I need alot of iformation regarding this paticular subject for a school report,anything anyone has will help. What i need to know is What it is, why it is done,where it is applied[residential,commertial]some examples would help, also please try explaining it as simple as possible for i am in my second year of school and get confused by the big words very easy. THANK YOU IN ADVANCE FOR WHO EVER GETS BACK TO ME, THE HELP IS VERY APPRECIATED! I was told to look under article[240.12] that might help a little.
selective coordination
- Thread starter korneddie
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bphgravity
Senior Member
- Location
- Florida
Re: selective coordination
I don't know if I can give you ALOT of information on this subject, but I will certainly provide a basic understanding of the idea behind system shutdown coordination.
Most overcurrent protection devices (breakers and fuses) are intended for immediate shutdown of the electrical circuit when a ground-fault or short circuit condition becomes present. In some applications, this action may do more harm to persons and property than the fault that exists.
In these situations, the code allows for monitoring and alarm systems to be used for fault notification so equipment can be shut down properly before the circuit is deenergized and repaired. These are usually industrial conditions where supervision and maintanence is assured. Good luck with your research!
I don't know if I can give you ALOT of information on this subject, but I will certainly provide a basic understanding of the idea behind system shutdown coordination.
Most overcurrent protection devices (breakers and fuses) are intended for immediate shutdown of the electrical circuit when a ground-fault or short circuit condition becomes present. In some applications, this action may do more harm to persons and property than the fault that exists.
In these situations, the code allows for monitoring and alarm systems to be used for fault notification so equipment can be shut down properly before the circuit is deenergized and repaired. These are usually industrial conditions where supervision and maintanence is assured. Good luck with your research!
Ed MacLaren
Senior Member
Re: selective coordination
Many larger (non-residential) installations have at least three "levels" of overcurrent protection, for the main service, the feeders, and the branch circuits.
In the event of a short circuit or ground fault in a branch circuit, if the system is not selectively coordinated, all three overcurrent devices, the main fuse(X), the feeder fuse (Y) and the branch circuit breaker (Z) could open, shutting down the entire system.
With a selectively coordinated system, only the branch circuit breaker (Z) will open, leaving the remainder of the system in operation.
Ed
Many larger (non-residential) installations have at least three "levels" of overcurrent protection, for the main service, the feeders, and the branch circuits.
In the event of a short circuit or ground fault in a branch circuit, if the system is not selectively coordinated, all three overcurrent devices, the main fuse(X), the feeder fuse (Y) and the branch circuit breaker (Z) could open, shutting down the entire system.
With a selectively coordinated system, only the branch circuit breaker (Z) will open, leaving the remainder of the system in operation.
Ed
Re: selective coordination
Thanks alot for the responce Bryon, your info definitly helps my understanding of at least one of the subjects ive read about, being that this procedure is placed in locations such as hospitals to prevent power loss to other areas where it may affect a patient or some type of equipment. If you or anybody else has some info on where to read up on the subject that would really help THANKS AGAIN MAN.
Thanks alot for the responce Bryon, your info definitly helps my understanding of at least one of the subjects ive read about, being that this procedure is placed in locations such as hospitals to prevent power loss to other areas where it may affect a patient or some type of equipment. If you or anybody else has some info on where to read up on the subject that would really help THANKS AGAIN MAN.
Re: selective coordination
.THANKS ALOT FOR THE REPLY ED,the pictures really help abunch. So what I see is the diagram you posted is of a uncoordinated system with a fault that opened all three over current protection devices. my question now is what makes this system coordinated. Is it as simple as adding a fused disconnect or something like that to this particular motor, also i dont understand why the circuit breaker [z] doesnt just trip and isolate this paticular fault to [panel b], Is it because the afc. How will a coordinated system change this, what is added, what is done diffrently.
.THANKS ALOT FOR THE REPLY ED,the pictures really help abunch. So what I see is the diagram you posted is of a uncoordinated system with a fault that opened all three over current protection devices. my question now is what makes this system coordinated. Is it as simple as adding a fused disconnect or something like that to this particular motor, also i dont understand why the circuit breaker [z] doesnt just trip and isolate this paticular fault to [panel b], Is it because the afc. How will a coordinated system change this, what is added, what is done diffrently.
Ed MacLaren
Senior Member
Re: selective coordination
Engineers coordinate overcurrent devices by comparing their time-current curves.
The sketch below illustrates the melting and clearing times for two fuses, let's say for example, the two fuses X and Y in my earlier sketch (above).
In order for them to coordinate, Fuse Y must have cleared the fault before Fuse X melts (dotted line in sketch). Keep in mind that they are both carrying the same fault current.
Ed
The time that each overcurrent protective device takes to clear a fault will determine which one opens first.
Engineers coordinate overcurrent devices by comparing their time-current curves.
The sketch below illustrates the melting and clearing times for two fuses, let's say for example, the two fuses X and Y in my earlier sketch (above).
