PI & DAR: Core Insulation Diagnosis for Electrical Equipment
Core Insulation Diagnostic Indicators: PI & DAR
When testing insulation resistance of high-voltage equipment such as motors and transformers, a single resistance reading often leads to inaccurate equipment condition evaluation.
PI (Polarization Index) and DAR (Dielectric Absorption Ratio) are two universal ratio tests in the electrical industry. By comparing insulation resistance values under different voltage application durations, they eliminate data interference caused by ambient temperature, humidity and equipment capacity, so as to intuitively judge insulation dampness, contamination and aging.
The rating chart below serves as a quick on-site judgment reference, complying with authoritative electrical maintenance standards IEEE Std 43 and NFPA 70B.
I. PI – Polarization Index
Application: In-depth insulation diagnosis for rotating equipment including motors, generators and large transformer
When to use PI: a full PI test requires 10 minutes of continuous DC voltage. It is suitable for equipment overhaul and annual preventive maintenance to conduct thorough insulation evaluation. Rotating machines have clear minimum acceptable PI thresholds specified by international standards, which acts as the core judgment basis.
II. DAR – Dielectric Absorption Ratio
Application: Fast on-site screening during routine inspections
When to use DAR: the whole test finishes within 1 minute with high efficiency, ideal for batch pre-inspection to quickly detect surface moisture and leakage. If DAR shows questionable results, supplementary PI testing is recommended to confirm internal insulation status.
III. Important Test Guidelines
1. Application differentiation: DAR for fast field screening; PI for deep diagnosis of large-scale equipment.
2. Comprehensive judgment: Never draw conclusions solely from PI or DAR values. Combine absolute insulation resistance, ambient temperature & humidity, and historical test data for full assessment.
3. Safety operation: Cut off all power and discharge residual charge before testing; fully discharge the tested equipment after measurement to avoid high-voltage electric shock hazards.
IV. FAQ
Q: Can I skip PI & DAR and only test insulation resistance?
Not recommended. Standalone insulation resistance fluctuates drastically with equipment size and ambient temperature, lacking unified comparison criteria for new and aged assets. PI and DAR adopt ratio calculation to exclude external interference, which are mandatory routine items for high-voltage equipment preventive tests.
Q: DAR passes but PI of the motor is low, is the equipment safe to operate?
No. DAR only reflects short-term surface insulation performance, while PI evaluates deep winding polarization characteristics. Internal dampness and deep aging cannot be detected merely by DAR, so PI is the primary criterion for rotating machinery.
Q: Current PI & DAR values are acceptable, but the equipment has run for years. Is maintenance needed?
We suggest establishing a test log and re-testing under fixed temperature & humidity every year to track index trends. If the ratios keep dropping year by year, pre-emptive cleaning and drying maintenance are recommended even if readings are still within acceptable ranges, to avoid sudden insulation breakdown failures.