For most of my life, my main experience with circuit breakers has been resetting the kitchen or bathroom Ground Fault Circuit Interrupters (GFCIs) when they occasionally trip. As a result, although I've experienced first-hand the tremendous improvement in other electronic components over the years - actually, decades - in everything from op amps to microcontrollers and even passives, I just naively assumed that circuit breaker (CB) technology was a sleepy backwater somehow unaffected by progress. Out of sight, out of mind, I suppose. Feel free to roll your eyes at this point.....
Recently, I had an opportunity to do a little research on CB technology, and it really opened my eyes. It's no surprise to many of you, I'm sure, but the field has moved on in the last 30 years.
Most designers (even low-voltage types like me) have an idea of what a circuit breaker is and what it does; here's a quick definition:
“A mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short circuit.” – IEC 60050
Seems pretty simple, right? But circuit breakers are used in a large number of situations with vastly differing voltage and current requirements. And when you're talking high voltages, a whole new set of considerations comes into play.
“High voltage” in this context refers to voltages of 72.5kV or above, as defined by the International Electrotechnical Commission (IEC). Primary applications for HV circuit breakers are in electricity generation and transmission: overhead transmission lines, bus transfer switching, generator switching, transformers, capacitor banks, protection, etc.
An ideal circuit breaker should act as an ideal conductor in the closed position, and as an ideal insulator in the open position; a real-world circuit breaker must carry its rated voltage and current when closed, and withstand its rated voltage