Buchholz Relay is a protective device or instrument specifically designed for use on oil filled transformers. The device operates very effectively and its reliability is beyond doubt. Generally three sizes of Relays are used. They are 25, 50 and 80 mm but in Europe exceptionally 100 mm is also used. The size of relay depends upon the MVA of transformer. 25 NB Buchholz is used below 1 MVA, 50 NB Buchholz is used upto 10 MVA and 80 NB above 10 MVA. The inclination of pipe work going towards conservator on which the Buchholz relay is mounted is kept as 5°
The body is a chill Aluminium casting and cover is Pressure Die Cast Aluminium. The cover holds the mechanism which includes the floats, switches and its operating systems. The leads from switches terminates on the terminal box. The main body holds this mechanism and is a oil tight fitment, generally the alarm functions on accumulation of air to a specific value and the trip functions on receiving a sudden surge or accelerated velocity of oil. The electrical faults of transformer results in formation of gases with or without velocity and the Buchholz employs this effect in its operations.
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