The document discusses the Buchholz relay, which is a gas-actuated relay used to detect incipient faults in oil-filled transformers. It is placed in the connecting pipe between the main tank and conservator. The relay contains upper and lower mercury switches - the upper element closes during less severe faults while the lower element trips the circuit breaker during more severe internal faults. It works by detecting the accumulation of gas bubbles in the oil, which rise and cause the float mechanism to trigger the switches based on the severity of the fault detected in the transformer.