Insulation Tester Guide for Motors and Cables

Why Insulation Testing Is Non-Negotiable

Insulation breaks down slowly and silently — until the day it fails catastrophically. A routine insulation tester reading flags moisture ingress, contamination, and thermal ageing months before the equipment trips on earth-fault. The investment is small; the asset protection is enormous.

1. Choose the Right Test Voltage

Voltage selection follows the system rating. Use too little and you cannot stress mediocre insulation; use too much and you over-stress healthy windings.

• ≤ 250 V system → test at 500 V DC.
• 440–600 V system → test at 1000 V DC.
• 1 kV–5 kV motors → test at 2.5 kV DC.
• 5 kV–15 kV switchgear → test at 5 kV DC.

For HV cable acceptance, IEC 60502 specifies even higher voltages and is the realm of dedicated VLF testers — not handheld megohmmeters.

2. Read More Than One Number

A single 30-second insulation-resistance (IR) value is the weakest possible measurement. Two ratios separate good insulation from contaminated insulation:

• Dielectric Absorption Ratio (DAR) — IR at 60 s ÷ IR at 30 s. Above 1.4 is good; 1.0–1.25 is poor.
• Polarisation Index (PI) — IR at 10 min ÷ IR at 1 min. Above 4 is excellent; below 2 indicates moisture or carbon tracking.

3. Motor-Specific Procedure

Disconnect the motor from its starter and ground all three phases for at least 5 minutes before testing. Measure phase-to-ground for each winding separately, then phase-to-phase. Watch the trend over years rather than absolute numbers — a 50 % drop is alarming even if the value is still in spec.

4. Cable-Specific Procedure

For long power cables, use a 1000V insulation resistance tester with a guard terminal to bypass surface leakage. Test conductor-to-screen, conductor-to-conductor and screen-to-ground. After the test, ground both ends of the cable for at least the test duration to discharge the capacitance — surprisingly often skipped, and a serious shock hazard.

5. Common Mistakes EK Engineers See

1. Testing energised circuits — instant meter destruction.
2. Forgetting to discharge after the test.
3. Reporting only IR without PI/DAR ratios.
4. Mixing test voltages between years and pretending the numbers are comparable.
5. Skipping the visual inspection that often finds the fault before the meter is even connected.