Learn why a Megger uses a DC generator for insulation testing. Learn how DC ensures accurate, interference-free, and safe measurements of insulation resistance in electrical systems.
Megger is a popular brand name for various types of insulation testers. Because of its popularity, the term “Megger” is often used with “insulation tester,” and “Megger value” is referred to as “insulation resistance value.”
Nowadays, many other brands of insulation testers are available, so it’s more accurate to call it an insulation tester instead of Megger.
Now, let’s understand why a Megger or insulation tester has a DC generator.
An insulation test is performed to measure the insulation resistance (IR). In this test, the high-voltage (HV) lead of the tester is connected to the conductor, and the low-voltage (LV) lead is connected to the ground.
For example, in the case of a cable, the HV lead is connected to the conductor, and the LV lead is connected to the ground.
The tester generates a high voltage, typically 250V, 500V, 1000V, or 5kV, depending on the voltage rating of the cable.
This high voltage causes a leakage current to flow through the insulation. The tester measures this leakage current to calculate the insulation resistance (IR) value of the connected equipment.
Megger Working Principle
The Megger has a built-in DC generator, which can be manual or electronic. It generates a high DC voltage.
The Megger’s positive lead is connected to a current-carrying part of the circuit, while the negative lead is connected to the equipment’s body. The positive lead is also connected to the equipment’s outer terminal.
Let:
- V = Voltage from the DC generator
- R = Insulation Resistance
The insulation resistance (IR) value is calculated using the formula:
R = V/I
This test can be performed with AC or DC voltage to measure insulation resistance. However, DC voltage is generally used for this purpose.
Importance of DC Generators in Meggers
A Megger has a DC generator because DC is essential for measuring insulation resistance.
Here’s why:
- Eliminates Capacitance Effects
- Prevents Reactive Component Influence
- Polarization and Dielectric Absorption
- Safety and Compatibility
- Compact Design and Durability
1. Eliminates Capacitance Effects
Another issue with using AC for insulation resistance measurements is the generation of capacitive charging currents in the insulation, which affects the accuracy of the measurement. DC eliminates this problem by providing a stable and steady voltage and provides accurate measurements.
2. Prevent Reactive Component Influence
AC introduces reactance due to capacitance and inductance, making it challenging to separate true resistance from the impedance. DC avoids this issue because reactance is not present in the process.
3. Polarization & Dielectric Absorption
Insulating materials have polarization and dielectric absorption properties when DC is applied. These effects provide valuable information about the condition of the insulation. AC cannot provide this information, and therefore, DC is more suitable for such measurements.
4. Safety & Compatibility
High-voltage DC is generally safer than AC for insulation testing because it produces steady leakage currents and reduces the risk of damaging the insulation during the test.
5. Compact Design & Durability
Older models of Meggers use a hand-cranked DC generator, making the device portable and ideal for powering tests in field conditions. This ensures reliable operation in various diverse situations.
The DC generator in a Megger allows for accurate, interference-free, and safe insulation resistance measurements.
6. Ability to Identify Defects
Using DC in the insulation test, you can observe and interpret leakage current, polarization index (PI), and dielectric absorption ratio (DAR), which provide the condition and aging of the insulation.
7. Minimizes Noise and Interference
AC signals are more prone to interference from surrounding electrical systems. DC Meggers are less affected by external electromagnetic interference, resulting in more accurate insulation resistance measurements.
Why is DC Voltage Used in Megger Testing?
To understand the use of insulation testers, it is important to first understand how the current flows in the insulator.
- When a DC voltage applied across an insulator is below its break-over voltage, no current flows through it. In this state, the insulator behaves like a capacitor with a very small capacitance.
- The reactance of the insulator is extremely high under such conditions, resulting in minimal current flow.
- However, when AC is applied, the insulator’s reactance decreases due to the high frequency of AC (as given by Xc=1/2πfC).
- This reduced reactance causes more current to flow through the insulator, which is a combination of leakage current and current due to the lower reactance.
- It is important to note that these currents do not accurately represent the insulation resistance of the insulator. For this reason, insulation resistance is measured using DC voltage.
- An ideal insulator exhibits very high reactance when subjected to AC voltage, effectively blocking current flow to zero.
- However, with DC voltage, the current passing through the insulator directly corresponds to its insulation resistance, allowing for an accurate measurement.
Why are DC Meggers Best for Winding Insulation Tests?
DC is used for insulation testing because it accurately measures the resistance of the insulation in the winding.
If AC were used for this test, the results would reflect impedance rather than pure resistance, as insulation has significant capacitive reactance. This reactance affects the measurement and makes it less reliable for assessing insulation health.
Thus, the condition of the winding is best evaluated by measuring its resistance, not impedance. For this reason, only DC is used for accurate insulation resistance measurements.
DC is used here because there is a need to measure only the resistance of the insulation of the winding.
Why are DC Meggers Preferred for Cable Insulation Tests?
Cables have capacitance. When AC is applied, the alternating polarity charges and discharges the cable capacitance, which can lead to fluctuating readings. DC eliminates this capacitive charging effect and provides stable and accurate insulation resistance measurements.
DC Meggers measure the true resistance by polarizing the cable insulation. This can detect insulation degradation, which is not possible with AC.
The insulation testing standards, such as those from IEEE and IEC, recommend DC voltage for cable insulation testing.
Conclusion:
DC generator in a megger delivers precise insulation resistance measurements by eliminating AC interference, minimizing reactance effects, and ensuring safe, stable, and accurate electrical testing.
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