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In the Forum: Audio Discussions
In the Thread: Input choke for Low Currents?
Post Subject: The difference between class A and B insulationPosted by Bud on: 1/17/2007
Jim,

UL has applied a compatability test to insulation materials since 1976. This test places representative amounts of each material, found within a potential transformer coil, in a test tube. This tube is then evacuated and subjected to infrared heating for a specific number of hours. One of the materials is a specific length of wire, made from two tightly twisted pieces, that has a coating of some specific chemical nature.

After this ageing test, and with the wires still hot, a dielectric withstand test is performed. Materials collections that pass this strenuous test are included in various reccomended temperature classification for usage in transformer construction. Class A materials are limited to 105 deg C total temperature and are expected to last at this temperature for about 300,000 hours without material change in properties. class B is 130 deg C, class F is 155 deg C and class C is 220 deg C. All of these classes allow the manufacturers of the various materials found within the coil of a transformer, to have their particular material added to an already known and tested collection for an ageing test. If the newly tested wire sample does not fail then their material is considered compatable with that particular group of materials and can be listed under one of the classes as rated for that level of operational temperature.

Transformer companies can buy into one of these materials collections, usually originally paid for by some major material manufacturer like Dupont, and use the "system" of materials to wind coils that will operate at total temperatures within the limits of the particular class of mateials. Total temperature includes the ambient temperature plus the temperature rise above the ambient testing temperature of the transformer in question. A class B transformer can have a 90 deg C rise in a 40 deg C ambient and be expected to have a 300,000 hour MTBF. This assumes that it is a well constructed device, i.e. it meets the various tests, both mechanical and electrical, from one of UL's overall classifications. These overall classifications cover all of the safety parameters applied to the entire device that the transformer operates within. These requirements reflect upon the transformers physical construction, i.e. creepage and clearance distances for spark suppression to ground or surfaces of opposite voltage polarity and dielectric withstand from the mains supply, provided by the transformer, to  the rest of the device. All of these various tests for usage, in a particular device, occur at the maximum rated temperature of the insulation system and are quite strenuous. These final usage tests confirm that the previous ageing tests have provided a stable insulation system and that the physical construction of the whole transformers is robust.

Bud

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