A Buchholz relay is a safety feature of some electrical transformers, choke coils, or high-voltage electrical capacitors and reactors. It is designed to prevent spreading damage in the case of a short circuit, arcing, or other dangerous electrical faults, such as an explosion or deteriorating condition of overheating. The concept for the relay was invented by Max Buchholz, a 20th century engineer and inventor whose ancestors emigrated to the US from Germany in the 1800s. He first developed the Buchholz relay in 1921, but it wasn't put into widespread use in the US until the 1940s.
Each Buchholz relay acts as a sort of circuit breaker, most often attached to the top of oil-filled electrical transformers where an oil reservoir tank known as a conservator sits. The chief role of the device is to maintain a dielectric constant or insulating property for the transformer, and it can do this by controlling the supply of circulating oil from the conservator, as well as detecting air leaks into the system. Safety switches like the Buchholz relay are an essential component of modern-day power distribution grids. They are designed to minimize damage to broader areas of the system in case of a localized fault, which could otherwise propagate and overload other transformers farther down the line.
The construction of such devices is heavy duty, so that they can withstand high electrical currents and varying climate conditions. The housing is dome-shaped and made from a weatherproof aluminum enclosure with built in mechanical test and trip circuit controls, as well as an inspection window of tempered glass to visually monitor insulating oil levels. The switches in a Buchholz relay are capable of handling voltages from 24 up to 250 volts of eitheralternating current (AC) or direct current (DC), and the insulation of the relay can handle 2,000 volt charges. The insulating oil itself is a form of mineral oil stable at high temperatures or silicon-based fluorinated hydrocarbon compounds which usually have a functional temperature range of between 77° to 239° Fahrenheit (25° to 115° Celsius).
A series of oil floats in a Buchholz relay are used to gauge fault levels in the transformer. Minor electrical faults will generate a small amount of gas in the oil, which will move an upper float and cause the relay to activate an external alarm. Large-scale faults will release enough gas that a tripping switch in the Buchholz relay is activated 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 reset of the system when the reason for the fault has been determined and corrected. If the transformer sustains a minor oil leak or a small amount of air enters the unit, the minor float assembly activates the alarm. When leaks become significant, the tripping switch is thrown by the larger float and the system is shut down.
Variations on the design can include a mercury switch attached to the rotating flap for the lower assembly instead of a float device. Some units have test cocks as well to check whether the floats and mercury switches are working correctly by channeling air through the system and monitoring their response. The relay assembly is often mounted on a heavy-duty cast iron plate and terminals are insulated with ceramics to give the Buchholz relay added strength and durability.
for example) and a wide range of supply voltages (9-20V for example). They are capable of switching much more rapidly than standard relays, up to several hundred times per second; but they can only switch low currents (500mA maximum for example). 
