move towards zero (resistance short-circuited). However, the pointer stops at a point on the scale determined by the current through the series resistor, which in turn is governed by the value of the resistance being measured.

3. The pointer moves over a scale showing zero resistance.

 

15. The dc generator in megger can generate EMFs ranging from ____ to ______ depending on the instrument.

  1. 10 V to 50 V
  2. 2V to 5 V
  3. 50V to 500V
  4. 5000V to 10000 V

Answer.3. 50V to 500 V

Explanation

The dc generator can generate EMFs ranging from 200 V to 1000 V, depending on the instrument.

The Megger has two main elements, a magnet-type de generator to supply current for making measurements, and an ohmmeter that measures the resistance value. The generator armature is turned by a hand crank usually through step-up gears, to produce an output voltage of 500 V. When the crank is turned, the gears turn the generator at high speed to generate an output voltage that may be 100, 50, 1000, 2500 or 5000 V, depending on the model.

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