A megohmmeter reading only means something once you know the winding temperature that went with it. The same motor can read 800 MΩ cold in the morning and 90 MΩ after a shift at full load, and nothing is wrong with it. Correcting to a fixed reference temperature is what makes today’s number comparable to last year’s number.… Read More: Insulation Resistance Temperature Correction Calculator »
Insulation testing in a wind turbine is not the same job as insulation testing in a plant motor room. The machine sits 80 to 150 meters up, inside a steel box that breathes damp air all night. It spends half its life stopped and cold. It is bolted to a power converter that a 500 V megohmmeter will… Read More: Wind Turbine Insulation Testing: Procedures, IEEE 43 Requirements, and Best… »
There is no single insulation resistance formula. There are five, and people mix them up constantly — using the measurement equation when they need the correction equation, or applying an acceptance formula built for 1950s asphalt to a modern epoxy winding. This page lays them out one at a time: what each equation is for, what the symbols… Read More: Insulation Resistance Formula: Every Equation You Need »
You meggered a motor. The display says 12 MΩ. Now what? Almost everyone in the field carries the same number in their head: one megohm. Anything above 1 MΩ, energise it. That rule is older than most of the motors it gets applied to, and for a modern machine it is far too generous. IEEE 43-2013 — the… Read More: Motor Megger Value: What Reading Is Acceptable? »
Short answer: DAR (dielectric absorption ratio) and PI (polarization index) are both ratios taken from a timed insulation resistance test with a megohmmeter. DAR is the resistance at 60 seconds divided by the resistance at 30 seconds — a one-minute check. PI is the resistance at 10 minutes divided by the resistance at 1 minute — a ten-minute… Read More: DAR vs PI: Reading Insulation Absorption Ratios »
A NETA-certified test company once failed a 138 kV transformer on polarization index. The IR measurement was healthy. The power factor was fine. The TTR, excitation current, DGA, leakage reactance, and SFRA all came back acceptable. Everything compared cleanly to previous tests. The transformer was returned to service over the contractor’s objection — and continued to run without… Read More: Transformer Insulation Resistance Testing: IR, PI, DAR, and Acceptance Values… »
If you’ve been doing motor insulation testing for any length of time, you’ve heard the standard cited a hundred times. “IEEE 43 says the PI minimum is 2.0.” “IEEE 43 requires correction to 40°C.” Most techs and engineers work from the rules without ever opening the document. That works fine until it doesn’t. Until an auditor asks where… Read More: IEEE 43-2013 Explained: A Clause-by-Clause Guide to Motor Insulation Testing… »
Most people who run a Polarization Index test can tell you the formula: 10-minute IR divided by 1-minute IR. Fewer can tell you why that ratio is meaningful, where it stops being meaningful, or what a result of 1.1 on a modern epoxy-mica winding actually implies — compared to the same reading on an asphaltic machine from 1965.… Read More: Polarization Index Testing of Motor Windings »
Insulation resistance testing is one of the most practical condition assessment tools in electrical maintenance. Apply DC voltage to a winding, measure the leakage current, divide one by the other — you get a number in megohms that tells you whether the insulation is clean and dry, wet and contaminated, or failing. IEEE 43-2013 is the standard that… Read More: Insulation Resistance Testing of Motors: A Complete Guide to IEEE… »
Choose the wrong insulation tester and you’ll either waste money on capabilities you don’t need or get stuck with equipment that can’t handle the job. Both happen often. A residential electrician buying a $3,000 5-kV tester to verify branch circuits, or an industrial maintenance team trying to test 13.8 kV motor windings with a 1 kV handheld —… Read More: How to Choose an Insulation Tester: A Complete Buyer’s Guide »