AFC/AIC

[michael king]

This is one of many references you might find interesting:

https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/technical-literature/bus-ele-br-10757-fuseology.pdf

AIC ignored = View attachment 2563430

That's awesome thank you. Which answers one of my early questions about the ratings, and in that we are trying to prevent arc flash, etc. Thanks again, and have a happy new year.

 

[electrofelon]

Okay, so now I'm seeing the picture. I couldn't wrap my brain around this for some reason. The AIC is the amount of fault current before failure of the device in the event of like a racoon or something. Not like it's really related to a breaker tripping under a heavy load, but the device itself being able to withstand a fault. Awesome!!

Remember the fault has to be downstream of the device for the device to see the fault current. An animal up on the transformer causing some sort of fault or damage wouldn't result in your main breaker seeing a fault.

A breaker that is underrated could catastrophically fail and explode, or have the contacts weld shut and not open the fault at all.

It sounds like you got the fault current down to 22k which the transfer switch is likely rated for, however note the branch breakers need to be rated for the fault current as well hopefully there's a seriea rating between them

 

[michael king]

Remember the fault has to be downstream of the device for the device to see the fault current. An animal up on the transformer causing some sort of fault or damage wouldn't result in your main breaker seeing a fault.

A breaker that is underrated could catastrophically fail and explode, or have the contacts weld shut and not open the fault at all.

It sounds like you got the fault current down to 22k which the transfer switch is likely rated for, however note the branch breakers need to be rated for the fault current as well hopefully there's a seriea rating between them

I went back and verified the GE panel has a 22kva rating, and the breakers as well. I confirmed with the utility company that the AFC they gave me was at the transformer, they have 3/0 AL conductor, and approximately 80 feet from transformer to meter can. Entered all this into the spreadsheet from this website for a new AFC of 21,007 amperes. The more input and feedback I'm getting, the more I'm learning and things are clearing up. The wrapping my brain around this is about there, because I couldn't see a transformer fault causing service device issues. It's the amount of potential current that is provided by the power company that can flow to/through the house at the time of a fault (shorted motor winding, etc), that a breaker can safely interrupt without that breaker failing to do so. Much appreciate all the help once again.

 

[suemarkp]

Perhaps one detail you are missing is circuit breakers dont trip instantly. Even on a full short (bolted fault) it may take one cycle of the 60hz wave to trip. One cycle is fast, but those available fault amps can flow instantly and blow things up before and while the breaker is tripping.

Putting additional fusing upstream doesnt change things, they all take time to trip.

 

[jim dungar]

Perhaps one detail you are missing is circuit breakers dont trip instantly. Even on a full short (bolted fault) it may take one cycle of the 60hz wave to trip. One cycle is fast, but those available fault amps can flow instantly and blow things up before and while the breaker is tripping.

Putting additional fusing upstream doesnt change things, they all take time to trip.

Actually many 'residential style' molded cars breakers begin to trip in under 1 cycle which is why series ratings, even with fuses, need to be tested rather than chosen from tables or charts.
When a breaker begins to open, dynamic resistance from arcing may cause a reduction in current resulting in the upstream device not entering its current limiting range resulting in the downstream device interrupting the fault.

 

[kwired]

I went back and verified the GE panel has a 22kva rating, and the breakers as well. I confirmed with the utility company that the AFC they gave me was at the transformer, they have 3/0 AL conductor, and approximately 80 feet from transformer to meter can. Entered all this into the spreadsheet from this website for a new AFC of 21,007 amperes. The more input and feedback I'm getting, the more I'm learning and things are clearing up. The wrapping my brain around this is about there, because I couldn't see a transformer fault causing service device issues. It's the amount of potential current that is provided by the power company that can flow to/through the house at the time of a fault (shorted motor winding, etc), that a breaker can safely interrupt without that breaker failing to do so. Much appreciate all the help once again.

Looks like you are starting to grasp what the concepts are.

Note that it doesn't take all that much conductor length to create enough impedance to make a significant change in the AFC at the load end of the conductor. Where you need to pay closest attention is when the point you are interested in is pretty close to the source output.

