Reducing AIC at a machine.

[petersonra]

I think a big problem here is that "specifiers" don't yet realize that they need to include an appropriate SCCR as a requirement. So vendors, fearful of adding cost and losing an order, take advantage of the omission and go with the cheapest thing they can find; an untested electrical assembly that gets the "courtesy" listing of 5kA SCCR.

I think by now most people building electrical equipment KNOW this is an issue, but some don't divulge it (willingly). Not a month goes by that I don't get a call from someone asking me how they can lower the AFC at a jobsite, because something came in with an SCCR label saying 5kA and they have 23kA, 44kA, 65kA or something like that available at the terminals, a fact that they discover only after the AHJ red tags it.

I think it is unfair to blame this on the equipment manufacturers. We have a form to be filled out and one of the things on the form is SCCR required. Very few customers bother to provide a number. A fair number write in "N/A".

It is not as big a deal these days, at least for us, but a few years ago there was a huge difference in cost between a UL508a panel rated at 10 kA SCCR and one rated 65 kA. The cost difference is somewhat less these days as long as end users are flexible on parts, especially if they are willing to accept fuses.

I mention 10 and 65 kA because I find that most of the time going above 10 kA gets you to 65 kA anyway. There are only a few cases where I have run into something in between. It happens but is not real common.

[Jraef]

I think it is unfair to blame this on the equipment manufacturers. We have a form to be filled out and one of the things on the form is SCCR required. Very few customers bother to provide a number. A fair number write in "N/A".

It is not as big a deal these days, at least for us, but a few years ago there was a huge difference in cost between a UL508a panel rated at 10 kA SCCR and one rated 65 kA. The cost difference is somewhat less these days as long as end users are flexible on parts, especially if they are willing to accept fuses.

I mention 10 and 65 kA because I find that most of the time going above 10 kA gets you to 65 kA anyway. There are only a few cases where I have run into something in between. It happens but is not real common.

Bob,
From my experience, the responsibility you show in asking for the AFC/SCCR is not the norm outside of the custom panel builder business. By equipment mfrs, I meant people selling compressors, blowers, pump skids, machine tools etc, who either build their own panels or farm them out to the lowest bidders. Would that they go to someone like you who does understand it and at least ask for the info. Sorry for having cast such a wide net...

But yes, I totally agree that it has to start at the specifier end first. Putting "N/A" in the field of your forms is indicative of the lack of understanding that exists out there. Hopefully it's getting better though. I just came from a meeting at a Google facility on some equipment upgrade projects, that was actually the first topic of discussion, because they were recently burned by it at another facility and a corporate edict went out to all of their facilities engineers. So hope springs eternal that one by one, people who get burned once will learn their lessons.

[jlee405]

I suspect you did not hear what they actually said. You cannot reduce the available short circuit current by the use of current limiting fuses. Many times though you can put them in series with another device and get a series rating of the devices as a pair that exceeds what the other component's rating is. But this has to be an NRTL tested pairing.

Just out of curiosity, how is that you selected the 14kAIC breakers in the first place?

Current limiting fuses do infact decrease the available fault current at the load side of the fuse. Fuses typically have a let-through current curve/graph on their datasheets.

[petersonra]

Current limiting fuses do infact decrease the available fault current at the load side of the fuse. Fuses typically have a let-through current curve/graph on their datasheets.

that is indeed so but you can't use that to calculate available short circuit current, unless possibly if you are a P.E. and are willing to put your seal on the calculations.

[packersparky]

Current limiting fuses do infact decrease the available fault current at the load side of the fuse. Fuses typically have a let-through current curve/graph on their datasheets.

That is true, but I had an engineer from a major fuse manufacturer tell me that "Current limiting fuses have never been tested or evaluated by a NRTL as a means to lower the available fault current."

