Hello, i have a question about circuit breaker coordination (MCBs). I thought I had a good understanding of the theory behind it, but after running some tests I got some different results than expected and was hoping someone could shed some light on this.
In order to test a proposed breaker setup for a system we are working on, I had a C32, then C20, then C10 breaker wired in series, downstream in that order. All breakers are siemens 5SY63XX-7 breakers, with the XX being the amperage ratings, i.e. 32, 20, 10. I downloaded the trip curves of each and compared. Between the C10-C20 breakers, there is almost no overlap. Between the C20-C32 there is some overlap in the short circuit range.
I then caused a short circuit by connecting this setup directly into the mains line (classroom, the main wall breaker is the 32A) to see if the coordination was ok (i.e. C10 trips first, the rest don't). All breakers are rated for 6kA. All 3 breakers immediately tripped.
I was expecting the C10 to trip and leave the C20/C32 closed. I am attaching an image of the curves superimposed on each other.
Can anyone provide insight into why this happened? Maybe it has something to do with the speed and short circuit current magnitude? And I'm curious what the correct way to interpret the curves in the range below 0.01s is. Is this even a realistic working range (due to the inertia of the devices)? Any info would be greatly appreciated.
In order to test a proposed breaker setup for a system we are working on, I had a C32, then C20, then C10 breaker wired in series, downstream in that order. All breakers are siemens 5SY63XX-7 breakers, with the XX being the amperage ratings, i.e. 32, 20, 10. I downloaded the trip curves of each and compared. Between the C10-C20 breakers, there is almost no overlap. Between the C20-C32 there is some overlap in the short circuit range.
I then caused a short circuit by connecting this setup directly into the mains line (classroom, the main wall breaker is the 32A) to see if the coordination was ok (i.e. C10 trips first, the rest don't). All breakers are rated for 6kA. All 3 breakers immediately tripped.
I was expecting the C10 to trip and leave the C20/C32 closed. I am attaching an image of the curves superimposed on each other.
Can anyone provide insight into why this happened? Maybe it has something to do with the speed and short circuit current magnitude? And I'm curious what the correct way to interpret the curves in the range below 0.01s is. Is this even a realistic working range (due to the inertia of the devices)? Any info would be greatly appreciated.