It is connected with pipes between transformer vessel and oil expansion vessel. It is used to protect transformer in a fault that may occur in electrical equipment in the transformer. When gas arises in insulation materials at the moment of fault, it causes that contacts work by accumulating in reservoir while passing through relay, pushing the float down, or, in sudden faults, activating the valve. There are two independent contacts in the relay for Opening and Alarm. They are 5 A-250 VAC or 0.2 A-250 VDC