In order for them to coordinate, Fuse Y must have cleared the fault before Fuse X melts (dotted line in sketch). Keep in mind that they are both carrying the same fault current.
Ed
- Location
- Massachusetts
Re: selective coordination
I have been the victim of un-coordinated overcurrent protection.
277 volt 20 amp lighting circuit with a short circuit took out a 2000 amp breaker feeding a bus duct.
Power outage on half of 4 floors, most lighting and all general purpose power.
It was not a good day!
I have been the victim of un-coordinated overcurrent protection.
277 volt 20 amp lighting circuit with a short circuit took out a 2000 amp breaker feeding a bus duct.
Power outage on half of 4 floors, most lighting and all general purpose power.
It was not a good day!
- Location
- Illinois
- Occupation
- retired electrician
Re: selective coordination
Bob,
That is somewhat unusual with that much difference in size between the branch and main breakers. Is it possible that the GFP setting on the main was too low? I have seen a bad ballast take out the main on ground fault when the GFP setting was too low.
However, it is almost impossible to coordinate standard breakers between 15 and 200 amps for short circuits and ground fault trips. The time/current trip curves are almost identical.
Don
I reread your post and I see that you said the branch circuit fault was a short circuit, so the GFP in the main should not be involved.
[ May 05, 2003, 09:14 PM: Message edited by: don_resqcapt19 ]
Bob,
That is somewhat unusual with that much difference in size between the branch and main breakers. Is it possible that the GFP setting on the main was too low? I have seen a bad ballast take out the main on ground fault when the GFP setting was too low.
However, it is almost impossible to coordinate standard breakers between 15 and 200 amps for short circuits and ground fault trips. The time/current trip curves are almost identical.
Don
I reread your post and I see that you said the branch circuit fault was a short circuit, so the GFP in the main should not be involved.
[ May 05, 2003, 09:14 PM: Message edited by: don_resqcapt19 ]
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: selective coordination
iwire,
Out of curiosity, is possible that the short circuit available at the 20A breaker was in excess of 20K, which is within most instantaneous characteristics of a 2000A breaker (10xtrip). In most cases, a short of this nature, will unlatch the 2000A breaker, and even if the 20A should clear it, the 2000A will complete the opening process anyway.
I am assuming that the 2000A breaker has an instantaneous characteristic.
[ May 05, 2003, 09:26 PM: Message edited by: ron ]
iwire,
Out of curiosity, is possible that the short circuit available at the 20A breaker was in excess of 20K, which is within most instantaneous characteristics of a 2000A breaker (10xtrip). In most cases, a short of this nature, will unlatch the 2000A breaker, and even if the 20A should clear it, the 2000A will complete the opening process anyway.
I am assuming that the 2000A breaker has an instantaneous characteristic.
[ May 05, 2003, 09:26 PM: Message edited by: ron ]
- Location
- Massachusetts
Re: selective coordination
I tell you what I can remember of this.
The short was caused by an apprentice wiring a 12/2 drop to a switch as it was an outlet. Still my bad judgment for not checking his work before energizing circuit.
I did get lucky in the fact building services told the other affected tenants it was a utility problem.
Bob
[ May 06, 2003, 04:46 AM: Message edited by: iwire ]
I tell you what I can remember of this.
The short was caused by an apprentice wiring a 12/2 drop to a switch as it was an outlet. Still my bad judgment for not checking his work before energizing circuit.
I did get lucky in the fact building services told the other affected tenants it was a utility problem.
Bob
[ May 06, 2003, 04:46 AM: Message edited by: iwire ]
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: selective coordination
I'll come from the other angle. The only CYA (or code) issue is the long time pickup of the breaker being set at 1, assuming the trip unit is the actual value needed. All other settings do not comprimise the code compliance of the installation, if set at the highest value, it just reduces the possible false trips until an actual "real" setting is made.
So, I think other than the long time pickup, all other values be set at max until a "real" value is determined. This especially applies to the instantaneous setting for breakers serving transformers or motors. Be sure to adjust them at least higher than the starting inrush, if not higher.
When the values are realy determined it will help with the selective coordination of the system. A coordination study does not always result in selective coordination, especially in the instantaneous regions.
I'll come from the other angle. The only CYA (or code) issue is the long time pickup of the breaker being set at 1, assuming the trip unit is the actual value needed. All other settings do not comprimise the code compliance of the installation, if set at the highest value, it just reduces the possible false trips until an actual "real" setting is made.
So, I think other than the long time pickup, all other values be set at max until a "real" value is determined. This especially applies to the instantaneous setting for breakers serving transformers or motors. Be sure to adjust them at least higher than the starting inrush, if not higher.
When the values are realy determined it will help with the selective coordination of the system. A coordination study does not always result in selective coordination, especially in the instantaneous regions.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: selective coordination
and, I don't like fuses, other than they are cheaper to install as compared to breakers. To have one blow, then scramble for a replacement is a headache for facilities folks. In addition, if there is any conflict between the characteristics (time current curves) of the fuse, you are out of luck, no adjustments.
