The M4110 Leakage Reactance Module from Doble Test Instruments

Doble M4110 Leakage Reactance Module

Leakage Reactance testing is highly sensitive to transformer winding deformation.  When these type changes occur, electrical losses increase and make the transformer less efficient.  Additionally, these type indications, as reflected in the impedance of a transformer, can be used as a diagnostic tool to help maintain the health of the transformer and ensure its continued service.


Transformers can be very expensive and replacing them not only puts a serious dent in your budget but can potentially cause production and operational delays.  To help diagnose transformer problems, use a leakage reactance module like the Doble M4110 Leakage Reactance Module.  This tool measures transformer impedance and lets you determine if there are any deviations from the transformer’s baseline data.  It has the ability to measure leakage reactance of 0.1 up to 700 Ω and inductance of 250 μH up to 1.8 H.  This lets you spot transformer problems early and extend the operating life of your transformers.

How do you know if the result of leakage reactance is passing or not?

The IEEE standard (C57.152-2013) addresses this question in section 7.2.12.5 Interpretation of the Impedance Test.  The result should be less than or equal 3% difference from nameplate impedance.

In this section of the standard, it gives and example of impedance changes from 5% to 5.4% as being too high because it represents an 8% change, where only +/- 3% tolerance is acceptable. From the example given in the standard the percentage of acceptable difference in short circuit impedance would be as follows.

Nameplate = 5% Impedance
5 x 0.97 = 4.85% Impedance (Lower Limit)
5 x 1.03 = 5.15% Impedance (Upper Limit)

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