Buchholz relay is a type of oil and gas actuated protection relay universally used on all oil immersed transformers having rating more than 500 kVA. Buchholz relay is not provided in relays having rating below 500 kVA from the point of view of economic considerations.
A latching relay is an electromechanical switch. Latching relays are electronic parts that are used to control large flow of electrical current with smaller flow of current. Relays are typically used when small continuous electrical currents must be used.
Conventionally, the term “machine tool relay” is applied to NEMA-style relays. Today, IEC relays, often called “control relays,” are also applied for the same purposes. In this article, the term machine tool relay will be used interchangeably for both NEMA-style and IEC-style relays.
Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays".