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 Post subject: IE of over-dutied equipment / devices
PostPosted: Fri Feb 10, 2017 6:39 am 
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All - I'm wading through a recently completed "arc flash survey" and my lack of knowledge combined with desire to understand things is making me crazy. :D

Per this report we have a lot of equipment/devices that are over-dutied. After they list the equipment as over-dutied they show an IE value.
E.g. - 10k fuses in bus switches that feed panels with screw in fuses "10k" 208v, BF-18.94ka, AF - 6.81ka, IE-.22

Now to my misunderstanding/ignorance. How do you arrive at an IE if the equipment is over-dutied? I could understand the IE if it was provisional based on a fuse upgrade but I don't see anything. Help show me the light.........


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Fri Feb 10, 2017 8:26 am 
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100questions wrote:
All - I'm wading through a recently completed "arc flash survey" and my lack of knowledge combined with desire to understand things is making me crazy. :D

Per this report we have a lot of equipment/devices that are over-dutied. After they list the equipment as over-dutied they show an IE value.
E.g. - 10k fuses in bus switches that feed panels with screw in fuses "10k" 208v, BF-18.94ka, AF - 6.81ka, IE-.22

Now to my misunderstanding/ignorance. How do you arrive at an IE if the equipment is over-dutied? I could understand the IE if it was provisional based on a fuse upgrade but I don't see anything. Help show me the light.........


From a purely mathematical basis;
The 'overduty' is based on the amount of available bolted fault current on the line side terminals of the device.
The "AF" is based on the arcing fault current flowing through the device.

For a practical use basis;
some companies act like the overdutied equipment is like a bomb waiting to explode, or at least it is likely to not clear an AF.
some companies view the arcing current as not causing a failure of the device, therefore the AF value is still valid.

One case I just worked with:
MV equipment rated at 22kA, available fault current (maximum design limit from utility) 22.7kA. Technically the gear is overdutied.
Each MV device feeds to a step down transformer therefore the arcing fault current is only 2kA on the MV side.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Fri Feb 10, 2017 10:52 am 
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Not sure I really follow the question but I'll take a stab and add to the discussion.

2 different issues. Bolted fault current is totally separate from arcing fault current and incident energy.

Protective device and/or equipment is rated at 10K but 18K is available under a bolted fault condition. Overdutied.

Calculated arcing fault current is 6K so it is less than 10K so the device should still operate and clear the fault in some amount of time that results in .22 cal/cm2 incident energy.

Did you already know this and I don't get the question?


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Fri Feb 10, 2017 12:04 pm 
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bbaumer wrote:
Not sure I really follow the question but I'll take a stab and add to the discussion.

2 different issues. Bolted fault current is totally separate from arcing fault current and incident energy.

Protective device and/or equipment is rated at 10K but 18K is available under a bolted fault condition. Overdutied.

Calculated arcing fault current is 6K so it is less than 10K so the device should still operate and clear the fault in some amount of time that results in .22 cal/cm2 incident energy.

Did you already know this and I don't get the question?


Playing devils advocate here, but how do you know that the breaker has not already been subjected to fault currents above it's duty rating and is therefore compromised but did not experience a complete failure. It may be compromised to where it will not even interrupt a lower arcing current safely or at all and therefore the IE will be higher than calculated. So why take the chance and not just ignore the overdutied breaker in the analysis?

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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Fri Feb 10, 2017 12:28 pm 
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Thanks for the input. I will try to ask a better question.

If I have 15ka available at my 3 ph switch (10k fuses) and my switch feeds my panel that has screw in fuses. How do I know what the IE is if the fuse reaction is unknown due to it being over-dutied?

In my ignorant state I see this as bbaumer stated - "should still operate and clear the fault in some amount of time that results in .22 cal/cm2 incident energy." BUT I guess I thought the survey would use a word like WILL rather than SHOULD.

Is the IE based on a bolted fault or arcing fault? I thought you had to go with the worst case scenario since you don't know. That being the case how do you know what the over-dutied device will do? Again, I'm learning and thanks in advance for the input.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Fri Feb 10, 2017 12:38 pm 
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wbd wrote:
Playing devils advocate here... It may be compromised to where it will not even interrupt a lower arcing current safely or at all and therefore the IE will be higher than calculated. So why take the chance and not just ignore the overdutied breaker in the analysis?


Based on this, all breakers should be ignored, as there is no way to know if they are compromised.

The UL listing for molded case breakers basically requires them to be removed from service once they have been exposed to a fault at their maximum rating. But, nothing is said about intermediate faults. There is no field testing available for AIC ratings.

NFPA70E is not about taking chances. However, it does require 'Assessments' to be made.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Sat Feb 11, 2017 10:54 am 
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JBD wrote:
The UL listing for molded case breakers basically requires them to be removed from service once they have been exposed to a fault at their maximum rating. But, nothing is said about intermediate faults. There is no field testing available for AIC ratings.


