Measurement of High Resistance MCQ Quiz - Objective Question with Answer for Measurement of High Resistance - Download Free PDF

Megger

A Megger (Megohmmeter) measures high insulation resistance in electrical circuits. It consists of:

• Current Coil (1 Coil): Produces torque proportional to the current flowing.
• Potential Coils (2 Coils): Produces torque proportional to the applied voltage.

The interaction between these coils ensures accurate insulation resistance measurement.

Construction of Megger

• The Megger has one current coil and two voltage coils V₁ and V₂.
• The voltage coil V₁ is passed over the magnet connected to the generator.
• When the pointer of the PMMC instrument deflects towards infinity, it means that the voltage coil remains in the weak magnetic field and thus experienced very little torque.
• The torque experienced by the coil increases when it moves inside the strong magnetic field.
• The coil experiences the maximum torque under the pole faces and the pointer is set at the zero ends of the resistance scale.
• To improve the torque, the voltage coil V₂ is used.
• The coil V₂ is so allocated that when the pointer deflects from infinity to zero, the coil moves into a stronger magnetic field.
• In Megger, the combined action of both the voltage coils V₁ and V₂ are considered.
• The coil comprises a spring of variable stiffness. It is stiff near the zero ends of the coil and becomes very weak near the infinity end of the spring.

Explanation:

Insulation Resistance Measurement of Domestic Appliances and Wiring

Definition: Insulation resistance is a critical parameter in electrical systems, representing the resistance between the electrical conductors and the insulating material surrounding them. It is essential to measure the insulation resistance to ensure the safety and reliability of electrical appliances and wiring, preventing electric shocks, short circuits, and potential fire hazards.

Correct Option:

The correct option for measuring the insulation resistance of domestic appliances and wiring is:

Option 3: Megger

A Megger, or insulation resistance tester, is a specialized instrument designed to measure high resistance values, typically in the range of megohms (MΩ). It is used to assess the integrity of the insulation in electrical systems by applying a high voltage and measuring the resulting current, which allows the calculation of the insulation resistance.

Working Principle of Megger:

The Megger operates by generating a high DC voltage (usually between 500V and 1000V) and applying it across the insulation material. The high voltage causes a small current to flow through the insulation, which is then measured by the Megger. The insulation resistance is calculated using Ohm's Law:

Insulation Resistance (R) = Voltage (V) ÷ Current (I)

The Megger displays the insulation resistance value, typically in megohms (MΩ). A high insulation resistance indicates good insulation, while a low value suggests potential problems with the insulation, such as moisture ingress, degradation, or damage.

Steps to Measure Insulation Resistance Using a Megger:

1. Ensure the appliance or wiring is disconnected from the power supply and other connected devices.
2. Connect the Megger's test leads to the conductors being tested (e.g., live and neutral wires or live and earth wires).
3. Set the Megger to the appropriate test voltage (typically 500V for domestic appliances and wiring).
4. Activate the Megger to apply the test voltage and measure the insulation resistance.
5. Observe the insulation resistance value displayed on the Megger and record the reading.
6. Interpret the results according to the relevant standards or manufacturer's guidelines to determine if the insulation is in good condition.

Advantages of Using a Megger:

• Accurate measurement of high resistance values, ensuring reliable assessment of insulation quality.
• Portable and easy to use, making it suitable for field testing and maintenance activities.
• Provides a direct reading of insulation resistance, simplifying the testing process.

Disadvantages of Using a Megger:

• High test voltages can potentially cause damage to sensitive electronic components if not used correctly.
• Requires careful handling and adherence to safety procedures to avoid electric shocks.

Applications of Megger:

• Testing the insulation resistance of electrical appliances, such as refrigerators, washing machines, and air conditioners.
• Assessing the insulation quality of domestic wiring systems to ensure safety and compliance with electrical standards.
• Periodic maintenance and inspection of electrical installations in residential, commercial, and industrial settings.

Analysis of Other Options:

Option 1: Voltmeter and Ammeter

This option is incorrect for measuring insulation resistance. A voltmeter and ammeter are used for measuring voltage and current, respectively, in electrical circuits. While they are essential instruments for various electrical measurements, they do not provide the direct measurement of insulation resistance. Additionally, using these instruments to measure insulation resistance would require a complex setup and calculations, making it impractical and less accurate compared to using a Megger.

