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RMS short circuit rating of fuses
- Thread starter ddderek
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Re: RMS short circuit rating of fuses
Different class fuses are listed to clear short circuits at different current levels
http://www.bussmann.com/library/docs/SafetyHandbook2004.pdf
http://www.bussmann.com/services/safety basics/
Try these sites for reading material
Basically a supply system may be capable of delivering thousands of amperes if a short occurs.
50k, 100k 300k ect. Fuses will be designed clear in a time frame measured in cycles if the OCP is current limiting it may clear in 1/4 cyc .004 sec.
If the fuse clears quickly the arcing and thermal damage is mitigated. Hold your hand in the flame of a candle.
Breakers clear atlonger duration 6-12 cyc. more energy more damage.
All electrical equipment is subject to short circuit current damage see 110.9 and 10. as well as definition in 240 for coordination.
You want to ensure the fuse installed will function at the fault level available and you probably do not want to buy more protection then you need to pay for.
thats a brief but incomplete overview.
Charlie
Different class fuses are listed to clear short circuits at different current levels
http://www.bussmann.com/library/docs/SafetyHandbook2004.pdf
http://www.bussmann.com/services/safety basics/
Try these sites for reading material
Basically a supply system may be capable of delivering thousands of amperes if a short occurs.
50k, 100k 300k ect. Fuses will be designed clear in a time frame measured in cycles if the OCP is current limiting it may clear in 1/4 cyc .004 sec.
If the fuse clears quickly the arcing and thermal damage is mitigated. Hold your hand in the flame of a candle.
Breakers clear atlonger duration 6-12 cyc. more energy more damage.
All electrical equipment is subject to short circuit current damage see 110.9 and 10. as well as definition in 240 for coordination.
You want to ensure the fuse installed will function at the fault level available and you probably do not want to buy more protection then you need to pay for.
thats a brief but incomplete overview.
Charlie
Re: RMS short circuit rating of fuses
Charlie may not be comparing apples to apples when he states a 1/4 cycle clearing time for vs 6-12 cycles for breakers.
I think it first must be emphasized that once the contacts of a "standard" breaker that is applied within its rating are told by the "trip unit" to open the contacts will clear the arc in less than 1 cycle. What is most misunderstood is that it can't do anything other than to do just that. Remember that you are dealing with alternating current that goes through point zero volts twice a cycle. There are "arc chutes" that the moving contact travels between when it opens which "de ionizes" the arc and then is extinguish on or before the voltage zero volts in the sign wave.
Where the 6-12 cycles comes from is a mystery to me.
A good example is an MCP vs dual element fuse for motor protection. It has been documented by Westinghouse that a motor circuit protector (MCP) type (mag only) breaker will blow a dual-element fuse away in its ability to clear a low level motor fault. Most motor fault beguine as low level faults and the goal is to limit them to just that which minim,izes the damage to the motor and reducing the chance of a fire. Unlike a fuse the magnetic pick-up of an MCP is adjustable and can be fine tuned to prevent is from nuisance tripping because of motor inrush. Remember that a magnetic trip element has no time delay and will open the contacts immediately, clearing the fault in less than 1 cycle where a dual element fuse may take many cycles to clear that same fault.
One example that I have is for a 3HP motor with approx 4.2FLA. A short circuit was applies to the load side of the combo starter of 150a which would be about 35x the FLA The dual element fuse device used was 5A, sized at 120% of the motor FLA, but could be sized to 175%. A 7a rated MCP was used and set at 12x the FLA or 51am for comparison. The test proved that the MCP cleared the fault consistently is less than 1 cycle where it took the fuse a minimum of 12-84 cycles to clear. And to think that there is no single phasing with a breaker as when one pole trips, all phases are cleared. Neat!! It took only the cost of one breaker but the cost of many fuses to verify these tests. And, as an added bonus, you don't have to stock replacement fuses when using a breaker. What's also neat about the adjustable trip MCP is that it is adjustable, you don't have to change or replace it with one of another value. The object is that you don't want it to nuisance trip when the motor starts but are allowed and settings up to that is included in NEC art 430-52(C)ratings and settings(3)Instantaneous trip breakers.
