Buchholz Relays

The primary function of the conservator is to provide space for the expansion of the oil in the transformer main tank during loading related heating of the oil. The Buch Holz Relay is mounted in a pipe from the transformer main tank to the consevator. The pipe is intentionally kept at a slope of about 2 to 7 degrees. As the oil in the transformer tank expands during loading or during faults, it finds its way to the conservator thro this pipe & the relay. As the pipe line is sloped, any gas accumulated is collected at the top of the pipe & the relay. As long as the volume of gas collected is within limits, nothing happens. But during incipient faults inside the transformer, the decomposition of oil and hence the gas collection is rapid that much volume of gas is collected in the relay. As the volume of the gas collected increases it pressurises the relay chamber and presses down the oil. The slantingly kept mercury switch straightens out and the mercury now links the two terminals, thus closing the contact. As you have rightly pointed out, the top mercury switch is for alarm and it gets activated first. But, if you ignore the alarm and the fault persists further gas accumulation will also push down the bottom mercury switch to trip the supply to the transformer, provided the logics are properly wired for the purpose.