[802.3_ISOLATION] Insulation resistance test
One of the groups I belong to has discussed insulation and the insulation resistance test. I thought this might be of interest to this group.
"The enamel provides functional insulation for the wire allowing the coil to work as intended. However, the enamel is not to be relied upon for the purposes of safety. Triple insulated wire (TIW) was developed partly to address this issue and it is used where there must be basic insulation between windings that are overlaid on top of each other."
"TIW is quite commonly used in the transformer of SMPS. For motor windings, enamel wire is still a mainstream material in use."
"You must work to separate the concepts in your thinking. Functional insulation is that insulation required for the device to function and has identified insulation properties. Insulation used for safety purposes has a set of properties that ensure the safety function is robust; it, too, has insulation properties which will most likely be different than those of the functional insulation.
Common usage mixes these two concepts together and the average engineer does not identify the difference in his application.
So, for your project, where are the safety insulations provided? Are they the part of the unit where the insulated wire is depended upon for the ? If not, then the usual insulated wire can be used but, if it is the safety isolation protecting the user from the electricity supply then the usual functionally insulated wire is not adequate and the needed protection must be supplied in another way.
TIW is a good solution to this latter problem. Perhaps this clarifies the concepts for you and you understand better why TIW is used in SMPS."
"Insulation resistance test is one of most common safety tests nowadays: Flash test, earth continuity test, leakage current test and insulation resistance test. Can someone share the history of this teat to use DC and 500 V. The products are working on AC and test voltage is higher than normal operating voltage but much lower than the flash test. It is a trend to skip this test on production line. What are the distinct benefits for this test?"
"The test is very old, and pre-dates the high-voltage test by very many years. You mentioned 'Megger' in your other post, yes, that was how it was done. (A Megger (brand name) was/is a type of magneto with an ohmmeter attached. The stable output voltage required for the ohmmeter is achieved by a mechanical governor, which limits the armature speed however fast you turn the handle.)
The test might not be totally irrelevant for modern electronic equipment, but the requirements in most standards are certainly in need of revision. These requirements are for a minimum insulation resistance of 1 or 2 megohms. A modern piece of electronic equipment typically has a resistance of hundreds of megohms (unless condensation occurs), so a measured value of a few megohms shows that something is in fact seriously wrong. I have raised this point in TC 108 before, but no-one was willing to take action. Tradition, you know."
Mick — If you have conducted an insulation voltage withstand test, following that with an insulation resistance test is a good way of finding out if anything untoward has happened that you missed during the withstand test.
Mick Maytum
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