With thermal magnetic breakers the thermal element respond to a given time/current curve as is the fuse design.
In addition, if current limiting is a hot button breakers are available the a current limiting that can do just as well at clearing in less than 1/4 cycle.
I'm not trying to throw dirt a fuses as they have worked well is the past and will continue to so in the future. What is boils down to is what type of protect we understand the best and can apply to provide the protection we are accustomed to, what are personal beliefs are.
Charlie may not be comparing apples to apples when he states a 1/4 cycle clearing time for vs 6-12 cycles for breakers.
I think it first must be emphasized that once the contacts of a "standard" breaker that is applied within its rating are told by the "trip unit" to open the contacts will clear the arc in less than 1 cycle. What is most misunderstood is that it can't do anything other than to do just that. Remember that you are dealing with alternating current that goes through point zero volts twice a cycle. There are "arc chutes" that the moving contact travels between when it opens which "de ionizes" the arc and then is extinguish on or before the voltage zero volts in the sign wave.
Where the 6-12 cycles comes from is a mystery to me.
A good example is an MCP vs dual element fuse for motor protection. It has been documented by Westinghouse that a motor circuit protector (MCP) type (mag only) breaker will blow a dual-element fuse away in its ability to clear a low level motor fault. Most motor fault beguine as low level faults and the goal is to limit them to just that which minim,izes the damage to the motor and reducing the chance of a fire. Unlike a fuse the magnetic pick-up of an MCP is adjustable and can be fine tuned to prevent is from nuisance tripping because of motor inrush. Remember that a magnetic trip element has no time delay and will open the contacts immediately, clearing the fault in less than 1 cycle where a dual element fuse may take many cycles to clear that same fault.
One example that I have is for a 3HP motor with approx 4.2FLA. A short circuit was applies to the load side of the combo starter of 150a which would be about 35x the FLA The dual element fuse device used was 5A, sized at 120% of the motor FLA, but could be sized to 175%. A 7a rated MCP was used and set at 12x the FLA or 51am for comparison. The test proved that the MCP cleared the fault consistently is less than 1 cycle where it took the fuse a minimum of 12-84 cycles to clear. And to think that there is no single phasing with a breaker as when one pole trips, all phases are cleared. Neat!! It took only the cost of one breaker but the cost of many fuses to verify these tests. And, as an added bonus, you don't have to stock replacement fuses when using a breaker. What's also neat about the adjustable trip MCP is that it is adjustable, you don't have to change or replace it with one of another value. The object is that you don't want it to nuisance trip when the motor starts but are allowed and settings up to that is included in NEC art 430-52(C)ratings and settings(3)Instantaneous trip breakers.
With thermal magnetic breakers the thermal element respond to a given time/current curve as is the fuse design.
In addition, if current limiting is a hot button breakers are available the a current limiting that can do just as well at clearing in less than 1/4 cycle.
I'm not trying to throw dirt a fuses as they have worked well is the past and will continue to so in the future. What is boils down to is what type of protect we understand the best and can apply to provide the protection we are accustomed to, what are personal beliefs are.
Ed MacLaren
Senior Member
Re: RMS short circuit rating of fuses
It just seems logical that it would take longer for parts of a mechanical assembly to move than it does for a fuse link to vaporize.
Ed
The basic reason why fuses can open a circuit faster than breakers can be summed up by one word - inertia.Where the 6-12 cycles comes from is a mystery to me.
It just seems logical that it would take longer for parts of a mechanical assembly to move than it does for a fuse link to vaporize.
Ed
Re: RMS short circuit rating of fuses
templdl
Yes I got a little off topic.
Thank you for pointing out the 6-12 cyc. That is information based upon Arc Flash Testing and I agree it is not indicative of all CBs . Information that I have read would suggest that larger frame inverse trip breakers could operate at rated interrupting levels within 3 cycles.(mechanical latching)
Also my reference to the Fuses operating at 1/4 cyc is also dependant upon available fault current and intensity of flow. Melting alloy devices that are designed as Current Limiting Attempt to clear Faults within their rating in this time frame as it is explained:
1. melt in the first 1/8 cyc,
2. clear arc in the next 1/8 cyc
3. total clear in 1/4 cyc.