Option 2: Multimeter

A multimeter is a versatile instrument capable of measuring voltage, current, and resistance in electrical circuits. However, standard multimeters are not designed to measure high insulation resistance values accurately. They typically measure resistance up to a few megohms, which is insufficient for assessing the insulation quality of domestic appliances and wiring. Meggers, on the other hand, are specifically designed for this purpose and provide more accurate and reliable measurements of insulation resistance.

Option 4: Energy Meter

An energy meter is used to measure the electrical energy consumption of a device or an entire electrical installation over time. It is not designed to measure insulation resistance. Energy meters are essential for monitoring energy usage and billing purposes, but they do not provide any information about the insulation quality of electrical systems. Therefore, this option is not suitable for measuring insulation resistance.

Conclusion:

In conclusion, the most appropriate instrument for measuring the insulation resistance of domestic appliances and wiring is the Megger (Option 3). Meggers are specifically designed for this purpose, providing accurate and reliable measurements of high resistance values necessary for assessing the insulation quality. The other options, such as voltmeters, ammeters, multimeters, and energy meters, are not suitable for this application due to their limitations in measuring high insulation resistance values and their different intended uses in electrical measurements.

For measuring current at high frequencies, we should use a thermocouple instrument.

Thermocouple Instrument

• A thermocouple instrument is used for measuring high-frequency currents and voltages by converting electrical energy into heat. 
• The instrument uses a thermocouple, which generates a small voltage when there is a temperature difference between two junctions. When current passes through a heating element, it heats up due to the Joule effect. The thermocouple measures the resulting temperature rise, which corresponds to the current or voltage in the circuit.
• Since the measurement is based on heat and not on electromagnetic principles (like inductance or capacitance), thermocouple instruments can accurately measure high-frequency currents without being affected by frequency changes, unlike moving iron or electromagnetic-based devices.
• Thermocouple instruments are ideal for RF (radio frequency) and high-frequency AC measurements, often used in radio transmitters, communication systems, and high-frequency power applications.

Additional Information

• Moving iron instruments are generally used for low-frequency AC measurements.
• Electrostatic instruments are typically used for voltage measurements, not current, and are not suitable for high-frequency applications.

Megger:

1.Definition:

• ​The Megger is the instrument uses for measuring the resistance of the insulation.
• A megger is an instrument that contains both a voltage source and a voltmeter: 
• Voltage source: A megger uses a hand-driven generator to produce a voltage of around 500, 1000, or 2500 volts. 
• Voltmeter: The voltmeter in a megger is calibrated in ohms, kilo-ohms, or megohms.
• It works on the principle of comparison, i.e., the resistance of the insulation is compared with the known value of resistance.
• The Megger has three coils two pressure coils (control coil) and one current coil. The pressure coil rotates the moving coil in the anticlockwise direction, whereas the current coil rotates it in the clockwise direction.
• If the resistance of the insulation is high, the pointer of the moving coil deflects towards the infinity, and if it is low, then the pointer indicates zero resistance.
• The accuracy of the Megger is high as compared to other instruments.

2.Construction of Megger:

• It is the combination of DC Generator and Ohm Meter.
• The Megger has one current coil and the two voltage coils V1 and V2.
• Deflecting coil or current coil connected in series and allows flowing the electric current taken by the circuit being tested.
• The control coil is also known as the pressure coil is connected across the circuit.
• D.C generator or Battery connection: The voltage is generated by connecting the hand-driven generator.
• Deflecting and Control coil: Connected parallel to the generator, mounted at a right angle to each other, and maintain polarities in such a way to produced torque in opposite direction.
• Permanent Magnets: Produce magnetic field to deflect pointer with North-South pole magnet.
• Pointer: One end of the pointer connected with coil another end deflects on a scale from infinity to zero.
• Scale: A scale is provided in the front-top of the megger from range ‘zero’ to ‘infinity’, enable us to read the value.
• Pressure Coil Resistance and Current Coil Resistance: Protect instrument from any damage because of low external electrical resistance under test.

3.Working Principle of Megger:

• The voltage coil V1 is passed over the magnet connected to the generator. When the pointer of the PMMC instrument deflects towards infinity, it means that the voltage coil remains in the weak magnetic field and thus experienced the very little torque.
• The torque experienced by the coil increases when it moves insides the strong magnetic field. The coil experience the maximum torque under the pole faces and the pointer set at the zero end of the resistance scale.
• For improving the torque, the voltage coil V2 is used. The coil V2 is so allocated that when the pointer deflects from infinity to zero coil moves into a stronger magnetic field.
• Resultant torque is directly proportional to voltage and inversely proportional to current.
• In Megger, the combined action of both the voltage coils V1 and V2 are considered. The coil comprises a spring of variable stiffness. It is stiff near the zero ends of the coil and becomes very weak near the infinity end of the spring.
• A current limiting resistor is connected in series with a control and deflecting coil to protect damage in case of very low resistance in the external circuit.
• The spring compresses the low resistance portion and opens the high resistance of the spring, which is the great advantage of the Megger because it is used for measuring the insulation of the resistance which is usually very high.
• The megger instrument simply connects across the insulation to be tested and operated it for a short, specific time period (60 seconds is usually recommended).
• For more than 60 seconds the temperature and humidity as well as the condition of insulation affect the reading.

