OLTC field testing has a new tools for the condition assessment of this moving transformer component. Dynamic testing can now provide information on the operational characteristics of the OLTC as well as problems with contacts, mechanism and other subassemblies. Reactor type tap changers are slow moving design and understanding principles and operational characteristics is essential for condition assessment. Dynamic DRM test is an off-line, non-destructive test in which a DC current is injected through a winding and tap changer as it moves through all of its positions. Results from the current signatures are examined and compared against previous tests or similar unit test results. Result of this tool is an exact position/component in trouble pinpointed when based on early warning provided by ratio of gasses analyzed using DGA.