I did neglect to mention that all of these points are subject to a tremendous amount or influence such as the ability of the source to deliver fault current (infinite buss)and premises wiring which obviously will effect circuit impedance.
Charlie
templdl
Yes I got a little off topic.
Thank you for pointing out the 6-12 cyc. That is information based upon Arc Flash Testing and I agree it is not indicative of all CBs . Information that I have read would suggest that larger frame inverse trip breakers could operate at rated interrupting levels within 3 cycles.(mechanical latching)
Also my reference to the Fuses operating at 1/4 cyc is also dependant upon available fault current and intensity of flow. Melting alloy devices that are designed as Current Limiting Attempt to clear Faults within their rating in this time frame as it is explained:
1. melt in the first 1/8 cyc,
2. clear arc in the next 1/8 cyc
3. total clear in 1/4 cyc.
I did neglect to mention that all of these points are subject to a tremendous amount or influence such as the ability of the source to deliver fault current (infinite buss)and premises wiring which obviously will effect circuit impedance.
Charlie
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: RMS short circuit rating of fuses
I would like to give you an actual experience that we had with the Bussmann Fuse Company, along with their Engineer Larry Eggert helping us with "Selective Fuse Co-ordination" on our plant site.
The plant that I am talking about at that time, had 8,000 employees,and 10,000 motors. The plant size was about 1-1/2 miles square. This is just to give you an idea of the enormity of this project.
We changed all of our fusing to Bussmann, Lo-Peak, Dual-Element fusing.
On our motor circuits, as you are all aware, that you can fuse to 175%. We fused all of our motors at 125%.
It took us about two-years to complete the project of "Selective Fuse Co-Ordination".
I must give the majority of the credit to Larry Eggert of the Bussmann Fuse Company for helping us with this task.
I could relate to you several incidents that have averted any damage to equipment or personnel, because of our "Selective Fuse Co-Ordination" project that we implemented on our site.
I would like to give you an actual experience that we had with the Bussmann Fuse Company, along with their Engineer Larry Eggert helping us with "Selective Fuse Co-ordination" on our plant site.
The plant that I am talking about at that time, had 8,000 employees,and 10,000 motors. The plant size was about 1-1/2 miles square. This is just to give you an idea of the enormity of this project.
We changed all of our fusing to Bussmann, Lo-Peak, Dual-Element fusing.
On our motor circuits, as you are all aware, that you can fuse to 175%. We fused all of our motors at 125%.
It took us about two-years to complete the project of "Selective Fuse Co-Ordination".
I must give the majority of the credit to Larry Eggert of the Bussmann Fuse Company for helping us with this task.
I could relate to you several incidents that have averted any damage to equipment or personnel, because of our "Selective Fuse Co-Ordination" project that we implemented on our site.
Re: RMS short circuit rating of fuses
OK, youse guys are talking way over my head... I am inquireing for the fuses installed in a residential sevice disconnect...
The disconnect lit says it accepts H or J fuses and has apparatus that you can buy to put in R type fuses... which is necesary?
Further, for this outside residential service disconnect, is there any reason not to put a 200A main breaker as opposed to a fused disconnect?
OK, youse guys are talking way over my head... I am inquireing for the fuses installed in a residential sevice disconnect...
The disconnect lit says it accepts H or J fuses and has apparatus that you can buy to put in R type fuses... which is necesary?
Further, for this outside residential service disconnect, is there any reason not to put a 200A main breaker as opposed to a fused disconnect?
Re: RMS short circuit rating of fuses
Class H fuses I believe have a short circuit current interrupting rating of 10kA, I think the J's and K's are between 50k to 300k depending on the fuse.
Most residential Main circuit breakers I am familiar with are rate for a ISCA of about 22k. Check with the manufacture. It would be rae for a single phase dwelling service ( In Mass. anyway) to exceed 5 to 10kA at the weather head.
One town near the Cape had all the pole mount transformeres changed out with low Z ratings. The POCO advised the inspection department that certain networks could provide up to 42kA at the service point. All the installed services needed Fused disconnects with fuses having an interrupting rating upwards of 50kA.
