Buchholz Relays | Mercury Switch Type Relays
Transformer Protection Up to 1001/10,000 litres 1001/10,000 kVA
Pbwel Mercury Switch Type Buchholz Relays consists of a lightweight container fitted with two pivoted elements. It is situated in the pipe line between the transformer and the conservator tank, so that under normal conditions it is full of oil.
The operating force relies upon the principle that when a body is immersed in a liquid it appears to lose weight. There are no floats or open containers which can be punctured or collect sludge, with consequent loss of buoyancy.
Mercury switches employed are of a special design to prevent maloperation due to excessive transformer vibration. A sample relay of this type has been submitted to a continuous 3000 hour vibratory type test. During this test the relay was vibrated to an
amplitude of 0.01 in. peak to peak at a frequency of 100Hz.
The Mercury switches were connected to sensitive detecting equipment and no maloperations were recorded. The Mercury switches are spring mounted within the switch cylinders and protected from possible damage.
Alarm and trip circuit Mercury switches will make, break and carry continuously 2 Amps at 250 Volts A.C or D.C. They will also make and carry for 0.5 sec. 10 Amps at 250 Volts A.C or D.C.
Principle of Operation
The operating mechanism consists of a solid non-metallic cylinder containing the Mercury switch, counter balanced by a
smaller solid metal cylinder. Both cylinders are jointed and free to rotate about the same axis, the amount of rotation being
controlled by stops. When the relay is empty of oil, the weight of the switch cylinder predominates and the switch system rests against the bottom stop, the Mercury switch being in the closed circuit position. When the relay is full of oil, both cylinders appear to lose weight.
Due to the different densities, the switch cylinder appears to lose enough weight of the counterbalance cylinder to
predominate and rotate the whole system until it reaches the top stop, with the Mercury switch in the open position.
When a slight or incipient fault occurs within the transformer, the gas generated will collect in the top of the relay housing. As gas collects, the oil level will fall and increasing amounts of the alarm switch will appear above the oil level. This results in gradual restoration of the apparent lost weight, until the weight of the switch cylinder predominates. The element rotates as the oil level continues to fall and eventually the alarm switch operates.
When a serious fault occurs, the generation of the gas is so rapid that an oil surge is set up through the relay. This oil flow will
impinge upon the flap fitted to the trip element causing it to rotate about its axis and so bring the Mercury switch to the closed position, which in turn operates the tripping devices.
In the event of serious oil loss from the transformer, both alarm and trip elements operate in turn, in the manner previously described for gas collection.
The oil level in the double element relay can be monitored against a graduated scale on the windows both sides.
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