On-load tap changers are used for changing the transforming ratio of power transformers without supply interruption. To reduce arcing at the contacts, the tap changer operates in a chamber filled with insulating transformer oil. Some arcing is unavoidable and both the tap changer oil and the switch contacts will slowly deteriorate. The arcing produces the same contaminants in the tap changer as in the transformer. These are the fault gases such as hydrogen and acetylene as well as carbon particles. Changes in ambient temperature causes condensation to accumulate in the tap changer compartment. This, combined with the other contaminants, slowly degrades the dielectric strength to such a degree that it, if left unattended, could cause a failure of the tap changer.