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Are there any established levels for Megger test results of low voltage dry type distribution transformers?

Megger testing of Low Voltage Transformers

An outside agency performed a Megger test per a NETA standard and found results below those noted in the standard, yet well above 1Mohm.  Are there any established levels for Megger test results of low voltage dry type distribution transformers?

Product Line:
LV Transformers

Applies to Low Voltage Transformers by SquareD/Schneider Electric

Testing of Transformers for suitability for use in Electrical Systems


Megger testing on Low Voltage Dry Type Distribution Transformers is, at best, a basic indication of the suitability of the insulation system. Megger testing also varies widely based upon current ambient conditions, how the Transformer was stored, and the cleanliness of the coils (dust/debris). Any value above 1Mohm, yes, that is one meg ohm, is easily sufficient, and even with values significantly below that level, the Transformer is likely still in satisfactory working order. Once the Transformer has been gently blown clean and thoroughly heated (usually by being energized for a day or more), and the Megger test is performed a second time, then the values typically increase, sometimes dramatically.

While Megger testing is basic indication, SquareD/Schneider Electric performs a high potential test (Hipot) on every Transformer manufactured. A Hipot is foolproof and a clear indication of the insulation system`s condition. The Hipot test is a pass/fail test, and SquareD/Schneider Electric only ships Transformers that have passed the test. As a Hipot test stresses the insulation system (too many tests, too long a test, or too high a voltage can be destructive) SquareD/Schneider Electric does not recommend additional Hipot tests be done on the Transformer (beyond the one performed at the factory).

To that end, a Megger test can be a useful tool when part of the annual routine MAINTENANCE performed on the transformer. Each year during the scheduled routine MAINTENANCE, the Megger test can be performed and the results recorded. The test should be performed in the same manner each time, and the recorded results should include the ambient conditions (date, time, temperature, humidity,...). From those records the customer will be able to evaluate whether there has been any significant degradation of each transformer`s insulation system.
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