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While we test electrical equipment like transformer the insulation readings goes higher linearly over time,And even to infinity which means the instrument could not measure beyond that, 15Tohm is the limit with test voltage 5000v DC for the equipment i use.

a) Here my question is if it have higher max limit does the curve drops at any point,if yes why and how?

b) Mostly we perform insulation test for predicting the life of machine,does it really gives an accurate prediction?

c)if we use same instrument model but different instrument to measure IR,why it gives difference in reading?

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    \$\begingroup\$ Difference in readings can be explained by the calibrated accuracy of the measurement devices. Have you considered this? If so then what difference do you refer to? \$\endgroup\$ – Andy aka Feb 18 '18 at 12:16
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    \$\begingroup\$ Using insulation resistance to predict life of a machine seems a little weird to me. We measure insulation of train motors on a regular base (series and induction motors) and I have had motors that measure a very high insulation (several GOhms) and once under load they go "bang" (and when it really goes wrong it's a huge "bang"). Then again we have motors that have 50-100MOhms and run fine. Catenary is 3000V DC. \$\endgroup\$ – Daniel P Feb 18 '18 at 12:52
  • \$\begingroup\$ PD is the only surefire method to test isolation capability. \$\endgroup\$ – winny Feb 18 '18 at 17:27
  • \$\begingroup\$ @winny PD is a insulation failure. Also don't confuse isolation and insulation. \$\endgroup\$ – Daniel P Feb 19 '18 at 1:15
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    \$\begingroup\$ For me its a useless test but how this test is included in international standards,The manufacturers of testing equipment claims this test is for early prediction of breakdown.I agree with What @Daniel P mentioned is the truth. \$\endgroup\$ – Eng. Jamshid Feb 20 '18 at 14:04

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