Insulation Resistance Tester — How PI and DAR Reveal Aging Cables

The Limit of Spot Readings

A 1-minute insulation reading on a 30-year-old motor might show 200 MΩ — the textbook "good" value. The same motor could fail next week. A modern insulation resistance tester answers this by trending the resistance over time: Polarisation Index (PI) and Dielectric Absorption Ratio (DAR) reveal aging that a single number hides.

1. What DAR Tells You

DAR = R(60 s) / R(30 s). Healthy insulation absorbs polarisation slowly; the 60-second reading is higher than the 30-second reading. A DAR < 1.25 means the insulation is contaminated or wet.

2. What PI Tells You

PI = R(10 min) / R(1 min). For new windings PI > 4 is excellent; PI < 2 is failing. PI is the gold-standard health number for motors above 100 kW.

3. Test Voltage Selection

• 250 V — telecom and instrumentation cables.
• 500 V — control wiring and 230 V appliances.
• 1 kV — 600 V distribution.
• 2.5 kV - 5 kV — medium-voltage motors and cables.

The EK-C350 covers 250 V to 5 kV in five steps with regulated output.

4. Why Voltage Regulation Matters

A cheap megohm meter drops from 5 kV to 2 kV when loaded by a wet cable — and the resistance reading is meaningless. The EK-C350 holds 5 kV ± 5 % into 5 GΩ load.

5. Built-in PI / DAR Automation

Modern testers auto-trigger the timer and compute PI and DAR. The EK-C350 stores up to 1000 records with timestamp, voltage, and ambient temperature — uploaded to PC via USB for trend reports.

6. Safety Discharge

1. After test, the meter must auto-discharge stored capacitance to < 50 V in < 5 s.
2. Look for a "capacitance discharge" indicator LED.
3. The EK-C350 does both as standard.

For motor-shop and substation work the EK-C350 insulation resistance tester is the right diagnostic tool. Talk to EK service teams for a sample.