Thursday, July 24, 2014

Preventive Maintenance test with Insulation Resistance Test, Part 1

Preventive maintenance is a predetermined task performed based on a schedule and its objective is to keep equipment in good condition to avoid breakdowns.
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.

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 first of three articles will describe what is insulation resistance testing, how it plays a part in preventive maintenance and factors that affect insulation resistance. The second article will focus on the methods of insulation resistance testing1  while the last article will detail test voltage selection guidelines and safety considerations.

1 Consulting the original product/equipment manufacturer for more detailed information is recommended.

What is Insulation Resistance Testing?
Insulation resistance is used to verify the integrity of the insulation material. It can be the 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 any flowing current. High DC voltages are used causing a small current to flow through the insulator surface. The total current consists of three components: capacitance charging current, absorption current, and leakage current (refer to Figure 1.)

  • Capacitance charging current is relatively high upon start-up and drop exponentially within a few seconds to a few ten 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 is constant over time.
Figure 1 Components of test current
How Insulation Resistance Testing Helps in Preventive Maintenance
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 by multiplying the measurement reading with the temperature correction factor at the corresponding temperature.

In the next article (Part 2), I will cover the insulation resistance test methods.


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