Fuses don't give you the oportunity to provide tigher protection when wanted or other protection for special circumstances, like reverse power, differential protection, under/over voltage, etc, etc. You could theoretically shunt trip a fused switch with standalone relays, but not always successful.
By tighter protection I mean protection closer to the characteristics of the load (to better protect the equipment) by adjusting the breakers (can't do it with fuses) with good knowledge of a predictable load.
and, I don't like fuses, other than they are cheaper to install as compared to breakers. To have one blow, then scramble for a replacement is a headache for facilities folks. In addition, if there is any conflict between the characteristics (time current curves) of the fuse, you are out of luck, no adjustments.
Fuses don't give you the oportunity to provide tigher protection when wanted or other protection for special circumstances, like reverse power, differential protection, under/over voltage, etc, etc. You could theoretically shunt trip a fused switch with standalone relays, but not always successful.
By tighter protection I mean protection closer to the characteristics of the load (to better protect the equipment) by adjusting the breakers (can't do it with fuses) with good knowledge of a predictable load.
jtb
Senior Member
- Location
- Pennsylvania
Re: selective coordination
Quick references are available online.
Google for Cooper Bussman.
Or try this link....
http://www.bussmann.com/apen/pubs/
You will see alot of good stuff here.
Quick references are available online.
Google for Cooper Bussman.
Or try this link....
http://www.bussmann.com/apen/pubs/
You will see alot of good stuff here.
Re: selective coordination
Bob,
This happened to me too. (sort of) I was a first year apprentice working on a re model of an insurance companies regional offices. A framer cut through a conduit with a 277V lighting circuit in it. It tripped a 3000A main and took out power to half of a 400,000 sq ft building. (Another service was on the other end of the site) In this case it was the ground fault protection that was set to low.
One thing to note about ground fault settings. When the testing company comes out to test your main breaker, [to comply with 230.95(c)] if you don?t have the settings from the engineer they can only test it ?as found.? Any modifications to the settings after that technically void the test.
Bob,
This happened to me too. (sort of) I was a first year apprentice working on a re model of an insurance companies regional offices. A framer cut through a conduit with a 277V lighting circuit in it. It tripped a 3000A main and took out power to half of a 400,000 sq ft building. (Another service was on the other end of the site) In this case it was the ground fault protection that was set to low.
One thing to note about ground fault settings. When the testing company comes out to test your main breaker, [to comply with 230.95(c)] if you don?t have the settings from the engineer they can only test it ?as found.? Any modifications to the settings after that technically void the test.
Re: selective coordination
If you want to learn more about selective coordination or any other overcurrent protection concept, perhaps the Bussmann SPD can help. The website address for this document is:
http://www.bussmann.com/apen/pubs/spd/
For selective coordination, see:
http://www.bussmann.com/library/docs/spd02/SPDSection08.pdf
In regards to selective coordination, it is relatively easy with fuses, just follow the ratio guide given by the fuse manufacture to achieve selective coordination. This is valid for ALL overcurrent conditions - overloads or short-circuits.
With molded case circuit breakers, it can be a little more involved, but basically, you just need to know the fault current and the instantaneous trip. If the fault level is about 10X or more than the upstream circuit breaker, you could have a problem with coordination because the upstream breaker could trip before the downstream device clears the fault. This is especially true if the downstream device is a molded case circuit breaker.
Ron, has brought up some other protection considerations as well in regards to fuse protection. In some special cases this may be true, but 80% or more of the applications are going to be a standard molded case circuit breaker or a fused disconnect switch. In this case, these "extra bells and whistles" are not used. What you sacrifice for not wanting to "change fuses" is often better protection and coordination of systems, components, and possibly people.
If you want to learn more about selective coordination or any other overcurrent protection concept, perhaps the Bussmann SPD can help. The website address for this document is:
http://www.bussmann.com/apen/pubs/spd/
For selective coordination, see:
http://www.bussmann.com/library/docs/spd02/SPDSection08.pdf
In regards to selective coordination, it is relatively easy with fuses, just follow the ratio guide given by the fuse manufacture to achieve selective coordination. This is valid for ALL overcurrent conditions - overloads or short-circuits.
With molded case circuit breakers, it can be a little more involved, but basically, you just need to know the fault current and the instantaneous trip. If the fault level is about 10X or more than the upstream circuit breaker, you could have a problem with coordination because the upstream breaker could trip before the downstream device clears the fault. This is especially true if the downstream device is a molded case circuit breaker.
Ron, has brought up some other protection considerations as well in regards to fuse protection. In some special cases this may be true, but 80% or more of the applications are going to be a standard molded case circuit breaker or a fused disconnect switch. In this case, these "extra bells and whistles" are not used. What you sacrifice for not wanting to "change fuses" is often better protection and coordination of systems, components, and possibly people.
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