The Buchholz relay is a safety feature for electrical transformers, chokes, and capacitors that prevents damage during short circuits or other dangerous electrical failures. It was invented by Max Buchholz in the 1800s and is a circuit breaker that controls oil supply and detects air leaks. The device is heavy and can handle high electrical currents and varying climatic conditions. It uses oil floats to measure fault levels and has a trip switch that shuts down the system during large-scale failures. Variations may include a mercury switch and test cocks.
A Buchholz relay is a safety feature of some electrical transformers, chokes or capacitors, and high voltage electrical reactors. It is designed to prevent the spread of damage in the event of a short circuit, arcing, or other dangerous electrical failure, such as an explosion or deteriorating overheating condition. The concept for the relay was invented by Max Buchholz, a 20th-century engineer and inventor whose ancestors immigrated to the United States from Germany in the 20th. He first developed the Buchholz relay in the 1800s, but it was not popularized in the United States until the 1921s.
Each Buchholz relay acts as a sort of circuit breaker, most often connected to the top of oil filled electrical transformers where there is an oil reservoir known as a conservator. The main role of the device is to maintain a dielectric constant or insulating property for the transformer, and it can do this by controlling the circulating oil supply from the conservator, as well as by detecting air leaks in the system. Safety switches such as the Buchholz relay are an essential component of modern energy distribution networks. They are designed to minimize damage to larger areas of the system in the event of a localized fault, which could otherwise propagate and overload other transformers further downstream.
The construction of such devices is heavy, so they can withstand high electrical currents and varying climatic conditions. The housing is dome shaped and made of a weather resistant aluminum enclosure with integrated test circuit controls and mechanical intervention, as well as a tempered glass inspection window to visually monitor insulating oil levels. The switches in a Buchholz relay are capable of handling voltages from 24 up to 250 volts of alternating current (AC) or direct current (DC), and the relay insulation can handle loads of 2,000 volts. Insulating oil itself is a high-temperature stable form of mineral oil or silicon-based fluorinated hydrocarbon compounds that usually have a functional temperature range of 77° to 239° Fahrenheit (25° to 115° Celsius) .
A series of oil floats in a Buchholz relay are used to measure fault levels in the transformer. Minor electrical faults will generate a small amount of gas in the oil, which will displace a top float and cause the relay to activate an external alarm. Large scale failures will release enough gas to activate a trip switch in the Buchholz relay when a flap on the larger, lower float is rotated by the rising gas and the relay cuts power to the transformer. An external button on the device is provided for a system reset when the reason for the failure has been determined and corrected. If the transformer experiences a small oil leak or a small amount of air enters the unit, the minor float assembly activates the alarm. When leaks become significant, the trip switch is activated by the larger float and the system is shut down.
Variations on the design may include a mercury switch connected to the rotary vane for the lower assembly instead of a float device. Some units also have test cocks to check if the mercury floats and switches are working properly by ducting air through the system and monitoring their response. The relay assembly is often mounted on a heavy-duty cast iron plate and the terminals are ceramic insulated to give the Buchholz relay added strength and durability.
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