The correct answer is option "3".

Explanation:-

Megger:

• Megger is a measuring instrument used for the measurement of the insulation resistance of an electrical system.
• As the open circuit has very high resistance, so the megger will so infinite resistance.
  Medgar does not need any controlling torque.
• An electrical system degrades its quality of insulation resistance with time and various environmental conditions including temperature, moisture, dust particles & humidity.
• Even mechanical and electrical stress affects the insulation resistance which adds to the necessity of checking insulation resistance at regular intervals to avoid fatal errors or electrical shocks
• Megger is used for measuring the electrical leakage in wires, and electrical insulation levels in generators, motors, etc.

Important Points Examples of High Resistance Materials:

1. Cable Insulation Resistance, all Motors, generators, and transformers winding's insulation resistance, all semiconductor devices when reverse biased.

Methods to measure High Resistance:

• ​Loss of charge method.
• Direct-Deflection method.
• Mega ohm Bridge.

Solution

Given 

• Galvanometer scale distance from mirror =0.4m
• deflection observed =44mm

Concept

the formula relating deflection, the distance of scale from the mirror, and coil turn angle is given by

⇒Deflection(d)=2 r θ_r  ,here

• d is the deflection observed
• r is the distance of scale from the mirror
• θ_r is the angle through which coil has tuned

​Calculation

⇒d=2 r θr 

⇒θ = \(\frac{d}{2r} \)

⇒θ = \(\frac{44 × 10^{-3}}{2× 0.4}\)

⇒θ = 55 × 10^-3 radian

Hence angle through which the coil turns = 55 × 10-3 rad

The correct option is 4

The Measurement of Resistance according to classification is shown below:

The Megger method is used for the measurement of the high value of resistance. And this is best suitable for the measurement of insulation resistance of cables.

Additional Information

To measure High resistance Three(3) terminals are required.

Where,

Terminals 1 and 2 are input terminals, Terminal 3 is called Guard Terminal

Guard Terminal is used to eliminate or bypass Leakage currents while measuring.

Examples of High Resistance Materials:

• Cable Insulation Resistance.
• All Motors, generators, and transformers winding's insulation resistance.
• All semiconductor devices when reverse biased.

Methods to measure High Resistance:

1. ​Loss of charge method.
2. Direct-Deflection method.
3. Mega ohm Bridge.
4. Meggar.

Megger

• The Megger is the instrument uses for measuring the resistance of the insulation.
• It works on the principle of comparison, i.e., the resistance of the insulation is compared with the known value of resistance.
• If the resistance of the insulation is high, the pointer of the moving coil deflects towards infinity, and if it is low, then the pointer indicates zero resistance.
• The accuracy of the Megger is high as compared to other instruments.

Construction of Megger

• The Megger has one current coil and two voltage coils V₁ and V₂.
• The voltage coil V₁ is passed over the magnet connected to the generator.
• When the pointer of the PMMC instrument deflects towards infinity, it means that the voltage coil remains in the weak magnetic field and thus experienced very little torque.
• The torque experienced by the coil increases when it moves inside the strong magnetic field.
• The coil experience the maximum torque under the pole faces and the pointer is set at the zero ends of the resistance scale.
• For improving the torque, the voltage coil V₂ is used.
• The coil V₂ is so allocated that when the pointer deflects from infinity to zero coil moves into a stronger magnetic field.
• In Megger, the combined action of both the voltage coils V1  and V₂ are considered.
• The coil comprises a spring of variable stiffness. It is stiff near the zero ends of the coil and becomes very weak near the infinity end of the spring.

• Megger is an electrical instrument used to determine the range of resistances between zero to infinity.
• Initially, the pointer is at the infinite position, it gets deflected when an emf is generated from infinity to zero, which depends on Ohm’s law.
• Megger works on the principle of electromagnetic induction.
• A permanent magnet dc generator driven by hand is used as a source of supply to the megger.
• When a primary coil that is carrying current is placed under the vicinity of a magnetic field it experiences a force. 
• This kind of force generates a torque that is made to deflect the pointer of the device which gives some reading.

