Series Rating to drive down available fault current

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davidr43229

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A customer has too much available fault current into his Industrial Control enclosure. I believe he has 65K. The main means of disconnect within his controller is a GE Spectra breaker, rated at 42K. Can he use series rating (combination of fuse and GE breaker) to reduce his availavle fault current?
Asked a different way, I know series rating calls out for both devices to "go" at the same time, but this would be a UL listed combination. So.....can we use the current limiting ability of the fuse to protect the breaker?
We have some motors and contribution is not a factor at this time...
Dave
 
davidr43229 said:
A customer has too much available fault current into his Industrial Control enclosure. I believe he has 65K. The main means of disconnect within his controller is a GE Spectra breaker, rated at 42K. Can he use series rating (combination of fuse and GE breaker) to reduce his availavle fault current?
Asked a different way, I know series rating calls out for both devices to "go" at the same time, but this would be a UL listed combination. So.....can we use the current limiting ability of the fuse to protect the breaker?
We have some motors and contribution is not a factor at this time...
Click to expand...

Series ratings do not call for both devices to go at the same time. While it is possible they both may trip it is not a requirement. For example, UL has standard ratings of 10k and 22kAIC, if a breaker by it self can safely open 21k SCA then it will only get a UL rating of 10k. Now put that breaker in series with the second breaker (this adds impedance to the circuit from the second protective device and the additional conductor) and it may pass 22k without any help from the upstream device. A fuse can only limit current if it is melting fast enough.

Series ratings do not lower the available fault current only impedance can do that. Series ratings are usually only for two devices in series so they can not be used to protect an entire control panel which contains many different devices.
 
Have you consulted with a GE applications expert to see if they have tested their breaker with upstream fusing or circuit breaker that would coordinate with the 42kaic breaker such that they would be UL series combination listed?
Also, it is a bit difficult to provide a constructive reply when very little information has been supplied.
Is 480y/277v to be assumed? Or ?
What is the P/N of the frame of the breaker.
Is 65ka being assumed or is that what the POCO has advised you that is or is that calculated?
Is it possible to replace the existing frame with one that has a 65kaic rating and then look at series rating possibility for down stream OCPDs?
 
templdl,
We are looking at a UL listed Series rated combination of a GE SED, SEH, SEL Spectra breaker 15-150amps at 480 volts, with a CLass J fuse.
http://www.bussmann.com/library/techspec/Ge.pdf
We have over 65k available, but less than 100kamps
What I am trying to do is to fuse the let through of the fuse to lower the available fault current.

Jim,
I read your reply and I would agree to lower the available fault currents, you need to introduce impedence, but aren't we using either the current limiting ability of the fuse or dynamic impedence of the GE Spectra breaker to do this same thing ?

Dave
 
davidr43229 said:
A customer has too much available fault current into his Industrial Control enclosure. I believe he has 65K. The main means of disconnect within his controller is a GE Spectra breaker, rated at 42K. Can he use series rating (combination of fuse and GE breaker) to reduce his available fault current?
Dave
Click to expand...
Can you tell us how the 65ka was calculated? Templdl asked this same question. You are considering a major change in the system. I just wanted to
be sure that the 65ka is the actual value and not some assumed one.
 
davidr43229 said:
I read your reply and I would agree to lower the available fault currents, you need to introduce impedence, but aren't we using either the current limiting ability of the fuse or dynamic impedence of the GE Spectra breaker to do this same thing ?
Click to expand...

A series rating does not there has been a change in the amount of fault current. A series rating simply says the two devices can be applied together regardless how the higher AIC is achieved. Every device, including current limiting fuses, exhibits dynamic impedance when extinguishing an arc. It is possible that the series rating is acheived because a breaker is so slow that it remains closed while the upstream fuse opens. In a listed series rating, we don't know and we don't care.

You mentioned an industrial enclosure in your OP. If you have more than two devices in series (i.e. current limiting fuse -> GE breaker -> motor starter) it is unlikely that a UL series rating exists.
 
You mentioned an industrial enclosure in your OP. If you have more than two devices in series (i.e. current limiting fuse -> GE breaker -> motor starter) it is unlikely that a UL series rating exists
Click to expand...
Jim, please explain. We are talking about a UL listed series rating between 2 OCPD
http://www.bussmann.com/library/techspec/Ge.pdf
It is possible that the series rating is acheived because a breaker is so slow that it remains closed while the upstream fuse opens
Click to expand...
I will find this out, but my bet is that the GE Spectra breaker within the customers panel is so slow and the UL Listed Series rating whould prove that out.
So if I can prove imperically that the breaker doesn't open, can I use the current limitation of the fuse as proof of lowering the avilable fault current. I know this is done with buss bar bracing all the time, with current limiting devices..( not to introduce another subject)

Dave
 
Dave,
Re: "So if I can prove imperically that the breaker doesn't open, can I use the current limitation of the fuse as proof of lowering the avilable fault current."
Why would you think that the breaker would not open? If a breaker is subject to a current equal or greater than it's trip calibration it will trip. If subjected to a fault current greater than its rating it probably would sustain damage. If great enough it could blow apart. That's where you must a consider series-conbination rating that you are so wisely exploring now.

