OT: Circuit Breaker Coordination Question

Jieve

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Join Date
Feb 2012
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USA
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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.

C10-C20-C32 Overlap Curves.jpg
 
A short circuit is infinite current draw. Draws the maximum current the supply can provide. The curves don't mean anything in that case. I think it would be more related to the 6KA rating.
 
The curves don't mean anything in that case.

If what you're saying is true, in that case, it would never be possible to coordinate breakers (MCBs) under a short circuit condition, only overload. Or are you saying that below the 0.01s mark there is no possibility for coordination?

I thought the whole purpose of coordinating breakers was not to trip upstream breakers under any fault condition, short circuit included.
 
In order for your coordination to work you first need to understand the capacity of the supply circuit. Then you need to understand how a particular type of CB or MCB reacts to the levels of current and the rise time of the current. Lastly you need to understand the "clearing" time of each CB.

It sounds like your supply is capable of a very large short circuit current, with a correspondingly fast rise time. In which case all of your MCBs see enough current to cause them to trip with what appears to be no coordination.

If you had the proper equipment you would be able to run your test and see that each MCB does trip in its proper order but that by the time the 1st one trips and "clears" the next one in line has already started tripping, and so on up the line.

CB coordination can be very tricky for even experienced engineers. I hope this helps you get a handle on it.
 
Thanks everyone for the helpful responses.

I did some more research immediately after my last post and recognized that while the C10 and C20 breakers only have a very slight overlap in the overload region, actually all of the breakers overlap along the short circuit line. From what I understand, the curves would actually have to be shifted far enough apart that the horizontal lines at the bottoms of each curve do not overlap, in order for the breakers to discriminate correctly under short circuit conditions. It MAY be possible to do this with something like a C4 & C16/C20 combination, but with more than 2 stages with low amperage breakers it doesn't look like this is gonna happen. Unfortunately we're somewhat limited on which breakers we can use based on the permanently installed wiring in our wall cabinet, and while it would have been nice to have all breakers discriminate, it's not a huge problem for our system, as it's only being used for education purposes.

I think I have a copy of that schneider manual, will have another good look at it.
 
If you wanted to try and demonstrate this for students, add a roll of 2.5mm2 wire downstream of your last breaker to simulate an actual circuit and generate your fault at the end of this . This will add some resistance which will reduce your prospective short circuit current. If you have a decent installation tester it will calculate PSC for you. If not, a good quality low range ohmmeter will do the job - measure the resistance of your wire (including return wire) and calculate PSC using ohms law. This assumes there is negligible resistance upstream and enough capacity to supply the full PSC.
 

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