You can get available short ratings from your POCO and then determine what you need for service equipment.
Charlie
Class H fuses I believe have a short circuit current interrupting rating of 10kA, I think the J's and K's are between 50k to 300k depending on the fuse.
Most residential Main circuit breakers I am familiar with are rate for a ISCA of about 22k. Check with the manufacture. It would be rae for a single phase dwelling service ( In Mass. anyway) to exceed 5 to 10kA at the weather head.
One town near the Cape had all the pole mount transformeres changed out with low Z ratings. The POCO advised the inspection department that certain networks could provide up to 42kA at the service point. All the installed services needed Fused disconnects with fuses having an interrupting rating upwards of 50kA.
You can get available short ratings from your POCO and then determine what you need for service equipment.
Charlie
Re: RMS short circuit rating of fuses
I'm not sure you can buy Class H (someone will respond)
If you are close to the transformer and not familiar with the supply network I strongly suggest an inquiry. It's probably not a problem but you should have an idea of what the system can deliver.
Circuit breakers or fuses either should be Code compliant.
Is this a new service or an upgrade.
This link will provide a lot of information regarding fuses.
[Edit for revised info,] I take it back Bussmans NON and NOS appears to be a Class H fuse. 10,000A interrupt and generally considered as non time delay.
Charlie
[ February 15, 2005, 12:09 PM: Message edited by: cpal ]
I'm not sure you can buy Class H (someone will respond)
If you are close to the transformer and not familiar with the supply network I strongly suggest an inquiry. It's probably not a problem but you should have an idea of what the system can deliver.
Circuit breakers or fuses either should be Code compliant.
Is this a new service or an upgrade.
This link will provide a lot of information regarding fuses.
[Edit for revised info,] I take it back Bussmans NON and NOS appears to be a Class H fuse. 10,000A interrupt and generally considered as non time delay.
Charlie
[ February 15, 2005, 12:09 PM: Message edited by: cpal ]
Re: RMS short circuit rating of fuses
Someone said I should use a 200A main breaker disconnect outside to save money... what is the reason for Breaker vs. fused disconnect at service interupting means? From what Iread I see no reason for anything greater than the 10000 rms rating of the class H fuses, but am not up to date on this topic. Please advise further.
An upgrade service combining several variously located subpanels into one 200A service. The pole mounted utility transformer is a minimum of 100 feet away. I am putting the 200A disconnecting means on the outside of the house next to the meter because the SE feed needs to go through a crawl space to the new 200A main breaker panel inside.Originally posted by cpal:
I'm not sure you can buy Class H (someone will respond)
If you are close to the transformer and not familiar with the supply network I strongly suggest an inquiry. It's probably not a problem but you should have an idea of what the system can deliver.
Circuit breakers or fuses either should be Code compliant.
Is this a new service or an upgrade.
This link will provide a lot of information regarding fuses.
[Edit for revised info,] I take it back Bussmans NON and NOS appears to be a Class H fuse. 10,000A interrupt and generally considered as non time delay.
Charlie
Someone said I should use a 200A main breaker disconnect outside to save money... what is the reason for Breaker vs. fused disconnect at service interupting means? From what Iread I see no reason for anything greater than the 10000 rms rating of the class H fuses, but am not up to date on this topic. Please advise further.
Re: RMS short circuit rating of fuses
Ed MacLaren, I agree with you with regard to the simple problem of inertia. However, this problem has been addressed in the design of the contacts. Most are inclines to visualize a moving contact being held simply against a stationary contact as commonly used. When called to open the moving contact simply moves up through the arc chutes which deionizes the arc gas and clears the arc.
But contact design has evolved to the point where the actual magnetic field caused by the current force the contacts apart, essentially being blown apart. These method often include the way the stationary contact is designed. Current limiting breakers commonly employ a pivoting lower contact that is allowed to move in the opposite direction as that of the upper moving contact. And then there is the "slot motor" design developed some years ago by Westinghouse where the upper and lower contacts move within a 'U' shaped slot of laminated steel.