Otherwise often times your service devices do have higher rating than your branch devices. In typical load centers, those branch devices are normally series rated with the main. If you have a service device, then a feeder to a sub panel, resistance of the feeder conductors does reduce the AFC at the feeder panel. You could be near the 22kA rating of the service breaker but a branch breaker at a distant sub panel might easily be seeing under 10kA AFC at it's location.

Arc flash is a different ball game though. If you are interested in determining incident energy, a high AFC doesn't automatically mean high incident energy. Incident energy has a time element involved in the outcome. Lower fault current in some cases might mean longer clearing time of the OCPD. That longer time at a lower but still pretty high current might mean more energy was released in the event than a higher current but for lesser time. Burns might be more severe if someone were in close proximity to the lower current than they would have in the higher current situation.

 

[ajb5102]

110.9 and 110.10 have been in the NEC for more than 50 years.

Available Fault Current needs to be calculated at the point where your device is. The length an size of the service conductors has a tremendous effect and so should be included in your calculations.

Every device which closes onto a fault and then tries to open, like a breaker, needs to be have an Amps Interrupting Capacity based on the fault current at its line side terminal, per 110.9.
Every device that does not interrupt a fault, like a manual switch, needs to be able to tolerate the current flowing through it per 110.10.

Many service entrance ATS are built using circuit breaker and so need to meet 110.9.

Trying to understand fault current phenomena and series ratings... why the is the fault calculated at the line side of a device? I thought faults only flow back to the transformer? so if we have a fault of 22k and the line side of a Loadcenter, or panel, do I need a main breaker to handle the 22k? I thought no, and the main breaker can be upstream, to handle the 22k fault. ( Not sure what other logic i am not understanding, so please pile on)

 

[kwired]

Trying to understand fault current phenomena and series ratings... why the is the fault calculated at the line side of a device? I thought faults only flow back to the transformer? so if we have a fault of 22k and the line side of a Loadcenter, or panel, do I need a main breaker to handle the 22k? I thought no, and the main breaker can be upstream, to handle the 22k fault. ( Not sure what other logic i am not understanding, so please pile on)

Current is the same at all points in a particular path, but (potentially) available current at the transformer terminals is higher than at the service disconnect because of resistance in the conductors between those two points.

For simple calculations we generally ignore resistance of conductors as it is often negligible for our intended purposes. But just an ohm or two can be significant when the current level is high to begin with like with a solid line to line or line to ground fault.

 

[electrofelon]

Trying to understand fault current phenomena and series ratings... why the is the fault calculated at the line side of a device? I thought faults only flow back to the transformer? so if we have a fault of 22k and the line side of a Loadcenter, or panel, do I need a main breaker to handle the 22k? I thought no, and the main breaker can be upstream, to handle the 22k fault. ( Not sure what other logic i am not understanding, so please pile on)

For AIC purposes, you assume the worst case which is a bolted fault right AFTER (load side) of the OCPD. If the fault was on the line side, the OCPD would not see it. Every OCPD needs to meet this requirement, but the available fault current is going to change has you add conductor length from the source. Series ratings may allow an OCPD with a lower AIC than AFC because they have been tested to work.

 

[ajb5102]

For AIC purposes, you assume the worst case which is a bolted fault right AFTER (load side) of the OCPD. If the fault was on the line side, the OCPD would not see it. Every OCPD needs to meet this requirement, but the available fault current is going to change has you add conductor length from the source. Series ratings may allow an OCPD with a lower AIC than AFC because they have been tested to work.

Thank you.

So, if i have panel A and another higher rated panel B upstream, why would a drawing show an AFC on the line side of Panel A...? shouldn't they be calculating the AFC at the load side of Panel B?

 

[electrofelon]

Thank you.

So, if i have panel A and another higher rated panel B upstream, why would a drawing show an AFC on the line side of Panel A...? shouldn't they be calculating the AFC at the load side of Panel B?

Yeah, but really a few feet of conductor or bussing isn't going to make much difference. Also they might want to show the AFC before the panel for SCCR purposes (but that is somewhat academic as it is unlikely that these days you would have a panel SCCR rating lower than the AIC of a breaker it can accept)

 

[d0nut]

Thank you.

So, if i have panel A and another higher rated panel B upstream, why would a drawing show an AFC on the line side of Panel A...? shouldn't they be calculating the AFC at the load side of Panel B?

