Current limiting fuses

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mshields

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Location
Boston, MA
Current limiting fuses have three curves describing their characteristics; peak let thru, the actual time-current curves and I squared t.

In determining what AIC rating of equipment is necessary down stream of these, do I need to consider anything other than the peak let thru.

i.e. do I merely need to take the "Up-over-and down" results from the peak let thru and make sure my equipment down stream is rated above the value I get.

OR - do I need to consider the I2t curve. And if so, how exactly do I need to take it into consideration?

Thanks,

Mike
 

jim dungar

Moderator
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Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Current limiting fuses have three curves describing their characteristics; peak let thru, the actual time-current curves and I squared t.

In determining what AIC rating of equipment is necessary down stream of these, do I need to consider anything other than the peak let thru.

i.e. do I merely need to take the "Up-over-and down" results from the peak let thru and make sure my equipment down stream is rated above the value I get.

OR - do I need to consider the I2t curve. And if so, how exactly do I need to take it into consideration?

Thanks,

Mike
First the Up=Over-Down method must never ever be used with circuit breakers that have dynamic impedance (a fancy term for saying any breaker with an Instantaneous trip function)

Second equipment does not have an AIC rating unless it is an actual protective device like a fusible switch or a circuit breaker. Equipment that does not interrupt a fault has either a Withstand Rating (the old term) or a Short Circuit Current Rating (SCCR is the preferred term).

So the answer depends on what you are trying to accomplish.
 

mshields

Senior Member
Location
Boston, MA
your clarifications

your clarifications

Thanks for those insights.

What I'm trying to do is properly protect an ATS with a "withstand" rating of 65kA" but with an upstream available fault current of over 100kA.

The switchgear that will feed the ATS is DS gear with limiters.

If I apply up over and down to the peak let thru curve of the limiters, I'm under 50kA. Am I done? Or is there something more I need to consider?


Thanks,

Mike
 

charlie

Senior Member
Location
Indianapolis
The up, over, and down method must never ever be used with circuit breakers

The up, over, and down method must never ever be used with circuit breakers

First the Up-Over-Down method must never ever be used with circuit breakers that have dynamic impedance (a fancy term for saying any breaker with an Instantaneous trip function) . . .
The up, over, and down method must never ever be used with circuit breakers that do not remain latched for the first ? cycle . . . period. This statement applies to almost all circuit breakers that are manufactured today.

Jim, I am concerned that you are giving the idea that the electrician can avail themselves of 240.86(A) to protect a circuit breaker. There are specific rules found in 240.86(A) that precludes almost all installations where anyone may want to make use of the provision. I am sure that this was not your intent but I wanted to make a clear statement that this method is not for general use. :)
 

ron

Senior Member
Since there will probably be breakers in the distribution downstream of the ATS's, they will make the impedance value dynamic. I do not believe that you can use the up over and down method unless the switch has been tested together with that fuse and has a "series" rating. I carefully whispered series rating because it is different than a traditional series rating between a fuse and a breaker, but you will find that ASCO, Russelectric and the others have withstand ratings tested with certain current limiting fuse ratings. I only use those published combinations.

I haven't used the up-over-down method in many years since the dynamic impedance issue has been in the trade journals and white papers.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Jim, I am concerned that you are giving the idea that the electrician can avail themselves of 240.86(A) to protect a circuit breaker. There are specific rules found in 240.86(A) that precludes almost all installations where anyone may want to make use of the provision. I am sure that this was not your intent but I wanted to make a clear statement that this method is not for general use. :)

It was not my intent.
As soon as I hear someone ask about the up-over-down method, my first impression is that they are unqualified to apply it.

For more than 30 years, Bussmann has published an example of how to perform this method in their SPD with the fine print disclaimer that its use is effectively very limited. I feel they would serve the industry better by removing this method from their general literature.
 

charlie

Senior Member
Location
Indianapolis
It was not my intent.
I was pretty sure of that, thanks.

. . . For more than 30 years, Bussmann has published an example of how to perform this method in their SPD with the fine print disclaimer that its use is effectively very limited. . .
The idea for 240.86(A) is from the IEEE Blue Book and promoted by Bussmann, ACC, and EEI (me) in the panel 10 meetings. It works well when it is needed but it is very restrictive.

. . . Bussmann . . .
It is interesting to me how many people use buss to refer to bus. Bussmann really likes everyone to refer to things like buss duct instead of bus duct (sorta like advertising for Bussmann). :smile:
 

mshields

Senior Member
Location
Boston, MA
your input

your input

gentlemen,

Thankyou for your input.

Ron - My initial design called for a fused switch ahead of the ATS in keeping with Russelectric tested combinations. Client does not want fuses however and has umpteen ATS's down stream of this switchgear indicating that previous engineers have sanctioned the limiter/ATS combination to be ok. WHICH DOESN'T MEAN IT IS OF COURSE.

My take away from all of this is as follows: I can use the let thru curve for the limiters to determine fault current at the ATS. What I need to worry about is the device down stream of the ATS, which in this case is a DP. I don't know if Cutler Hammer has tested combinations. I'm looking into it.

thanks,

Mike
 

ron

Senior Member
Mike,
I can understand you situation. My response to the client is that the Russelectric instructions show a particular withstand rating for the equipment, I will follow it.
Your OP indicated that you are looking into fuse curves, so it sounds like you will have fuses upstream of the ATS. It doesn't matter if it is in a switch immediately upstream of the ATS or at the service, as long as it is the correct size and type.
 

ron

Senior Member
Pierre,
There are several that I've found when searching for the terms
dynamic impedance up over down current limit
I don't have a favorite, but be care to know who wrote the article as fuse manuf are somewhat biased toward their product.

Essentially the concept is that if there is equipment in the distribution that will change its impedance during a fault, the current limiting capabilities of the current limiting fuse (or current limiting CB), cannot be accurately predicted.

Circuit breakers change their impedance during a fault as they open. My theory is that ATS switches change their impedance during a fault. This is why I only go by the published withstand ratings from the manufacturer. Each has a rating on it's own, then a rating with "any" upstream CB, then they have a rating with an upstream fuse of a particular class/size.
 
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