Early digital relays appeared around 1980, with numerical relays following by 1985. These devices transformed relay protection by using analog-to-digital conversion and advanced digital signal processing. The ability to have a device that could directly monitor the changing voltage and current and make control deci time, demands on the power system grid increased as generators grew in size and capacity. This. Today, digital relays provide features such as self-testing, waveform analysis, and rapid fault response, which far surpass the capabilities of early devices. In 1901, the induction-type overcurrent relay was introduced, followed by ASEA (now ABB) launching the first time-delay overcurrent relay, TCB, in 1905, enabling graded protection. The current differential protection principle. The rectangular devices are test connection blocks, used for testing and isolation of instrument transformer circuits. The following sections detail the origins and development history of various types of electrical protection devices. The origins of the fuse date back to the early 19th century when.