[Ingenieur]

That is true, but I had an engineer from a major fuse manufacturer tell me that "Current limiting fuses have never been tested or evaluated by a NRTL as a means to lower the available fault current."

these are UL listed/tested for current limiing ?

https://www.omega.com/auto/pdf/REF_FuseSizingGuide.pdf


for mv iirc they all are IEC rated, possibly UL

good sizing info
http://www.cooperindustries.com/cont...ons_Charts.pdf

[Ingenieur]

many of these are listed 600 vac class
http://www.cooperindustries.com/cont...tion_Fuses.pdf
Limitron (Fast-Acting)
KTS-R (600Vac), KTN-R (250Vac), 1 to 600A, 200,000AIR ac Current-Limiting
STD 248-12 Class RK1
UL Guide #JDDZ, UL File #E4273, CSA Class #1422-02, CSA File #53787
Single-element, fast-acting fuses with no intentional time-delay. The same basic performance of the 601-6000A KTU fast-acting Limitron fuses. Provide a high degree of short-circuit current limitation (component protection). Particularly suited for circuits and loads with no heavy surge currents of motors, transformers, solenoids, and welders. Limitron fuses are commonly used to protect circuit breakers with lower interrupting ratings. If used in circuits with surge currents (motors, etc.), must be oversized to prevent opening and, thus, only provide short circuit protection. Incorporate Class R rejection feature. Can be inserted in non-rejection type fuse holders. Thus, can physically and electrically replace fast-acting Class H, K1, K5, RK5, and other RK1 fuses.

[Jraef]

It's not that the fuses are not current limiting. CL fuses are quite OFTEN used in series listing of equipment, through TESTING of the equipment.

U.L. testing and listing of equipment protected by current limiting
fuses is done with fuses that have the same current limiting
performance as the maximum allowable peak let-through current when
any fuse manufacturer submits current limiting fuses for testing and
listing. The result is that any brand of U.L. listed fuses may be used
to protect U.L. short-circuit rated equipment within the specified limit
of short-circuit rating

That testing is a destructive test and it's expensive, not something a contractor can do in the field. It is POSSIBLE that a licensed PE can use the fuse mfrs charts to determine that the let-through of the fuses, under the specific conditions of the installation and any expected fault conditions, can act fast enough to protect the down stream equipment if the PE also has information on the damage curve of the weakest link in the chain, etc. etc.. THEN the PE must be willing to sign off on the system design, staking their professional reputation and future income against it. Has it happened? I don't know, I've just herd of it anecdotally. But I have been in that situation twice, both times a PE was hired, both times when they looked into the details, they declined to sign off on it (and still sent the bill). There are a LOT of variables and if they miss one and someone gets hurt, they are accepting the responsibility.

[Ingenieur]

data sheet http://www.cooperindustries.com/cont...1044_KTS-R.pdf

assume 100 a fuse
avail rms 30 ka
avail peak is 70 ka

let thru rms 3.5 ka
let thru peak 8 ka

will trip in 1/2 cycle

[Ingenieur]

It's not that the fuses are not current limiting. CL fuses are quite OFTEN used in series listing of equipment, through TESTING of the equipment.

That testing is a destructive test and it's expensive, not something a contractor can do in the field. It is POSSIBLE that a licensed PE can use the fuse mfrs charts to determine that the let-through of the fuses, under the specific conditions of the installation and any expected fault conditions, can act fast enough to protect the down stream equipment if the PE also has information on the damage curve of the weakest link in the chain, etc. etc.. THEN the PE must be willing to sign off on the system design, staking their professional reputation and future income against it. Has it happened? I don't know, I've just herd of it anecdotally. But I have been in that situation twice, both times a PE was hired, both times when they looked into the details, they declined to sign off on it (and still sent the bill). There are a LOT of variables and if they miss one and someone gets hurt, they are accepting the responsibility.

if I personally calculated the avail sc i at the machine based on util info or inf bus
and knew the machine load i and mtr hp
and the machine rated aic
I would have no issue sizing the fuses with the stipulation they are soley used for sc i limitation and not equip toc protection or conductor protection

no issue sealing the calcs if a fuse could be applied within spec
obviously fee vs liability would be considered