OT: Circuit Breaker Coordination Question | PLCS.net

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.

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.

What Ron said notwithstanding, there is a problem with your question as worded. You are asking for a comparison between what you perceive to be two options, but in fact you have mixed up three related, but not comparable things, because they are three ASPECTS of circuit breakers that could potentially exist in the SAME breaker.

Molded case relates to the CONSTRUCTION of the breaker, officially a “Molded Case Circuit Breaker or “MCCB” as opposed to Insulated Case Circuit Breaker (ICCB) or Power Circuit Breaker (PCB) or Miniature Circuit Breaker (MCB).

Thermal Magnetic has to do with the TRIP SYSTEM of the breaker, as opposed to Magnetic Only or Electronic Trip or Hydraulic Trip.

100% has to do with the USE RATING of an MCCB, as opposed to a “normal” rating. Ron describes that well except there is no such thing as an “80% rated” MCCB. There are “normal” MCCBs and there are 100% rated MCCBs. All MCCBs that are in a panel next to other breakers are normal breakers and all breakers are tested and listed at 100% of their rated value. But you cannot USE them that way. Breakers protect the conductors. Conductors are to be sized for 125% of their continuous load. The inverse of 125% is 80%, meaning that since the conductors are rated for 125% of the load, the maximum load is 80% of the rating of the conductors and since the breakers are sized to protect the conductors, you would never see more that 80% of the load on those breakers; continuously, which is where the “80% rated” concept comes from. Then BECAUSE of that, panel mfrs take advantage of that fact in how the panel and MCCBs are designed with regard to heat dissipation, which is why MCCBs in a panel can be mounted next to each other, touching.

But it if you need to use an MCCB at 100% of its rating continuously, it CANNOT be used in a panel like that, it must have free air around it to dissipate the added heat. So because of THAT, breakers intended to be used at 100% continuously must be specifically LABELED as such, which will mean it cannot be used in a panel, except as a main. If it is not a Main, it must be separately enclosed by itself.

Then when you start discussing POWER Circuit Breakers (PCBs), where each PCB has its own totally separate cubicle in switchgear, those are ALL rated 100%, so it’s unnecessary to mention it.

SO back to your original question; you CAN have a Molded Case Thermal Magnetic 100% rated breaker. So that’s why asking for a comparison is technically spurious.

I just don't follow the 125% rule

Well, here's my understanding, in case it helps you (and so someone can correct me if I'm wrong). It's all about limitations of standard breakers; a wire can carry current at its ampacity indefinitely without damage.

A standard thermal-magnetic MCCB tested by itself (in an enclosure? in open air?) at 40C ambient is supposed to hold its rated current indefinitely. That's a spatial arrangement that maximizes the breaker's ability to cool off by rejecting heat to its environment. Under these conditions, the thermal trip unit will reach some steady-state temperature, due to an equilibrium between the self-heating from the current through the breaker and the breaker's heat rejection to the environment. The thermal trip unit is calibrated to not trip at this steady-state temperature (but presumably to trip at a temperature just a bit higher).

Now take that same breaker and put it in a panel full of other breakers. The nearby breakers inhibit any convective cooling off the case of the breaker, plus the nearby breakers are themselves heat sources, so their temperature may be higher than ambient. The result is a lower ability of the breaker to reject heat to its environment. That means that if you run the full rated current through the breaker, the steady-state temperature of the thermal trip unit will be higher than previously; the equilibrium has shifted. That temperature may now be above the trip point, causing the breaker to trip when you don't want it to.

A work-around for this is to prohibit continuously running the full rated current through the breaker. Somehow the factor of 80% was arrived at, it was judged that reducing the current to at most 80% of the rating would shift the equilibrium temperature back sufficiently to avoid unintended tripping. In other words, you need to oversize the breaker by a factor of 125%.

Code-wise, that factor of 125% requires upsizing the conductors by 125% as well; otherwise the conductors would be underprotected by the breaker. [And upsizing the conductors has the further benefit of improving the breaker's ability to reject heat via conduction through the conductors.] The code writers chose to put the requirements in terms of upsizing the conductors as the first matter, with the upsized breaker as a side effect. I don't find that particular enlightening, since the limitations of thermal trip units are the whole reason for the requirement, so I prefer to think of it as upsizing the breaker as the first matter, with the upsized conductors as a side effect.

Cheers, Wayne

It’s not necessarily just the trip unit that’s the issue though, it’s the ability of the entire breaker to dissipate heat. Most (if not all) breakers with ETUs can be ordered as 100% rated for the same price, but once it has that 100% rated label on it, your use of it becomes restricted. One of those restrictions will be that it must be mounted with limitations on what is next to it. Other breakers on either side will violate that.

OK, but I'm trying to understand the technological limitations, as opposed to the listing limitations. It sounds like the listing limitations are based on the behavior of thermal trip units. My questions:

1) Am I correct in my understanding that an ETU's trip curve is independent of ambient temperature (to first order)?

2) Can the electronics in an ETU be made to withstand the internal temperature of a breaker carrying its full rated load continuously, when packed into a load center with adjacent breakers?

3) If so, is there any other reason a manufacturer couldn't make ETU MCCBs for regular small panelboards that function as 100% breakers, without limitation on breaker spacing?

I understand, of course, that even if technologically possible, there's little incentive or market for manufacturers to develop this product and get the UL listing standards adjusted to accommodate them.

Thanks,
Wayne