The article discusses the essential steps and considerations when drying out wet electrical equipment, emphasizing the importance of insulation resistance tests during the drying process.
Tests During Drying Out of Equipment
1. Hazard of Wet Electrical Equipment:
Wet electrical equipment poses a common hazard for maintenance engineers. The approach to drying out the equipment depends on whether it’s wet from fresh water or saltwater. Saltwater requires washing with fresh water to remove corrosive salt deposits.
2. Removing Oil and Grease:
In addition to moisture, it’s crucial to remove any oil or grease from the insulation, which can be done using an appropriate solvent.
3. Drying Techniques:
Various methods can be used to dry electrical equipment, including hot air, ovens, current circulation, or a combination of techniques. The choice of method depends on the equipment’s size, portability, and manufacturer recommendations.
4. Insulation Resistance Tests:
Insulation resistance tests are vital during the drying process. If you have records of previous insulation resistance tests, they can help determine when the insulation is moisture-free.
5. Use of Low-Voltage Megger Tester:
Wet equipment is susceptible to voltage breakdown, so it’s advisable to use a low-voltage Megger tester (e.g., 100 or 250 VDC) in the initial stages of drying. Slow cranking of a 500-volt tester can be a substitute if a low-voltage instrument isn’t available.
6. Kilohm Range Measurement:
Some testers have a kilohm range that measures below the Megohm range. This range is ideal for an initial measurement on flooded equipment and can serve as a benchmark for monitoring the drying progress.
7. Importance of Past Readings:
Past insulation resistance readings are crucial for evaluating current measurements. A significant drop in resistance compared to past records indicates that moisture is still present.
8. Drying-Out Curve:
The typical drying-out curve for a DC motor armature is explained. Initially, resistance may decrease due to higher temperature, but it eventually rises to a high value as drying progresses.
9. Periodic Testing:
Periodic insulation resistance tests during the drying process, along with readings from dry equipment, help determine when the insulation reaches a safe value for the unit. Time-resistance tests, such as the dielectric absorption ratio or polarization index, can be useful for monitoring dry-out progress without temperature correction.
In summary, proper drying and insulation resistance testing are essential steps in restoring electrical equipment that has been exposed to moisture. Monitoring the insulation resistance values and comparing them to past records are key to ensuring the equipment’s safety and functionality.
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.