If I may, and please correct me if I?m wrong, I would like to make a distinction between available fault current and current limiting.
Available fault current can be calculated if a qualified person has done the calculations and that the OCPD applied accordingly.
However, regarding a CL device as it would relate to protecting a down stream device by limiting fault current, curves and any calculations regarding same don't count. The devices to be used in series-combination (integrated rating) must be tested together and UL listed as such. No exceptions. Using the "up-over-down" method with "Prospective Peak Let-Through Current in Amperes" fuse curves may be invalid as they are based upon passive bus systems which do not take into account the dynamic nature of a circuit breaker. Always refer to the recommendations by the circuit breaker manufacture, which are based on actual test data.

It is interesting to note that CL fuses are focused on. But all breakers are also CL to some degree that makes them suitable for being series-combination tested with other breakers down stream. In addition there are specifically breakers which are UL listed and classified as current limiting that have been tested to limit the let through current to a specific level or less. As I can recall that level is less than 1/4 cyc.
Bottom line? Identify the available fault current, go to the GE series-combination rating chart as refered to by a previous poster and compare it to the exising gear. It may be that simply changing a breaker of two may be the solution dependent upon what the exisiting breakers are and if they are included on the series-combination rating chart of the kaic required.
 
davidr43229 said:
Jim, please explain. We are talking about a UL listed series rating between 2 OCPD
http://www.bussmann.com/library/techspec/Ge.pdf

I will find this out, but my bet is that the GE Spectra breaker within the customers panel is so slow and the UL Listed Series rating whould prove that out.
So if I can prove imperically that the breaker doesn't open, can I use the current limitation of the fuse as proof of lowering the avilable fault current. I know this is done with buss bar bracing all the time, with current limiting devices..( not to introduce another subject)
Click to expand...

A NEMA starter has a UL listing of only a few kSCA. UL has listed combinations (these are series ratings) of protective devices and starters that are into the 10's of kSCA. Now you want to use an additional series rating to protect the entire panel. This would effectively be trying to "add" series ratings together, which is not allowed.

My guess is that the GE breaker will begin opening way before the fuse finishes melting.
 
breakers which are UL listed and classified as current limiting that have been tested to limit the let through current to a specific level or less. As I can recall that level is less than 1/4 cyc.
Click to expand...
I made a statement a few months ago that Breakers will not toally clear within 1/2 (.008) seconds and I was proved wrong, however I have never seen a breaker totaly clear within 1/4 cycle (.004) seconds..
.
If a breaker is subject to a current equal or greater than it's trip calibration it will trip
Click to expand...
Not in a UL listed Series Rating,, could be that the fuse total clear, even before the fault current even reaches the instantious trip region of the breaker, breaker could have a total clearing of 2-3 cycles, which then the fuse has done it's job and is booking a trip to Florida.
But all breakers are also CL to some degree that makes them suitable for being series-combination tested with other breakers down stream
Click to expand...
Not all breaker combinations can be series rated.
Using the "up-over-down" method with "Prospective Peak Let-Through Current in Amperes" fuse curves may be invalid as they are based upon passive bus systems which do not take into account the dynamic nature of a circuit breaker
Click to expand...
Can still be used if the breaker does not initate within the first 1/2 cycle.
 
davidr43229

A) CL breakers have contacts that are designed to increase the speed at which they open. Some are designed as a slot motor concept with (2) moving contacts which are forced apart, some where the stationery contact is reverse looped such as the magnetizing field caused be the fault current assists in forcing the contacts apart as the breaker trips. Yet another design has both moving contacts, contacts that move away from one another in opposite directions. Some breakers even incorporate a CL fuse that has a rod that extends to hit the trip bar of the breaker at the same time the fuse is clearing the fault. Yes, CL breakers are more than a run of the mill breaker.
B) The dynamics of faults are different so one can not go so far as to assume that the breaker and fuse will react in the same way in all situations.
C) As I stated all breakers are current limiting to some degree which makes them suitable "to be tested" for series-combination rating. It was not my intention to infer that all breaker can be assumed to be included in series-combination as some may not be current limiting enough to pass the applicable tests to coordinated with a down stream device because some just do not pass muster.
D) How is it to be determined when to or not to use the up-over-down method based upon the 1/2 cycle you had referred to? It will depend upon the dynamics of the fault. Would that be bolted fault? It is to my understanding that arcing faults are more common.
?The reason is that the Up-over-down method ignores dynamic impedance (the inherent current-limiting of the down stream molded case circuit breakers). This impedance is developed directly by the forces of the let-through current created when the contacts are blown apart.
Always refer to the recommendations by the breaker manufacturer, which are based upon actual test data for the proper application of current limiting fuses.
 
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