• Megger is a portable instrument to measure high insulation resistance
• It basically works on the principle of electromagnetic induction.
• The electrical power to a megger is provided by a permanent magnet D.C. generator.
• The operating voltage of a megger is about 50 to 100 V.
• The speed of megger is kept around at 160 rpm.
• Generally, megger is the combination of DC generator and series type ohmmeter.
• Series type ohmmeter is used due to the measurement of a high range of resistance i.e. insulation resistance in (0 - infinity) Ω range.

Additional Information

Megger: 

1.Definition:

• ​The Megger is the instrument uses for measuring the resistance of the insulation.
• It works on the principle of comparison, i.e., the resistance of the insulation is compared with the known value of resistance.
• The Megger has three coils two pressure coils (control coil) and one current coil. The pressure coil rotates the moving coil in the anticlockwise direction, whereas the current coil rotates it in the clockwise direction.
• If the resistance of the insulation is high, the pointer of the moving coil deflects towards infinity, and if it is low, then the pointer indicates zero resistance.
• The accuracy of the Megger is high as compared to other instruments.

2. Construction of Megger:

• It is the combination of DC Generator and Series type Ohm Meter.
• The Megger has one current coil and two voltage coils V1 and V2.
• Deflecting coil or current coil connected in series and allows flowing the electric current taken by the circuit being tested.
• The control coil is also known as the pressure coil is connected across the circuit.
• D.C generator or Battery connection: The voltage is generated by connecting the hand-driven generator.
• Deflecting and Control coil: Connected parallel to the generator, mounted at a right angle to each other, and maintain polarities in such a way to produce torque in opposite direction.
• Permanent Magnets: Produce magnetic field to deflect pointer with North-South pole magnet.
• Pointer: One end of the pointer connected with coil another end deflects on a scale from infinity to zero.
• Scale: A scale is provided in the front-top of the megger from range ‘zero’ to ‘infinity’, enabling us to read the value.
• Pressure Coil Resistance and Current Coil Resistance: Protect instrument from any damage because of low external electrical resistance under test.

3.Working Principle of Megger:

• The voltage coil V1 is passed over the magnet connected to the generator. When the pointer of the PMMC instrument deflects towards infinity, it means that the voltage coil remains in the weak magnetic field and thus experienced very little torque.
• The torque experienced by the coil increases when it moves insides the strong magnetic field. The coil experience the maximum torque under the pole faces and the pointer set at the zero ends of the resistance scale.
• For improving the torque, the voltage coil V2 is used. The coil V2 is so allocated that when the pointer deflects from infinity to zero coil moves into a stronger magnetic field.
• Resultant torque is directly proportional to voltage and inversely proportional to current.
• In Megger, the combined action of both the voltage coils V1 and V2 are considered. The coil comprises a spring of variable stiffness. It is stiff near the zero ends of the coil and becomes very weak near the infinity end of the spring.
• A current limiting resistor is connected in series with control and deflecting coil to protect damage in case of very low resistance in the external circuit.
• The spring compresses the low resistance portion and opens the high resistance of the spring, which is the great advantage of the Megger because it is used for measuring the insulation of the resistance which is usually very high.
• The megger instrument simply connects across the insulation to be tested and operated for a short, specific time period (60 seconds is usually recommended).
• For more than 60 seconds the temperature and humidity as well as the condition of insulation affect the reading.

Megger:

1.Definition:

• ​The Megger is the instrument uses for measuring the resistance of the insulation.
• It works on the principle of comparison, i.e., the resistance of the insulation is compared with the known value of resistance.
• The Megger has three coils two pressure coils (control coil) and one current coil. The pressure coil rotates the moving coil in the anticlockwise direction, whereas the current coil rotates it in the clockwise direction.
• If the resistance of the insulation is high, the pointer of the moving coil deflects towards the infinity, and if it is low, then the pointer indicates zero resistance.
• The accuracy of the Megger is high as compared to other instruments.

