The article discusses the differences between AC (alternating current) and DC (direct current) testing for insulation resistance and highlights the advantages of using DC testing for high-voltage maintenance and field testing. Here’s a breakdown of the key points:
AC Testing vs. DC Testing
1. Types of Current Produced:
- DC Testing: DC testing primarily focuses on the conduction or leakage current in insulation. It measures the steady-state current, which provides valuable information about insulation resistance.
- AC Testing: AC testing results in a charging current that is significantly larger compared to the leakage current. Leakage current is relatively minor in AC testing.
2. Destructiveness:
- DC Testing: DC testing is generally considered non-destructive to insulation. It allows for measuring insulation resistance without causing significant harm to the insulation material.
- AC Testing: AC testing, often used for high-potential testing, is more of a GO/NO-GO test. It can potentially cause deterioration of insulation due to the high charging current involved.
3. Suitability for Proof Testing:
- DC Testing: DC testing provides a more qualitative picture of insulation resistance. It allows for metering leakage current as voltage increases, providing specific values of insulation resistance.
- AC Testing: AC testing can be suitable for proof testing, where equipment needs to meet specific standards. It involves applying a chosen voltage level to determine if the equipment passes or fails.
4. Economic Advantages:
- DC Testing: DC testing is more economically advantageous as equipment size increases. AC test equipment becomes bulkier and more expensive with higher test voltages due to the need to supply and handle the high charging currents.
DC test sets are preferred for high-voltage maintenance and field testing due to their lower cost, lighter weight, smaller size, non-destructive nature, and ability to provide better quality and quantity of information about insulation resistance.
In essence, DC testing is favored in high-voltage maintenance and field testing because it offers practical and economic benefits while providing valuable insights into insulation conditions. AC testing, on the other hand, is more suitable for certain specific applications such as proof testing.
Related Articles:
What is Good insulation?
What Makes Insulation Go Bad?
How Insulation Resistance is Measured.
How to Interpret Resistance Readings.
Factors Affecting Insulation Resistance Readings.
Types of Insulation Resistance Tests.
Test Voltage vs. Equipment Rating.
AC Testing vs. DC.
Use of DC Dielectric Test Set.
Tests During Drying out of Equipment.
Effect of Temperature on insulation Resistance.
Effects of humidity.
Preparation of Apparatus to test.
Safety Precautions.
Connections for testing insulation resistance of electrical equipment.
Additional Notes About using A Megger Insulation Tester.
Interpretation-Minimum Values.
Minimum Values for Insulation Resistance.
Tests Using Multi-Voltage Megger Insulation Testers.
Step-Voltage Method.
Use of a Guard Terminal.
Outdoor Oil Circuit Breaker.