Where does it say that? All UL listings for molded case and insulated case breakeres all reference NEMA AB-4 for maintenance which requires a visual inspection that can be done by a competent person after EVERY fault regardless of how severe it is. The inspection takes all of about 10 seconds and only takes longer because for instance lighting panels are almost impossible to open for inspection without powering it down and taking it apart.

Quote:
Is the IE based on a bolted fault or arcing fault? I thought you had to go with the worst case scenario since you don't know. That being the case how do you know what the over-dutied device will do? Again, I'm learning and thanks in advance for the input.


Incident energy can be calculated in several ways but using IEEE 1584 it's based on an arcing fault, which is in turn calculated based on the available fault current (ie, bolted fault current). In the case of an over-dutied breaker it most likely will not actually open under fault conditions. Since an arc has impedance and thus arcing faults have a lower current than bolted faults, it is certainly possible to have a circuit breaker which can open under an arcing fault condition but not a bolted fault one.

But you specifically mentioned fuses which is an entirely different animal. An over-dutied circuit breaker can often handle much higher currents but not be able to open. That's why for instance with high current switchgear I've seen some circuit breakers that have backing fuses...the circuit breaker itself might only be rated for say 35 kA but the fuse is rated for 100 kA, making the series combination device rated for 100 kA. This is why the circuit breaker can have different making, breaking, and withstand current ratings especially in medium voltage switchgear.

I've asked however several times with different vendors about medium voltage fuses. Except for the backup types I just mentioned, the ratings seem to be either around the 20-25 kA range or 10-12 kA. My question was about 10-12 kA expulsion (cutout) fuses in particular in the overdutied scenario. What I was told by the fuse manufacturers that I checked with at the time is that the fuse holder (not just the fuse) would be destroyed in attempting to interrupt in an over-dutied scenario. So in that case it becomes clear that the fuse will still "work" but that fuse "replacement" is a lot more expensive than simply replacing the fuse cartridge. I have yet to locate a fuse vendor that stated that their fuses wouldn't actually function in the event of an over-dutied scenario, including an engineer that did a lot of the testing when the company started producing their own clone of the SMU/DBU/CMU-20 series fuses from S&C, Eaton, or Cooper.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Tue Feb 14, 2017 9:29 am 
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I look at your question slightly differently.
The IE at your fuse panel, should be the IE of the line side of those fuses in any case, so the IE at THAT location is independent of the fuse operation.
It gets trickier down stream of it. There, at the downstream locations, the arcing current may be smaller than the fuse rating, so the fuse can clear it, and the IE can be calculated.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Tue Feb 14, 2017 9:45 am 
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All - Thanks for all of the input. I do understand some aspects a lot better know. We have reached back out to our consultant for some additional input. All of this equipment should have been replaced years ago but differed maintenance has us here now. I just want to make sure I'm yelling "asking questions" loud enough and as to the point as possible so it will get the attention it needs before the "water starts going over the emergency spillway" and someone asks why I didn't let someone know things were in this condition beforehand.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Tue Feb 14, 2017 4:56 pm 
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This are the rules I follow regarding overdutied overcurrent protection devices and arc flash. If you have any comments about this, I would appreciate to receive your ideas.

Overdutied (from now on, OD) overcurrent protection devices (from now on, OCPD) are considered us such when the available short circuit current (from now on, ASCC) of the system is larger than the interrupting rating (from now on, IR) of the OCPD. The OCPD could also be considered as OD even if the ASCC is less than the IR, when the ASCC X/R is larger than the TESTED X/R of the OCPD and a multiplication factor is applied to the ASCC to compensate for this low TESTED X/R ratio.

When an OCPD is installed in a location that has an ASCC higher than its IR, it could fail catastrophically. In other words, explode.

This is a hazard that is not DIRECTLY related to arc flash hazard. But many consultants, like me, believe that it is a real danger. Therefore, we classify the location where this OCPD is DIRECTLY connected, as Dangerous!. So even if the arc flash incident energy (from now on, AFIE) is less than 40 cal/cm2, this location is still classified as Dangerous! due to the OD OCPD device connected DIRECTLY to that location. Again, this classification of Dangerous! has nothing to do with AFIE or arcing currents.

Some consultants will not consider a location as Dangerous! if the arcing current that an OD OCPD (directly connected to that location), sees an arcing current that is less than its IR.

They will assign the calculated AFIE to this location even though the location has DIRECTLY connected OD OCPD's). As far as arc flash analysis is considered, I cannot argue that this practice is faulty. I just go one step forward in analyzing risk (not just hazard), and I do classify the location as Dangerous! due to the DIRECLY connected OD OCPD's. independently if the arcing current is less or more than the IR.

It could be argued that if the arcing current of an OD OCPD is less than its IR, then the OCPD will open under normal operation in case of an arc flash, and therefore, the actual AFIE should be shown on the arc flash label for this location. That is true. But the OD OCPD could violently explode due to a short circuit (not an arc flash) and this also represents a substantial electrical hazard. That is why I classify this location as Dangerous!.

