How to Perform Insulation Resistance Testing

From Fluke Corp.

Manufacturing plants face the risk of unscheduled shutdowns, which can greatly impact production and profitability. To minimize this risk, implementing a Preventative Maintenance Program (PMP) is crucial.

Insulation measurement is an important aspect of a PMP, especially for the many motors used in manufacturing facilities. By regularly testing insulation resistance and analyzing historical data, it’s possible to detect potential failures in advance. This article can serve as a guide for the process of testing insulation resistance to make sure equipment runs smoothly.

Equipment Types

Consider the following equipment types commonly found in manufacturing plants and their significance in daily operations that can benefit from testing insulation resistance and analyzing historical data:

1. Pumps.
2. Conveyors.
3. Compressors.
4. Fans.
5. Mixers.
6. Grinders.
7. HVAC Systems.
8. Refrigeration Units.

Importance of Insulation Resistance Testing

The insulation coating within motors deteriorates over time due to wear and tear. Factors such as moisture and contamination can also lead to premature motor failure.

Detecting degraded insulation in electromechanical equipment is important to prevent motor failures and subsequent production losses. By incorporating regular insulation checks into a preventive maintenance schedule, you can identify issues and prevent unexpected shutdowns.

How It Works

Insulation resistance testing helps identify leaks in wire insulation that can be dangerous and costly. Over time, insulation resistance gradually decreases, indicating normal deterioration. However, sudden drops and returns in current during the test can indicate more severe issues.

This testing method isn’t limited to motors. It’s also applicable to critical electrical equipment such as airport runway lighting and alert-monitoring system cables.

Insulation Testers

Insulation testers are highly suitable for testing capacitance and leakage current in high-voltage equipment such as switchgears, motors, generators and cables. Timed ratio tests, such as the polarization index (PI) and dielectric absorption rate (DAR), detect insulation resistance.

The PI is the ratio of the 10-minute resistance value to the 1-minute resistance value, while the DAR is the ratio of the 60-second resistance value to the 30-second resistance value. These tests analyze current changes over specific time intervals and provide ratios for comparison.

Note that factors like voltage and temperature affect current flow, and they need to be compensated for accurate insulation-resistance performance evaluation.

Temperature Considerations

Temperature significantly affects insulation resistance values. Deviations from the baseline temperature can drastically change resistance. For every 10°C (18°F) increase above the baseline temperature, the resistance value halves. Conversely, for every 10°C (18°F) decrease below the baseline temperature, the resistance value doubles.

To achieve accurate measurements, the Institute of Electrical and Electronics Engineers (IEEE) recommends correcting resistance measurements to a constant, compensated temperature of 40°C (104°F). This establishes a consistent baseline for historical comparisons.

Capturing Testing Data

To use insulation resistance testing effectively, maintain a record of the collected data. Historical tracking and trending of equipment performance facilitates identification of degradation patterns over time. This allows you to predict maintenance needs and repairs, preventing costly disruptions in plant operations.

When collecting insulation-resistance testing data, include the following information at a minimum:

• Insulation resistance values.
• Testing time stamps.
• Contextual information (e.g., output test voltages, test durations, temperature compensations).

Continuous Testing and Maintenance

Insulation resistance testing should be performed during equipment installation and continue throughout its lifetime. Regularly scheduled preventive maintenance is vital to identifying and addressing problems before they lead to failures so you can take early action to prevent them.

By following these steps and implementing insulation resistance testing as part of a PMP, it’s possible to minimize the risk of manufacturing line shutdowns, optimize equipment performance and achieve consistent production output.

Based in Everett, Washington, Fluke Corp. manufactures, distributes and services electronic test tools.

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