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

Concept:

Low resistance measurement requires precise techniques that minimize the effect of lead and contact resistances. Common methods for such measurements include the Kelvin Double Bridge and Ammeter-Voltmeter method. However, the Loss of Charge method is typically used for measuring high insulation resistance, not low resistance.

Explanation of Methods:

- Potentiometer Method: Used for accurate low voltage and low resistance measurements.
- Loss of Charge Method: Used for high resistance or insulation resistance measurements.
- Ammeter-Voltmeter Method: Simple method for measuring low to medium resistance.
- Kelvin Double Bridge Method: Most accurate method for very low resistance measurements.

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.

Explanation:

Kelvin's Double Bridge Method

Definition: The Kelvin's Double Bridge method is a precise technique used for measuring low resistances, typically in the range of micro-ohms (μΩ) to milli-ohms (mΩ). This method enhances the accuracy of low resistance measurements by eliminating the effect of lead and contact resistances, which are significant when measuring very low resistances.

Working Principle: Kelvin's Double Bridge method is an extension of the Wheatstone bridge principle, incorporating a second set of ratio arms to compensate for the resistance of the connecting leads. This method involves a primary bridge (main bridge) and a secondary bridge (auxiliary bridge). The primary bridge measures the unknown resistance, while the secondary bridge compensates for the lead resistances. By balancing both bridges, the unknown low resistance can be accurately determined.

Advantages:

• High accuracy in measuring very low resistances.
• Minimizes errors due to lead and contact resistances.
• Suitable for laboratory and industrial applications requiring precise low resistance measurements.

Disadvantages:

• More complex setup compared to simpler methods like the Wheatstone bridge.
• Requires precise calibration and careful handling to maintain accuracy.

Applications: Kelvin's Double Bridge method is widely used in applications where accurate low resistance measurements are critical, such as in the testing of electrical components, calibration of measurement instruments, and quality control in manufacturing processes.

Correct Option Analysis:

The correct option is:

Option 4: Kelvin's double bridge method

This option correctly identifies the Kelvin's Double Bridge method as a widely used technique for the precise measurement of low resistances. The method effectively compensates for lead and contact resistances, ensuring accurate measurements.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Wheatstone bridge method

The Wheatstone bridge is a well-known method for measuring electrical resistances. While it is highly effective for medium to high resistance measurements, it is less suitable for low resistance measurements due to the significant influence of lead and contact resistances. The Wheatstone bridge is not as precise as Kelvin's Double Bridge for low resistance measurements.

Option 2: Megger method

The Megger method is typically used for measuring insulation resistance and high resistance values, rather than low resistances. It employs a high-voltage generator to measure the resistance of electrical insulation materials. Therefore, the Megger method is not appropriate for low resistance measurements.

Option 3: Ohm's law method

The Ohm's law method involves measuring voltage and current to calculate resistance (R = V/I). While this method can be used for various resistance measurements, it is not particularly suited for very low resistances due to the difficulty in accurately measuring small voltage drops and the significant impact of lead and contact resistances.

Conclusion:

Understanding the appropriate methods for measuring different ranges of resistance is crucial for obtaining accurate results. Kelvin's Double Bridge method stands out as the optimal choice for measuring low resistances due to its ability to compensate for lead and contact resistances, ensuring precise measurements. Other methods like the Wheatstone bridge, Megger method, and Ohm's law method are better suited for medium to high resistance measurements and specific applications, but they do not provide the same level of accuracy for low resistance measurements as Kelvin's Double Bridge method.

The resistances are classified depending upon the values as:

• The resistances of the order of 1 Ω (or) less than 1 Ω are classified as low resistances.
• The resistances from 1 Ω to 100 KΩ are classified as medium resistances.
• The resistances of the order of 100 KΩ (or) higher are classified as high resistances.
   

The methods used for the measurement of low resistances are:

Kelvin's Double Bridge Method: Kelvin's Double Bridge Method is a modification of the Wheatstone bridge method. 

The circuit diagram is shown below:

R is the unknown resistance and S is the standard resistance.

Ammeter Voltmeter Method:

Connecting voltmeter across the load is suitable for low resistance measurements.

Ammeter reading is:

\(I+I_V=\frac{E(R+R_V)}{R.R_V}\)

Voltmeter reading = E

The calculated resistance is:

\(\frac{E}{I+I_V}=\frac{R.R_V}{R+R_V}\)

Potentiometer Method:

• It works by comparing the unknown resistance with the standard resistance.
• The voltage drop across the known and unknown resistance is measured and by comparison the value of known resistance is determined.

Wheatstone is used for the measurement of a medium value range of resistance from 1 Ω to a few M Ω.

