Dissolved gas analysis (DGA) is the most important tool in determining the condition of a transformer. It is the first indicator of a problem and can identify deteriorating insulation and oil, overheating, hot spots, partial discharge, and arcing. The “health”of the oil is reflective of the health of the transformer itself. Dissolved gas analysis consists of sending transformer oil samples to a commercial laboratory for testing. A four-condition DGA guide to classify risks to transformers with no previous problems has been published in the Standard IEEE Standard (Std). C57-104™. The guide uses combinations of individual gases and total combustible gas concentration as indicators. It is not universally accepted and is only one of the tools used to evaluate dissolved gas in transformers. The dielectric strength test measures the voltage at which the oil electrically breaks down. The test gives an indication of the amount of contaminants (water and oxidation particles) in the oil. Acid number is the amount of potassium hydroxide (KOH) in milligrams (mg) that it takes to neutralize the acid in 1 gram (gm) of transformer oil. The higher the acid number, the more acid is in the oil. New transformer oils contain practically no acid. Oxidation of insulation and oils form acids as the transformer ages. Oxidation products form sludge particles in suspension in the oil which rains (precipitates out) inside the transformer.