2.Construction of Megger:

• It is the combination of DC Generator and Ohm Meter.
• The Megger has one current coil and the two voltage coils V1 and V2.
• Deflecting coil or current coil connected in series and allows flowing the electric current taken by the circuit being tested.
• The control coil is also known as the pressure coil (voltage coil) is connected across the circuit which provides controlling torque to the instrument.
• D.C generator or Battery connection: The voltage is generated by connecting the hand-driven generator.
• Deflecting and Control coil: Connected parallel to the generator, mounted at a right angle to each other, and maintain polarities in such a way to produced torque in opposite direction.
• Permanent Magnets: Produce magnetic field to deflect pointer with North-South pole magnet.
• Pointer: One end of the pointer connected with coil another end deflects on a scale from infinity to zero.
• Scale: A scale is provided in the front-top of the megger from range ‘zero’ to ‘infinity’, enable us to read the value.
• Pressure Coil Resistance and Current Coil Resistance: Protect instrument from any damage because of low external electrical resistance under test.

3.Working Principle of Megger:

• The voltage coil V1 is passed over the magnet connected to the generator. When the pointer of the PMMC instrument deflects towards infinity, it means that the voltage coil remains in the weak magnetic field and thus experienced the very little torque.
• The torque experienced by the coil increases when it moves insides the strong magnetic field. The coil experience the maximum torque under the pole faces and the pointer set at the zero end of the resistance scale.
• For improving the torque, the voltage coil V2 is used. The coil V2 is so allocated that when the pointer deflects from infinity to zero coil moves into a stronger magnetic field.
• Resultant torque is directly proportional to voltage and inversely proportional to current.
• In Megger, the combined action of both the voltage coils V1 and V2 are considered. The coil comprises a spring of variable stiffness. It is stiff near the zero ends of the coil and becomes very weak near the infinity end of the spring.
• A current limiting resistor is connected in series with a control and deflecting coil to protect damage in case of very low resistance in the external circuit.
• The spring compresses the low resistance portion and opens the high resistance of the spring, which is the great advantage of the Megger because it is used for measuring the insulation of the resistance which is usually very high.
• The megger instrument simply connects across the insulation to be tested and operated it for a short, specific time period (60 seconds is usually recommended).

For more than 60 seconds the temperature and humidity as well as the condition of insulation affect the reading.

Important Points

• Megger is a portable instrument to measure high insulation resistance
• It basically works on the principle of electromagnetic induction
• The electrical power to a megger is provided by permanent magnet D.C. generator
• The operating voltage of a megger is about 50 to 100 V
• The speed of megger is kept around at 160 rpm

Concept:

Megger:

• It is a measuring instrument that is used for the measurement of high resistance.
• It is used for the measurement of the insulation resistance of an electrical system.
• Resistance (R) for any circuit is given by:

           R = \({V_R \over I_R}\)

where I_R = Current

V_R = Voltage

R = Resistance

Case 1:   Open-circuit condition​

In this condition, I_R = 0 A

R = \({V_R \over 0}\)

R = ∞ Ω 

Case 2:   Short-circuit condition

In this condition, VR = 0 A

R = \({0 \over I_R}\)

R = 0  Ω ​

From the above conditions, we observe that the reading of megger during open-circuited condition will be equal to infinity.

High resistance:

• Generally, if the resistance is more than 100 kΩ, it is called high resistance.
• Cable insulation resistance, machine winding insulation resistance, reverse-biased semiconductors are some examples of high resistance.
• Usually, to measure high resistance three terminals are required.
• The third terminal is the guard terminal which is used to eliminate leakage currents.

Methods used for measuring high resistance are 

1. Loss of charge method
2. Megger method
3. Mega ohm bridge method
4. Direct deflection method

Important Points

• Among all the above methods loss of charge method is an accurate method to measure high resistance.
• Loss of charge is used to find cable insulation in research laboratories.
• Megger is also can be used for the measurement of insulation resistance of the cable.
• In megger, spring control is not required because it does not need any controlling torque.

Multimeter:

• A multimeter consists of a voltmeter, current meter, and Ohm meter
• A multimeter can be used to measure voltage, current and resistance.
• The current drawn by an electronic circuit is in the order of milliamperes. We can measure this using multimeter
• The supply voltage to DC motor is around 240 V, we can measure this voltage by using a multimeter
• We can do continue test of motor winding by using a multimeter
• We cannot measure insulation resistance using a multimeter

Megger:

• Megger is a portable instrument to measure high insulation resistances.
• It works on the principle of electromagnetic induction.
• The electrical power to a megger is provided by a permanent magnet D.C. generator.
• The test voltages are usually of order 500, 1000, or 2500 V are generated by a hand-driven generator (permanent magnet D.C. generator).
• The operation of a megger is based on a moving coil meter.
• Earth pit resistance is measured with an earth tester, also called earth megger which can test the resistance across a range of currents and distances.
• Insulation megger is used to measure the insulation resistance of the cables.