Charlie, When referring to the 1/4 cycle clearing time at what current levels are you referring to? The question is if the fuse clearing time will be consistent with low level arcing faults as on would expect when motor winding failures occur as compared to a bolted fault. A breaker will be ?consistent when called to trip. The fuse may be fast in clearing bolted faults but what about those low level arcing faults?
Yes, I did error in overlooking the fact of the additional cycle time that it takes to unlatch and open the breaker.
friebel, What advantages did you end up with by refusing the entire plant rather than using properly set MCPs? Are you implying that the fuse protection will further reduce motor damage when a motor is in a failure mode if that was the intent?
Also, how does one address the cost of stocking spare fuses and having them handy for a qualified person to install them.
And what about single phasing the motor? Does one now have to add single phase protection or rely of the OLR for single phase protection?
Ed MacLaren, I agree with you with regard to the simple problem of inertia. However, this problem has been addressed in the design of the contacts. Most are inclines to visualize a moving contact being held simply against a stationary contact as commonly used. When called to open the moving contact simply moves up through the arc chutes which deionizes the arc gas and clears the arc.
But contact design has evolved to the point where the actual magnetic field caused by the current force the contacts apart, essentially being blown apart. These method often include the way the stationary contact is designed. Current limiting breakers commonly employ a pivoting lower contact that is allowed to move in the opposite direction as that of the upper moving contact. And then there is the "slot motor" design developed some years ago by Westinghouse where the upper and lower contacts move within a 'U' shaped slot of laminated steel.
Charlie, When referring to the 1/4 cycle clearing time at what current levels are you referring to? The question is if the fuse clearing time will be consistent with low level arcing faults as on would expect when motor winding failures occur as compared to a bolted fault. A breaker will be ?consistent when called to trip. The fuse may be fast in clearing bolted faults but what about those low level arcing faults?
Yes, I did error in overlooking the fact of the additional cycle time that it takes to unlatch and open the breaker.
friebel, What advantages did you end up with by refusing the entire plant rather than using properly set MCPs? Are you implying that the fuse protection will further reduce motor damage when a motor is in a failure mode if that was the intent?
Also, how does one address the cost of stocking spare fuses and having them handy for a qualified person to install them.
And what about single phasing the motor? Does one now have to add single phase protection or rely of the OLR for single phase protection?
Re: RMS short circuit rating of fuses
templdl
My discussion would lean towards worst possible scenario. Time trip characteristic curves and let thru charts are available when actual fault levels are know. That being said I should have qualified my responses indicating so!!
ddderek
I see no disadvantage to installing a breaker. It will work good and last a long time. It is also convenient for those rare occasions when it may tip,
A Question ???
Are you a homeowner or are you in the business.
Charlie
templdl
My discussion would lean towards worst possible scenario. Time trip characteristic curves and let thru charts are available when actual fault levels are know. That being said I should have qualified my responses indicating so!!
ddderek
I see no disadvantage to installing a breaker. It will work good and last a long time. It is also convenient for those rare occasions when it may tip,
A Question ???
Are you a homeowner or are you in the business.
Charlie
Re: RMS short circuit rating of fuses
[ February 15, 2005, 01:47 PM: Message edited by: ddderek ]
A homeowner, and licensed Electrician. I do mostly Commercial and Industrial but that has been slow and I now have time to do service changes for the family members that have been after me to do so.A Question ???
Are you a homeowner or are you in the business.
Charlie
[ February 15, 2005, 01:47 PM: Message edited by: ddderek ]
Re: RMS short circuit rating of fuses
Just a few reasons why I would be in favor of fuses at this service disco:
-Have come across a few really old main breakers that once turned off, will not hold closed once you turn them back on. Resulted in having to do a pnael upgrade since those old mains are hard/expensive to get
-Fuses are "failsafe", ie they WILL with out a doubt trip on OL or short circuit, while a circuit breaker is not "guaranteed" to trip. Now, today's CB's are very well designed, but again, have run across some old CB's in residences that had not been moved since they were installed and seemed like they would not blow open on a trip.
