Loading Effects Due to Shunt Connected and Series Connected Instruments

Introduction

In electrical measurements, it is important to consider the loading effects caused by the instruments used. Loading effects refer to the changes in the measured quantity caused by the presence of the measuring instrument itself. This can lead to inaccuracies in the measurements and affect the overall performance of the measurement system.

To understand the loading effects, it is necessary to have a clear understanding of shunt connected and series connected instruments.

Loading Effects

Loading effects occur when the impedance of the measuring instrument affects the circuit being measured. This can result in a change in the voltage or current being measured, leading to inaccurate readings. The magnitude of the loading effects depends on various factors such as the impedance of the instrument, the impedance of the circuit being measured, and the type of connection (shunt or series).

Shunt Connected Instruments

Shunt connected instruments are connected in parallel with the circuit being measured. They are primarily used for measuring current. The shunt connected instrument diverts a portion of the current away from the circuit, causing a voltage drop across the shunt resistor. This voltage drop can affect the accuracy of the measurement.

The loading effects in shunt connected instruments can be observed in two ways:

1. Voltage drop across the shunt resistor: The presence of the shunt resistor creates a voltage drop, which can affect the voltage measurement across the circuit being measured.

2. Change in current flowing through the shunt resistor: The shunt connected instrument diverts a portion of the current away from the circuit, resulting in a change in the current flowing through the shunt resistor.

Shunt connected instruments are commonly used in applications such as ammeters and current transformers.

Series Connected Instruments

Series connected instruments are connected in series with the circuit being measured. They are primarily used for measuring voltage. The series connected instrument introduces a series resistor into the circuit, which can affect the current flowing through the circuit and the voltage drop across the series resistor.

The loading effects in series connected instruments can be observed in two ways:

1. Change in voltage drop across the series resistor: The presence of the series resistor can cause a change in the voltage drop across the circuit being measured.

2. Change in current flowing through the series resistor: The series connected instrument affects the current flowing through the circuit, resulting in a change in the current flowing through the series resistor.

Series connected instruments are commonly used in applications such as voltmeters and voltage transformers.

Calibration Curve

A calibration curve is a graphical representation of the relationship between the input quantity and the output reading of a measuring instrument. It is used to determine the accuracy and linearity of the instrument.

Loading effects can impact the calibration curve of a measuring instrument. The presence of loading effects can cause shifts or distortions in the calibration curve, leading to inaccurate measurements.

To create a calibration curve considering the loading effects, the instrument should be tested and calibrated under different loading conditions. The measurements obtained at different loading conditions can then be used to create a calibration curve that takes into account the loading effects.

Testing and Calibration of Instruments

Testing and calibration of instruments are essential to ensure accurate measurements. When testing and calibrating shunt connected instruments, it is important to consider the loading effects caused by the instrument itself. The instrument should be tested and calibrated under different loading conditions to determine its accuracy and performance.

Similarly, when testing and calibrating series connected instruments, the loading effects caused by the series resistor should be taken into account. The instrument should be tested and calibrated under different loading conditions to ensure accurate measurements.

To minimize the loading effects during testing and calibration, it is important to use calibration standards with known impedance values and to carefully select the test setup.

Advantages and Disadvantages

Shunt connected instruments have the following advantages:

• They can measure high currents accurately.
• They have a low burden voltage.

However, they also have some disadvantages:

• They can cause a voltage drop across the shunt resistor, affecting the accuracy of voltage measurements.
• They require a separate power supply.

Series connected instruments have the following advantages:

• They can measure high voltages accurately.
• They have a low input impedance.

However, they also have some disadvantages:

• They can cause a change in the current flowing through the circuit being measured, affecting the accuracy of current measurements.
• They require a separate power supply.

Conclusion

In conclusion, loading effects due to shunt connected and series connected instruments can significantly impact the accuracy of electrical measurements. It is important to understand and mitigate these effects to ensure accurate and reliable measurements. By considering the loading effects during testing, calibration, and instrument selection, engineers and technicians can improve the overall performance of the measurement system.

Summary

Loading effects occur when the impedance of the measuring instrument affects the circuit being measured, leading to inaccuracies in the measurements. Shunt connected instruments are connected in parallel with the circuit being measured and can cause voltage drop and change in current. Series connected instruments are connected in series with the circuit being measured and can cause change in voltage drop and current. Loading effects can impact the calibration curve of a measuring instrument, leading to inaccurate measurements. Testing and calibration of instruments should consider the loading effects to ensure accurate measurements. Shunt connected instruments have advantages such as accurate measurement of high currents, but they can cause voltage drop and require a separate power supply. Series connected instruments have advantages such as accurate measurement of high voltages, but they can cause change in current and require a separate power supply.

Analogy

Imagine you are trying to measure the flow of water in a pipe using a flow meter. The flow meter itself has some resistance to the flow of water, which can affect the accuracy of the measurement. If the flow meter is connected in parallel with the pipe, it will divert some of the water flow, causing a decrease in the measured flow rate. On the other hand, if the flow meter is connected in series with the pipe, it will introduce some resistance to the flow, causing an increase in the measured flow rate. Similarly, in electrical measurements, shunt connected instruments divert current away from the circuit being measured, while series connected instruments introduce resistance into the circuit, both of which can affect the accuracy of the measurements.