Calculate Short-Circuit Currents

[coulter]

I think the point is, that you can't install a fully rated fuse or circuit breaker that is listed below the calculated fault current. ...

Yes that would be true. Again, I don't see where this is news. I re-read my posts, I don't see where I ever suggested that. I certainly have tried to adamantly suggest that one must pay attention to: drum roll ....(there were three items - this is a test to see if you read the posts - without looking back - Name That Criteria!)

... The installation of a random current limiting fuse upstream does not reduce fault current for the underrated over current device ...

Wow. "Random" is an interesting word choice. I don't know how to answer that. I guess I never considered any equipment selection I made to be "random".

...when the lower rated device begins opening before the higher rated device. This can happen instantaneously, even before you can begin seeing it on a time current coordination curve characteristic curve. ...

This is an excellent example of claims that elude me. What I am hearing you say is one can't believe the mfg trip curves. That clearing times can happen much faster than is shown on the mfg curves. Just out of curousity, do you have any published test data that supports that claim?

...It must be a series rated tested combination if you wish to use one to protect the other in a fault condition.

Another interesting claim. My reading of 240.86(A) is certainly different. But you're in good company - that's my intrepretation of Jim's thinking.

"Random" selection of equipment ??. Ouch - That was a slap in the face.

 

[hmspe]

This is one of the things I am trying to figure out. What changed? Did we as a group forget how to do coordination and equipment specification? Was there a regulatory change?

I don't have documentation, but I suspect a lot of this was better instrumentation, better understanding of the dynamics of the components and the systems, and significant changes in the types and amounts of load in typical buildings which can brought problems to light that we never saw with simpler systems.

Now for the dark side: I'm not much of a conspiracy theorist, but I think this was more driven by marketing than by anything technical. Does "dynamic impedance" exist? Sure. Are there documented cases where an arc in a breaker has caused a current limiting fuse not to work? Maybe, but I've never seen a documented case in the field. Were there equipment failures or injuries caused by the use of UOD and applying current limiting fuses? Not that I know of.

As I said, the only difference I see between applying CLF the old way and series rating is that the latter has been "verified" by a lab test. Whether the controlled conditions in a lab adequately mimic real life, well... Series ratings probably reduce our liability, which is a good thing. I do wish they'd raise the series rating limit on motor contribution to 2% -- 90% of the panels I specify are 200A. I see a lot of panels with 150A of A/C units, and the panels can't be series rated with 10K breakers. I find it hard to believe that 100A of motor contribution on a system with all equipment rated at least 10K is safe but 200A of motor contribution is not.

Yes, I knew about that. In my previous post, this is exactly the same section and link I listed. I guess you missed that.

Just trying to give a complete answer so that anyone new to the thread would get the whole picture even if they hadn't read all the earlier posts. I didn't mean to offend...

Martin

 

[coulter]

...In the reference you posted, there is a statement that even with the inst setting turned off this breaker will try to open in order to protect itself during a high fault condition.

yes, that's true. My thinking is one would have to actually look at the trip curves to see the effect. (Yes, I actually did that.)

...In conclusion, in order to even attempt a successful engineered implementation of fuse curves/charts in a "series rating", the downstream ciruit breakers must be guaranteed not to have any dynamic impedance.

Yes, that's pretty well been my point. Again, my thinking is one can use the mfg trip curves.

...I'm not sure that the downstream device can be guaranteed to not have ANY dynamic impedance, but at least not enough to effect the operation of the tested combination with the upstream OCPD.

My intrepretation of Ron's comment is there are things that happen that the mfg doesn't show on the trip curves. I don't tend to follow that thought process without peer reviewed, published test data.

So, I don't know where to go with this. You and Ron appear to be locked up with only tested combinations can be used. And I fully admit I am locked up with the idea that one can use the mfgs data, in conjunction with NEC 240.86(A) to arrive at an engineered, safe, reliable series combination. This would be what Ron calls my "random selection".

I would absolutely agree that today, I would tend to do as you and ron mandate - select a series combination from a mfg bulletin. But then I've already said that.

 

[jim dungar]

This is an excellent example of claims that elude me. What I am hearing you say is one can't believe the mfg trip curves. That clearing times can happen much faster than is shown on the mfg curves.

Coulter,
Don't focus so much on CLEARING times, series ratings are much more dependent on OPENING times. If the downstream device begins to open an increasing amount impedance is added to the circuit which will reduce the current flowing through the fuse. If the reduction in the current is sufficient enough both the clearing time of the fuse and it's resultant let-through current may be increased.

