Preventive maintenance is a predetermined task performed based on a schedule and its objective is to keep equipment in good condition to avoid breakdowns. With properly scheduled monitoring and data collection, this testing can be very useful in analyzing and predicting the current and future behavior of equipment. Early problem detection helps avoid major repairs, resulting in cost savings when compared to a run-to-failure maintenance practice. Preventive maintenance has the added benefit of pre-planning for necessary parts and resources.
This article describes the insulation resistance testing method commonly used for preventive maintenance activities. Insulation resistance testing is commonly performed as part of electrical testing in a preventive maintenance program for rotating machines, cables, switches, transformers, and electrical machinery where insulating integrity is needed. Insulation resistance testing in the preventive maintenance program helps identify potential electrical issues to reduce unpredictable, premature equipment repair and replacement cost (see Figure 1).
Insulation resistance is used to verify the integrity of the insulation material, whether cable insulation or motor/generator winding insulation. Insulation resistance testing is carried out by applying a constant voltage to the equipment under test while measuring the flowing current. High DC voltage is used, causing a small current to flow through the insulator surface. The total current consists of three components (see Figure 2):
- Capacitance charging current: relatively high upon start-up, drops exponentially within a few seconds to dozens of seconds. It is normally negligible when the reading is taken.
- Absorption current: decays at a decreasing rate. It may require up to a few minutes to reach zero depending on the insulation materials.
- Leakage current: constant over time.
For an effective test, results should be regularly recorded over a period of time and compared with earlier recorded values taken when the equipment was new and in good condition. The trend of the readings over a period of time will help identify the presence of anomalies. Insulation resistance values that are consistent over time indicate that the equipment’s insulation properties are good. If the resistance values are decreasing, it indicates that potential issues can occur sometime in the future and more thorough preventive maintenance should be scheduled soon.
Factors that affect the insulation resistance
The factors that commonly affect the insulation resistance are:
- Surface condition. For example oil or carbon dust on the equipment’s surface that can lower the insulation resistance.
- Moisture. If the equipment’s surface temperature is at, or below, the dew point of the ambient air, a film of moisture forms on its surface would, lowering the equipment’s resistance value.
- Temperature. The insulation resistance value may vary inversely with the change of the temperature. Its influence on readings can be mitigated by performing preventive maintenance testing at the same temperature each time. If the temperature cannot be controlled, normalizing to a base temperature such as 40 °C is recommended. This is commonly done using the estimation rule, “Every 10 °C increase in temperature halves the insulation resistance, while a 10 °C reduction doubles the resistance”. As different materials may have different degrees of resistance change due to temperature, for more precise temperature correction, some may adopt a temperature correction factor; the measurement reading should be multiplied bythe temperature correction factor at the corresponding temperature.
What are the test methods for insulation resistance test?
There are three types of tests for measuring insulation resistance, and each test applies its own methodology that focuses on a specific insulating property of the devices being tested. Users need to choose the one that best fits the test requirements.
Spot test: This test is suitable for a device with a small or negligible capacitance effect, e.g. short wiring run.
A test voltage is applied for a short interval until a stable reading is achieved, or for a fixed period of time, normally 60 seconds or less. The reading is collected at the end of the test. For the historical record, a chart is plotted based on the history of the readings. Observation of the trend is taken over a period of time, normally over years or months (see Figure 3).