Important Points

Wheatstone Bridge:

• A Wheatstone bridge is used to measure an unknown electrical resistance.
• By balancing two legs of a bridge circuit, the unknown resistance of any one leg can be measured easily.
• It provides extremely accurate measurements.

The circuit is balanced when:  

\(\frac{R_a}{R_x}=\frac{R_1}{R_2}\)

Calculation:

Given bridge,

When R = 10 ohm:

 By bridge balance equation:

⇒\(\frac{P}{10}=\frac{Q}{S}\)

⇒\({P}=\frac{10\times Q}{S}\) ........(i)

When R is changed to 20 ohms and S is changed by 5 ohms:

By bridge balance equation:

⇒\(\frac{P}{20}=\frac{Q}{S+5}\)

⇒\({P}=\frac{20\times Q}{S+5}\) ........(iI)

Equating equations (i) and (ii)

\(\frac{10\times Q}{S}=\frac{20\times Q}{S+5}\)

\(\frac{1}{S}=\frac{2}{S+5}\)

\(2S=S+5 \)

S = 5 Ω

• Megger is a portable instrument to measure high insulation resistances
• It basically works on the principle of electromagnetic induction
• The electrical power to a megger is provided by permanent magnet D.C. generator
• The test voltages are usually of order 500, 1000, or 2500 V are generated by hand driven generator (permanent magnet D.C. generator)

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

• Megger is a portable instrument to measure high insulation resistances
• It basically works on the principle of electromagnetic induction
• The electrical power to a megger is provided by 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 moving coil meter

Kelvin's double bridge is used for measuring low values of resistance.

Note:

CONCEPT:

Wheatstone bridge:

It is an arrangement of four resistance which can be used to measure one of them in terms of rest. Here arms AB and BC are called ratio arm and arms AC and BD are called conjugate arms.

Where R is unknown resistance & S is standard resistance.

The bridge is said to be balanced when deflection in the galvanometer is zero i.e. no current flows through the galvanometer or in other words VB = VD.

In the balanced condition

\(\frac{P}{Q} = \frac{S}{R}\)

Explanation:

We know that the balanced condition of a bridge

\(P \times S = R \times Q\)

\(R = \frac{{P \times S}}{Q} = \frac{{3 \times 6}}{5} = 3.6\;k{\rm{\Omega \;}}\)

Now take minimum value of S = 1 kΩ

\(R = \frac{{P \times S}}{Q} = \frac{{3 \times 1}}{5} = 600\;{\rm{\Omega }}\)

Now take the maximum value of S = 8 kΩ

\(R = \frac{{P \times S}}{Q} = \frac{{3 \times 8}}{5} = 4.8\;k{\rm{\Omega }}\)

The Measurement of Resistance according to classification is shown below:

Measurement of Low, Medium & High Resistance:

Resistance is classified into three categories. Different categories of Resistance are measured by different techniques. They are classified as

• Low Resistance:  Resistance having a value of 1 Ω or less than 1Ω is kept under this category.
• Medium Resistance: This category includes Resistance from 1 Ω to 0.1 MΩ.
• High Resistance: Resistance of the order of 0.1 MΩ and above is classified as High resistance.

Important Points

Variable resistors:

• It is a resistor that can change its resistance value through the control of a person or himself.
• A variable resistor is a resistor of which the electric resistance value can be adjusted.
• There are three types of variable resistors. Potentiometer, rheostat, and trimmer.
   

Fixed resistance:

• It is a type of resistance that cannot change its value.
• The fixed resistance has only one value and never changes (except through temperature, age, etc.).
• Examples are carbon composition resistors, wire resistors, thin film resistors, and thick film resistors.

Additional InformationThe methods employed for measurement of low resistances are:

• Kelvin’s Double Bridge Method
• Potentiometer Method
• Ducter Ohmmeter.
   

The different methods used for Medium resistance are as follows: 

• Ammeter-Voltmeter Method
• Wheatstone Bridge Method
• Substitution Method
• Carey- Foster Bridge Method
• Ohmmeter Method
   

The different methods used for High resistance are as follows: 

• Loss of Charge Method
• Megger
• Megohm bridge Method
• Direct Deflection Method

Murray loop test is the most common method for locating high resistance faults in low resistance conductor circuit. It employs the principle of Wheatstone bridge to determine the fault location.

Important Points:

• Hopkinson’s Test is a method of determining the efficiency of a DC machine. It is a full load test and it requires two identical machines which are coupled together. One of the two machines is operated as a generator and the other is operated as a motor to drive the generator. It is also called a back-to-back test or regenerative test.
• Open circuit test is done on the transformer to determine core losses in transformer and the parameters of the shunt branch of the equivalent circuit of the transformer. This test is done on LV side while keeping HV side of the transformer open-circuited.