What happens when the OD OCPD is the first device upstream from a main lugs only (MLO) equipment (i.e. without a main device) or the first device downstream from a tap from the location? This topic is very interesting, but not time enough to discussed now.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Tue Feb 14, 2017 8:48 pm 
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Again...there are really 2 or more ratings for an overcurrent protective device. There is both an interrupting rating and a withstand rating, although the terminology is somewhat different from one manufacturer to another and depending on standards. The withstand rating is the maximum current which the device can withstand without being destroyed. I can't tell what the specific failure mode is but most likely it's like bus bars...they fly apart due to the magnetic force which is proportional to the square of the current.

The interrupting rating is usually lower than the withstand rating. With fuses this would be the same number but with circuit breakers, it can be a much lower number. So we can for instance have a 35 kA withstand rated circuit breaker but say only 15 kA interrupting rating. The breaker is not inherently "dangerous" as has been suggested under a 25 kA fault...it simply cannot trip anymore and another upstream device would have to trip instead.

As mentioned, X/R ratio also plays a role in this and there are standard IEEE calcualtions for taking this into account.

A third factor not mentioned is the method by which short circuit current is calculated. The ANSI method as well as multiple IEC methods are all popular. The difference is that some of the assumptions that are made create different results. The major reason for the differences are that before everything was done with software, various estimation methods (particularly the ANSI method) were developed with some approximations which make it easier to do the calculations by hand. The simpler methods also result in higher values than would actually occur in reality. This is the reason that most software programs use one of the IEC methods (I do not remember the standard number) for doing arc flash calculations even though they can give different (higher) values for short circuit current studies. Thus simply changing the short circuit calculation method can result in lower (sometimes significantly lower) short circuit ratings, although the incident energy rating will remain the same.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Wed Feb 15, 2017 6:24 am 
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Low voltage breakers have only interrupting ratings.

Medium and high voltage breakers have both, interrupting ratings and withstand rating (which is also called momentary ratings).

Is any body out there classifying locations as Dangerous! When a directly connected over current protection device is overdutied?


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Wed Feb 15, 2017 7:22 am 
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RECS wrote:
Low voltage breakers have only interrupting ratings.

Medium and high voltage breakers have both, interrupting ratings and withstand rating (which is also called momentary ratings).

Is any body out there classifying locations as Dangerous! When a directly connected over current protection device is overdutied?


Whenever I discover that equipment is overdutied I find ways to remedy the issue by either replacing the equipment, finding a series rating if possible and making that happen or adding conductor length if it makes sense to do that if no series rating can be found and cost and logistics makes equipment replacement unfeasible.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Wed Feb 15, 2017 11:07 am 
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Thanks again for all of the input. I am still reading and learning. RECS - I am now going back and digesting your post. Great info from my vantage. I'm less ignorant today because of you guys. Thanks for sharing your wisdom.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Mon Feb 20, 2017 10:47 am 
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When we find overdutied (we call it underrated) equipment in our short circuit studies, we always list the equipment and recommend that it be replaced. It must be replaced because it is a hazard and NEC code violation.

If we are also conducting an AF study, we will not provide labels for the underrated equipment. A picture of the label is attached. We are not sure how this underrated breaker may operate, it could explode and cause another fault and arc flash event.

To be conservative, In the SKM model, we will remove the circuit breakers in the underrated equipment and then run the AF module. The equipment located downstream, the SKM AF program will look upstream (above the underrated equipment) for device clearing time that is needed for the AF energy calculation.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Mon Feb 20, 2017 11:01 am 
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Robertefuhr wrote:
When we find overdutied (we call it underrated) equipment in our short circuit studies, we always list the equipment and recommend that it be replaced. It must be replaced because it is a hazard and NEC code violation.......


Must be replaced? I agree it is a code violation but you can often do things to reduce the amount of available fault current or create a series rating for the condition by adding an upstream device.

Are you saying that since the equipment MIGHT have been subjected to a higher than rated for fault current at some point in its life it must be replaced or it says so somewhere in the code?

I've come across overdutied/under rated equipment many times and have "fixed" it by adding certain fuses or breakers ahead of the equipment to achieve published series ratings many times or simply done some digging to see if there was already a published series rating for the existing combination of devices. There oftentimes is. I've even artificially increased feeder lengths a few times and replaced transformers with higher impedances than the existing ones a couple times to lower the available fault current when it was faster, cheaper and easier to do that than replace a switchboard and have several days of outages we couldn't schedule. There are many ways to skin the cat unless all of these are not allowed by code.

Please explain and thanks for your input.


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 Post subject: Re: IE of over-dutied equipment / devices
PostPosted: Mon Feb 20, 2017 11:11 am 
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Sorry for the confusion, yes there are many things that can be done to fix the problem. The term replaced should be "must correct". In new construction projects and if the study has been completed before the equipment has been installed, then the term "Replace" is acceptable since it is cheaper to replace the equipment before it is installed.

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