-if your 200A main on your house tripped/blew, do you just want the HO to go out there and simply reset it? Usually, something is wrong if it blows. Better to have them call an electrician (since they can't buy the fuses at Home Depot) to check out the problem. Closing a breaker on a bolted fault might damage / de-sensitize the breaker, not to mention burn off one's knuckle hair!
-It's not that expensive to put fuses rated 100 or 200 kaic into the disconnect. You wuld KNOW that you are covered as far as available short circuit from the utility.
Not saying CB's are bad, use them all the time, but fuses are still a viable option these days. I found little to no cost difference when I priced out 200A meter mains fused vs CB, not sure on a disco vs a CB+Enclosure.
Oh, and fuse types:
Class R kit rejects all but Class R fuses (so you are sure that they are replaced with a high AIC rating
Class H is the standard that most are set up for, around 10kAIC
Class J sets you up for 100,000 or 200,000 AIC as well, depending on who makes the switch, and size of switch.
That's for square D, YMMV
Good Luck
Mike
[ February 15, 2005, 03:02 PM: Message edited by: mhulbert ]
Just a few reasons why I would be in favor of fuses at this service disco:
-Have come across a few really old main breakers that once turned off, will not hold closed once you turn them back on. Resulted in having to do a pnael upgrade since those old mains are hard/expensive to get
-Fuses are "failsafe", ie they WILL with out a doubt trip on OL or short circuit, while a circuit breaker is not "guaranteed" to trip. Now, today's CB's are very well designed, but again, have run across some old CB's in residences that had not been moved since they were installed and seemed like they would not blow open on a trip.
-if your 200A main on your house tripped/blew, do you just want the HO to go out there and simply reset it? Usually, something is wrong if it blows. Better to have them call an electrician (since they can't buy the fuses at Home Depot) to check out the problem. Closing a breaker on a bolted fault might damage / de-sensitize the breaker, not to mention burn off one's knuckle hair!
-It's not that expensive to put fuses rated 100 or 200 kaic into the disconnect. You wuld KNOW that you are covered as far as available short circuit from the utility.
Not saying CB's are bad, use them all the time, but fuses are still a viable option these days. I found little to no cost difference when I priced out 200A meter mains fused vs CB, not sure on a disco vs a CB+Enclosure.
Oh, and fuse types:
Class R kit rejects all but Class R fuses (so you are sure that they are replaced with a high AIC rating
Class H is the standard that most are set up for, around 10kAIC
Class J sets you up for 100,000 or 200,000 AIC as well, depending on who makes the switch, and size of switch.
That's for square D, YMMV
Good Luck
Mike
[ February 15, 2005, 03:02 PM: Message edited by: mhulbert ]
Re: RMS short circuit rating of fuses
Mike,
I understand what you are describing regarding breakers. It is not uncommon to have panel located in a cool damp basement for years without being even opened. Yes, it is best to exercise breakers to distribute the lubrication and the contacts actually rub against one another to provide better contact which is seldom done.
But I've also experienced fuse holder jaws of a main fused pullout becoming so week and corroded that the end of the fuse gets hotter than a pistol presenting a fire hazard. So it may not be the fuse which you have confidence in as being reliable but the fuse holder that can fail.
Also, yes, fuses are an economical solution when one must address kaic issues. But if one believed that using a higher kaic OCPD, either breaker or fuse is better, doesn't gain a thing.
Anyway, all that stuff is based one the worst case scenario bolted fault which is a rare event at best.
Mike,
I understand what you are describing regarding breakers. It is not uncommon to have panel located in a cool damp basement for years without being even opened. Yes, it is best to exercise breakers to distribute the lubrication and the contacts actually rub against one another to provide better contact which is seldom done.
But I've also experienced fuse holder jaws of a main fused pullout becoming so week and corroded that the end of the fuse gets hotter than a pistol presenting a fire hazard. So it may not be the fuse which you have confidence in as being reliable but the fuse holder that can fail.
Also, yes, fuses are an economical solution when one must address kaic issues. But if one believed that using a higher kaic OCPD, either breaker or fuse is better, doesn't gain a thing.
Anyway, all that stuff is based one the worst case scenario bolted fault which is a rare event at best.
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