A gas sample from above the insulating liquid in transformers and other electrical power apparatus can be used to give an early indication of developing fault conditions. Combustible gases are formed from the breakdown of the insulating materials under thermal and electrical stress. Excessive concentrations of combustible gases indicate the need to take appropriate action.

It is important that gas samples from equipment be taken from the gas space directly above the liquid level, and not from remote expansion compartments, such as tank bracing bands.

The procedure described should be carefully followed to ensure that the sample is not diluted by air or nitrogen, or contaminated by residual materials in the sampling bottle.