The drawing shows the fault current on the line side of the downstream panel because that location takes into account the conductors between the upstream and downstream panel. If you used the value on the load side of the upstream panel (ahead of the feeder conductors) that value could be overly conservative for the downstream panel.

 

[ajb5102]

The drawing shows the fault current on the line side of the downstream panel because that location takes into account the conductors between the upstream and downstream panel. If you used the value on the load side of the upstream panel (ahead of the feeder conductors) that value could be overly conservative for the downstream panel.

thank you... your last statement (If you used the value on the load side of the upstream panel (ahead of the feeder conductors) that value could be overly conservative for the downstream panel.) is what I do not understand yet... this downstream panel won't see any faults that are on its line side, right? And in this case, a fault between panel A (downstream) and panel B (upstream); will result in Panel B seeing the fault and panel A will see none of the fault.

 

[jim dungar]

thank you... your last statement (If you used the value on the load side of the upstream panel (ahead of the feeder conductors) that value could be overly conservative for the downstream panel.) is what I do not understand yet... this downstream panel won't see any faults that are on its line side, right? And in this case, a fault between panel A (downstream) and panel B (upstream); will result in Panel B seeing the fault and panel A will see none of the fault.

We use the available fault current at the line side terminal because it can be calculated easily. For all intents the internal impedance of a closed protective device is fairly insignificant so we consider its load side current to be equal to its line side current. This allows us to select the AIC of protective device as if a fault occurred directly on its load side lugs.

 

[ajb5102]

We use the available fault current at the line side terminal because it can be calculated easily. For all intents the internal impedance of a closed protective device is fairly insignificant so we consider its load side current to be equal to its line side current. This allows us to select the AIC of protective device as if a fault occurred directly on its load side lugs.

ok - simple enough.... onto series ratings.... If Device U (upstream) is 22k Rated Breaker and enclosure, and Device D (downstream) is a MLO load center, 10k rated branch breakers and enclosure... and there is an AFC on the line side terminals of Device D (MLO LC) ... do i need to add a main breaker to Device D to handle 22k AFC, or will Device U protect the entire system form the AFC?

 

[jim dungar]

ok - simple enough.... onto series ratings.... If Device U (upstream) is 22k Rated Breaker and enclosure, and Device D (downstream) is a MLO load center, 10k rated branch breakers and enclosure... and there is an AFC on the line side terminals of Device D (MLO LC) ... do i need to add a main breaker to Device D to handle 22k AFC, or will Device U protect the entire system form the AFC?

Typically series rating do not care if there are conductors between the upstream and downstream devices. They do care if there are in between protective devices.

 

[electrofelon]

Typically series rating do not care if there are conductors between the upstream and downstream devices. They do care if there are in between protective devices.

Eaton does state the following:

■ Any FULLY RATED breaker can be applied upstream,
downstream, or in the middle of any of the series ratings
stated in the tables

 

[ajb5102]

Eaton does state the following:

■ Any FULLY RATED breaker can be applied upstream,
downstream, or in the middle of any of the series ratings
stated in the tables

Which makes sense. The additional breaker would only improve the performance of the already proven to be acceptable series rating combination. Can you share the literature that states this is true?

 

[electrofelon]

Which makes sense. The additional breaker would only improve the performance of the already proven to be acceptable series rating combination. Can you share the literature that states this is true?

Panelboard and Switchboard series rating information manual - Eaton

https://www.eaton.com/content/dam/eaton/products/low-voltage-power-distribution-controls-systems/panelboards/panelboard-and-switchboard-series-rating-information-manual-1c96944h02.pdf

 

[bmw270372]

I went back and verified the GE panel has a 22kva rating, and the breakers as well. I confirmed with the utility company that the AFC they gave me was at the transformer, they have 3/0 AL conductor, and approximately 80 feet from transformer to meter can. Entered all this into the spreadsheet from this website for a new AFC of 21,007 amperes. The more input and feedback I'm getting, the more I'm learning and things are clearing up. The wrapping my brain around this is about there, because I couldn't see a transformer fault causing service device issues. It's the amount of potential current that is provided by the power company that can flow to/through the house at the time of a fault (shorted motor winding, etc), that a breaker can safely interrupt without that breaker failing to do so. Much appreciate all the help once again.

where can I find this table? I have a similar problem. please share your calculations.