Why aren't current limiting fuses integrated in software modeling?

As most softwares contain the loads of information regarding circuit breakers, couldn't they easily integrate and incorporate fuses melting times, current limiting curves and there affects after 1/2cy, 5cy, 30cy...

for example, its easy for current limiting fuse curves to be applied to see how a 30kA fault is reduced to 12kA for a particular fuse type. In addition, the 5cy and 30cycle should have roughly 5kA and 0kA respectively.

It seems to be a gap in my software, easy power, and is really a huge part in understanding power systems for protective coordination.

Any thoughts?

What software are you referring to?

In EasyPower there is an option for Current Limiting fuses.

Easy Power. I'm sorry maybe i don't know how to utilize the function, but i know how to choose current limiting fuses. The problem is the software calculating the affects of those such fuses, for example at 30cycles the fuse should be clearly melted and there should be 0 current through the device. At 5 cycles the current limitation should be seen in the calculations, violations, etc.



For example i have a TRS80R fuse in an MCC line up feeding a control panel. in that control panel is 3 breakers with 6kA interupting max. My software has 7kA on that bus and therefore overdutys those three breakers by 17%. However, referencing the current limiting chart that from a Ferraz Shawmut TRS80 fuse, a 7kA SC will be lowered to roughly an RMS of 3kA. allowing it to be under duty.

The software doesn't seem to take this into account.

Series Ratings vs. Up-Over-Down "Let thru" Charts



The problem is not with the software, it is with the application method for what is known as a “series rated� combination. Prior to the early 1980’s, using fuse charts often known as let thru charts, was common. This was known by many as the “up-over-and down� method because of how you draw lines on the current limiting chart to determine the limiting effect of the fuses. You draw a line “up� then “over� then back “down� on the chart / graph. The concept was that you could “predict� how much a fuse would limit the short circuit current.

Over time, it was determined that although these charts give a good prediction of how a current limiting fuse might limit the current by itself, it was not how it may respond when in series with a circuit breaker. The problem was when the breaker and fuse opened together - the whole circuit dynamic changed. Things like the arc impedance across the breaker contacts were not considered by the charts and this changed how the series rating behaved. There were many cases where using a series rating determined by the charts simply would not work as expected.

Underwriter’s Laboratories got involved to sort out the problem and the series rating “rules� were re-written. I was at Square D Company at the time and involved from the inside. Manufacturer’s were now required to physically test combinations of devices in series under short circuit conditions. i.e. a specific type/size of current limiting fuse upstream with a specific circuit breaker downstream. The fuses were UL “umbrella� value fuses that had let thru characteristics of that fuse’s size and class. That way if it passed with the umbrella fuses, any manufacturer’s fuses could be used.

As an example, perhaps a downstream breaker with a 22kA rating would be tested in series with a 200A RK5 fuse. They would place the two devices in series, and test the combination with a much greater fault current, let’s say 100 kA for example. If the combination of devices passed the tests, it would receive a 100kA series rating. No let thru charts permitted - just look up the series rating of the combination of devices and compare it to the calculated fault current.

So, EasyPower (and all the other software packages) do not account for current limitation because that keeps them in compliance with the standars defined by U.L. about 30 years ago as well as the requirements of NEC 240.86 which requires tested combinations.

In your case, you would need to see if there is a series rating between the fuse and downstream breakers. This would be obtained from the breaker manufacturer.

Article 240.86 of the NEC does allow the use of let thru charts “under engineering supervision in existing locations� but places all kinds or requirements such as the combination being selected by a licensed P.E., proper documentation, stamping by a P.E. etc. This language was added because there would be existing older locations where there were no tested combinations of equipment.

For older systems with no tested series ratings you would be “between a rock and a hard place� This NEC language permits a person to stick their neck out and come up with their own series rating in the absence of tested combination. I don’t know of anyone that would do this because of the liability involved - including the software companies.

Quite a lengthy explanation but I hope it helps.

You are discussing the combination rating of a fuse and a circuit breaker in series. You cannot use the current limiting effect of fuses to reduce the let through current to a downstream protective device that has an instantaneous trip function. I do not know of a fuse or breaker manufacturer that sanctions what your methodology

There are several reasons, but probably the biggest is often called 'dynamic impedance'. As soon as a fault initiates the instantaneous trip function of the breaker may cause its contacts to begin to open prior to the fuse clearing. As the breaker contacts open an arc is created adding additional impedance and lowering the current flowing through the fuse. This lowered current may mean the fuse takes longer to clear the fault and therefore is not as current limiting. Minature molded case circuit breakers are extremely fast, it is not uncommon for them to begin operation in the first 1/4 cycle.

OK, i see the issues here. I wondered why it was never included in such expensive software, but i can see the hangups when it comes to such details.


thank you all..

"Current-limiting" Circuit Breakers?




When you say "miniature molded case circuit breakers" are you referring to CB's labeled as "current limiting" CB's only?

I have been struggling as well with how one should interpret and apply this "current-limiting" characteristic of breakers that are labeled as-such, to the incident energy results of the arc flash calculations, and also for protective device coordination in facilities with a mixture of these and other breakers that do not contain the current-limiting label.

One manufacturer's claim on these current limiting breakers states that the "peak let-through current is held to less than 45% of the maximum available peak fault current, resulting in a tremendous reduction in the amount of energy that the fault delivers to the conductors and connected load".

If this is true then for arc flash incident energy calculations it should be expected that this would be reflected in the output when these breakers are involved but, at least in SKM, it is not.

I contacted SKM and they told me that in order to model a breaker's current limiting ability the user must themselves enter in the data in the breaker model's arc flash calculations and set the model to use the equations. Sounds like someone's passing the (liability) "buck" on down the line, perhaps.

I am left seemingly no choice but to ignore the current-limiting claim, which for arc flash analysis will lead to reporting of a higher incident energy / hazard category, a more conservative and arguably wiser choice.

Selective coordination, however is altogether another issue. It seems odd that the manufacturers are claiming in brochure statements that these devices interrupt the circuit at around .004 seconds, yet the SKM models and the manufacturer's published trip curves do not include plot times prior to a .01 second threshold. How is anyone supposed to interpret this conflicting information? Seems all one can do is say that selectivity cannot be guaranteed unless the combinations of breakers in the study have been tested together by the manufaturer for selectivity.

Check my post above. You no longer model the current limiting effect. Current limiting devices are tested in combinations according to U.L. The NEC requires that you use the tested combinations.

Instead you use the tested "series rating" of a CL device in series with a lower rated downstream device. The series rating of the two together tenda to be quite high. Then you compare the series rating with available fault current.

This is the way we do it.

And this rational is consistent with how fuse ratio tables have been built. Anything below the .01s portion of a curve must be confirmed through testing.

As far as the phrase 'miniature molded case breaker'. I was speaking in generalities of breakers with 100A frame size.