This Bussmann link states the situation very clearly:
"With series rated combinations, the up-over-down method is not able to be used to determine the protection of newer style circuit breakers with blow-apart contacts (which exhibit dynamic impedance). For these applications, listed equipment which has been labeled for use with tested combinations must be installed. Despite the inability to predict protection for circuit breakers using the up-over-down method, the method is still (as it always has been) valid for static components (components passive within first half-cycle). Some applications which can utilize the up-overdown method include:

1. Bus-Bracing Requirements
2. Wire and Cable Protection"

I have highlighted the key phrase. Bussmann makes it sound like these types of breakers are "new", but they have been the most common breaker used in the US for at least 40 years.​

 

[coulter]

...Just trying to give a complete answer so that anyone new to the thread would get the whole picture even if they hadn't read all the earlier posts. I didn't mean to offend...

Martin -

I apologize. You are the only fresh air in this thread. I appreciate your comments.

 

[ron]

Sorry for the "random" comment. I didn't mean it as a jab.
I meant that picking a current limiting fuse and researching the manufacturers trip curve, only lets you see as fast as approx. 1 cycle.
A breaker may begin to unlatch or fuse may begin to melt within that 1 cycle but might not clear the fault before exhibiting dynamic impedance and virtually reduce the fault current that the upstream device may "see".
Without series rating (manufacturers testing), I wouldn't feel comfortable to know that the current limiting fuse/breaker selection was anything but "random".
There are folks that are willing to bet the house on that selection without the manufacturers combination testing, and that's why 240.86(A) was added to the 2005 code.

 

[coulter]

... series ratings are much more dependent on OPENING times.

Yeah, I knew that. I sloppily used the term "clearing time' to refer to the time from start of opening to end of opening - which is of course - well -- sloppy.

...If the reduction in the current is sufficient enough both the clearing time of the fuse and it's resultant let-through current may be increased. ...

That is consistant with my understanding of the mfg curves, which is where I would tend to get that information.

 

[coulter]

Ron -

Thanks for the explanation of your thought process. I did not have a clear picture of your intent or meaning - but you knew that.

That is the best commentary I heard so far. My translation is: Often (most of the time), (often enough) the opening time of the downstream CB compared to the CLF clearing time is just too close to be comfortable.

Jim likely thinks he said this a dozen times in a dozen ways - and he likely did.

I'm not convinced I couldn't do it in some cases, but I am convinced it could be near impossible in others.

I am also convinced there is little reason to not follow the mfg series rating test bulletins.

 

[LarryFine]

I never noticed I was one bit better as an engineer the day after I got my PE ticket than I was the day before.

Agreed. I felt the same way when I first attained my Journeyman and Master licenses. (I did my first service change, for an uncle, when I was 16.)

However, just like a Journeyman or Master license, others notice the difference in a way that takes less time than watching you work for a while.

 

[kingpb]

To help clarify, Bussmann provides the following information about Up-Over-Down Method:
With series rated combinations, the up-over-down method is not able to be used to determine the protection of circuit breakers with blow-apart contacts, which exhibit dynamic impedance (All new breakers). For these applications, listed equipment which has been labeled for use with tested combinations must be installed. Despite the inability to predict protection for circuit breakers using the up-over-down method, the method is still valid for static components (components passive within first half-cycle). Some applications which can utilize the up-overdown method include Bus-Bracing Requirements, and Wire and Cable Protection.

Further, the following is an excerpt from an article that apppeard in EC&M Magazine:
Prior to 1970, series ratings had been applied by means of calculations in what were termed ?cascade ratings.? No formal method had been specified for these cascade ratings, and they were unreliable. The formal test program replaced these informal ratings in order to avoid unreliable installations. The requirement for marking of tested series ratings on end-use equipment first appeared in the 1990 NEC.

The ?up-over-down method? was promoted by some fuse manufacturers and applied as an alternative to testing until about 1993 when a paper titled ?Interplay of Energies in Circuit Breaker and Fuse Combinations? appeared in the May/June 1993 issue of IEEE Transactions on Industry Applications and cast doubt on the viability of the method.

This paper, authored by engineers Bernie DiMarco and Steve Hansen, clarified that the up-over-down method didn't accurately determine protection for the downstream MCCB, especially for faster operating MCCBs. One reason cited was that the method couldn't predict performance if the downstream MCCB contacts separated prior to the time the fuse cleared. Information isn't available to predict when MCCB contacts will separate, even though time-current curves show the longest time that the MCCB will take to clear the circuit by itself. With contacts beginning to open, the MCCB is, in fact, absorbing a portion of the energy of the short circuit that the upstream fuse won't see. Without the full energy, the fuse will operate slower than anticipated by the method. So the degree of protection is indeterminate.

Finally, Thermal-Mag Breakers have both instantaneous (I), whic is the magnetic portion, and Long Time (LT) protection, the thermal portion. In addition, sometimes you will see Molded Case Circuit Protectors (MCP) in motor control centers for protection of motors. These are Magnetic Only breakers, wherein the thermal protection comes from the motor overloads.