Introduction to the History of Selective Protection

Author: Walter Schossig, Germany

The Relay

The word "relay" was established in the time when stage coaches were a means of transportation. A "relay" was a station for the change from tired horses to fresh ones. In the military the term "relay chains" refers to a chain of annunciators for passing on messages and commands. The introduction of protection relays was of course only possible after the creation of suitable switching devices that were capable of extinguishing the fault current arc and could also withstand the onerous short circuit currents generated under fault conditions. Brown proposed an oil circuit-breaker in 1892. With the introduction of direct release (directly initiated by the primary current) and secondary relays (supplied by current transformers or voltage transformers) "oil self switching circuit breakers" were developed.

 

Figure 2 Oil Circuit-Breaker with Direct Release (Elektra Birsek, CH)

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