Insulation Resistance (IR) Testing in Bangladesh

What does an IR reading actually tell you?

Insulation is never perfect; a tiny current always leaks through it. The tester measures that leakage at a set DC voltage and reports it as resistance, usually in megohms. A high, stable reading means the insulation is dry and sound. A low reading, or one that has fallen sharply since last year, points to moisture, contamination, heat damage or a crushed cable.

A single number says less than people think, because readings change with temperature, humidity and cable length. So we record the conditions, apply temperature correction where the method calls for it, and compare against earlier results. A steady downward trend is often the most useful warning in the whole report.

When is a polarization index test worth doing?

For motors, generators and transformers, a one-minute reading can mislead. The polarization index compares the reading at ten minutes with the reading at one minute. Clean, dry insulation keeps absorbing charge, so its reading rises over time; wet or contaminated insulation stays flat. IEEE 43 describes the test for rotating machines.

It takes longer, so we use it where the equipment justifies the time: large motors driving production lines, fire pump motors, standby generators and distribution transformers. For ordinary building cables, spot readings at the right test voltage, compared year on year, are usually enough.

Why test after flooding or a long shutdown?

Monsoon flooding, roof leaks and a factory standing idle through the Eid holidays all let moisture into cables, motor windings and panels. Switching back on without testing risks a flashover at the worst moment. An IR test before re-energizing shows which circuits are safe, which need drying out and which need replacing.