The tap changer drive (6) and switching contact assembly (7) are housed in a container (2,3) which separates the oil in the tap changer from that in the transformer tank (1). Attached to that part of the housing (2) outside the transformer tank is an oil circulating system. It comprises a suction tube (16) from the bottom of the tap changer tank, a circulating pump (20), flow monitoring relay (21), and filter unit (19) together with the necessary connecting piping (17,18). When the filter becomes choked, typically with carbon formed during tap changing operations, the reduced flow causes the relay contacts to close and give an alarm. The use of a flow instead of a pressure relay ensures more definite indication of filter state. The application is typically to industrial transformers making a large number of tap changes per year.
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