Insulation resistance in electrical equipment is influenced by various factors, including equipment design, the type of insulating materials used, insulation thickness and area, cleanliness, moisture, and temperature. To use insulation resistance readings as a reliable measure of equipment condition, these variables must be taken into account.
After electrical equipment is put into service, factors like design, insulating material type, thickness, and area become constant, allowing minimum insulation resistance values to be established with reasonable tolerances. However, post-service variables, including cleanliness, moisture, temperature, and mechanical damage (like fractures), need consideration when taking insulation resistance measurements.
Good Housekeeping:
Cleanliness and the prevention of moisture penetration into insulation are crucial for reliable electrical equipment operation. This can be seen as good housekeeping and is vital for maintaining all types of electrical equipment. The fact that insulation resistance is affected by moisture and dirt, considering temperature, makes the Megger insulation tester invaluable. It serves as both an indicator of cleanliness and good housekeeping and a detector of degradation and potential issues.
Expected Readings for Periodic Tests:
Several criteria have been developed to define minimum insulation resistance values for equipment in service. These criteria act as guidelines. However, periodic tests on in-service equipment often yield readings significantly higher than the suggested minimum values. Therefore, maintaining records of periodic tests is strongly recommended. Consistent decreases in insulation resistance readings can be an early warning sign of trouble, even if the actual values are above the suggested minimum safe levels.
Conversely, if in-service equipment shows periodic test values below the suggested minimum safe levels but remains stable, this might not be a cause for immediate concern. It could be due to uniformly distributed, harmless leakages rather than a dangerous localized weakness. Nevertheless, monitoring changes in insulation resistance test data over time is essential, as trends can be more significant than the numerical values.
The One-Megohm Rule:
For many years, the one-megohm rule has been widely used as an allowable lower limit for insulation resistance in ordinary industrial electrical equipment rated up to 1000 volts. It is also recommended for those who are not deeply familiar with insulation resistance testing practices. For equipment rated above 1000 volts, the rule states a minimum of one megohm per thousand volts. While somewhat arbitrary, this rule has proven effective in practical experience, providing assurance that equipment is not excessively wet or dirty and preventing many unnecessary breakdowns.
Recent studies have led to formulas for minimum insulation resistance values based on insulating material types and the electrical and physical dimensions of specific equipment types. These formulas offer a more tailored approach to insulation resistance 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.
Minimum Values for Insulation Resistance.
Tests Using Multi-Voltage Megger Insulation Testers.
Step-Voltage Method.
Use of a Guard Terminal.
Outdoor Oil Circuit Breaker.