Available Fault Current - 110.24 (B) Exception

I am working with a consulting engineer to provide the 110.16(B) labeling on a project. As a part of their analysis, they shared a different take on the SCCR or "withstand rating" of inverters. The consulting engineer interprets the "AC Surge Protection" spec as a withstand rating that specifies a maximum available fault current (AFC). In the case of the CPS 60kW string inverter, this value is 15kA. Here is a link to the spec sheet.

I'm curious to hear from the folks on this forum if this is a valid interpretation, given that other posts on this thread seem to be of the opinion that inverters do not have a max AFC spec. In my specific case, I am being asked to install a current-limited, fused, AC disconnect upstream of my AC inverter combiner panel in order to keep the AFC below the AC surge protection rating of 15kA.

Assuming this is a valid interpretation and important spec to design around, has anyone used any other method (aside from a fused disco) to achieve the same level of protection?
 
solarmatt said:
I am working with a consulting engineer to provide the 110.16(B) labeling on a project. As a part of their analysis, they shared a different take on the SCCR or "withstand rating" of inverters. The consulting engineer interprets the "AC Surge Protection" spec as a withstand rating that specifies a maximum available fault current (AFC). In the case of the CPS 60kW string inverter, this value is 15kA. Here is a link to the spec sheet.

I'm curious to hear from the folks on this forum if this is a valid interpretation, given that other posts on this thread seem to be of the opinion that inverters do not have a max AFC spec. In my specific case, I am being asked to install a current-limited, fused, AC disconnect upstream of my AC inverter combiner panel in order to keep the AFC below the AC surge protection rating of 15kA.

Assuming this is a valid interpretation and important spec to design around, has anyone used any other method (aside from a fused disco) to achieve the same level of protection?
Click to expand...
I just don't know if it is practical. An impulse surge rating of xxkA may not have any thing in common with a true SCCR even though they use the same units of measurement.

Standard sized fuses would likely never operate fast enough to limit current to a surge device before its MOVs failed.
 
solarmatt said:
I am working with a consulting engineer to provide the 110.16(B) labeling on a project. As a part of their analysis, they shared a different take on the SCCR or "withstand rating" of inverters. The consulting engineer interprets the "AC Surge Protection" spec as a withstand rating that specifies a maximum available fault current (AFC). In the case of the CPS 60kW string inverter, this value is 15kA. Here is a link to the spec sheet.

I'm curious to hear from the folks on this forum if this is a valid interpretation, given that other posts on this thread seem to be of the opinion that inverters do not have a max AFC spec. In my specific case, I am being asked to install a current-limited, fused, AC disconnect upstream of my AC inverter combiner panel in order to keep the AFC below the AC surge protection rating of 15kA.

Assuming this is a valid interpretation and important spec to design around, has anyone used any other method (aside from a fused disco) to achieve the same level of protection?
Click to expand...
Can you limit AFC with a fused disco? I thought it was a series impedance thing.
 
A fuse cannot be used to reduce the available fault current at an inverter. Two OCPD can be series rated but it does not work with OCPD and equipment. An upstream OCPD can reduce arc fault incident energy at a downstream device. SCCR and AIC ratings have nothing to do with arc fault protection. They are ratings for the available fault current.
 
pv_n00b said:
A fuse cannot be used to reduce the available fault current at an inverter. Two OCPD can be series rated but it does not work with OCPD and equipment. An upstream OCPD can reduce arc fault incident energy at a downstream device. SCCR and AIC ratings have nothing to do with arc fault protection. They are ratings for the available fault current.
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Is there an online course somewhere that I could take that could make all of this clear and calculable?
 
jim dungar said:
This is often referred to as a 'series combination' rating. You can find this used with motor starters and control panels.
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Series combination rating only applies to two OCPD. UL shops who build power panels can do things that the NEC does not support.
 
ggunn said:
Is there an online course somewhere that I could take that could make all of this clear and calculable?
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Eaton does a webinar series called Fuse Fridays. One of the topics they talk about is series rating. The webinars are very good. Contact:
Christy (Rosati) McElhinny
Field Application Engineer
Bussmann Division, Eaton
christymcelhinny@eaton.com

to get on the email list for upcoming webinars. I will add though that manufacturers of OCPD provide tables for series rating their OCPD if the downstream OCPD does not have a high enough AIC rating. It's straight forward to do series rating using the tables. While the NEC does allow series rating to be done outside of this under engineering supervision I've never run across an engineer that would take on this kind of liability. It would require overlaying TCCs on a light table and eyeballing the series protection capability. Very old school stuff that I just don't see done anymore.
 
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pv_n00b said:
Eaton does a webinar series called Fuse Fridays. One of the topics they talk about is series rating. The webinars are very good. Contact:
Christy (Rosati) McElhinny
Field Application Engineer
Bussmann Division, Eaton
christymcelhinny@eaton.com

to get on the email list for upcoming webinars.
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Thanks, but what I am looking for is something more encompassing about available fault current and arc flash calculations and how kAIC, OCPD ratings, and SCCR figure into them. I have looked at the curricula of some on line training companies but I haven't found anything.
 
pv_n00b said:
Series combination rating only applies to two OCPD.
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No.

There is one rating for OCPD in series. This is implied in NEC 110.9 as well as specifically addressed in the 240 for breakers.
There is an entirely different combination rating for an OCPD and non-OCPD. This is implied in 110.10 and other places.

I know of no UL tested combination rating that is not also supported by the NEC.
 
ggunn said:
Thanks, but what I am looking for is something more encompassing about available fault current and arc flash calculations and how kAIC, OCPD ratings, and SCCR figure into them. I have looked at the curricula of some on line training companies but I haven't found anything.
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You would need to take a power systems type course.
Look at those offered by Jim Phillips at Brainfiller.com
https://brainfiller.com/.
 
jaggedben said:
If a PV inverter allows short circuit current to pass through it (like, from the AC side to the DC side) then basically the inverter has failed already. So denoting an SCCR would make little sense. The short circuit would happen because the inverter failed, rather than vice versa.
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pv_n00b said:
Very uncommon to have a rating on an inverter, unless the inverter has some kind of AC OCPD at the AC terminals.
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jim dungar said:
I just don't know if it is practical. An impulse surge rating of xxkA may not have any thing in common with a true SCCR even though they use the same units of measurement.

Standard sized fuses would likely never operate fast enough to limit current to a surge device before its MOVs failed.
Click to expand...
Given these responses, it seems reasonable that I question the interpretation of the AC Surge Protection spec as equivalent to a max Available Fault Current.
I think my engineer is taking me down a path of solving a problem that doesn't need to be solved.

pv_n00b said:
A fuse cannot be used to reduce the available fault current at an inverter.
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And not only does it appear that we are trying to a problem that doesn't need to be solved, it also sounds like the proposed solution wouldn't even solve it.

Also, CPS publishes an AC Site Design guide that does not mention an SCCR or withstand rating for the inverters.

ggunn said:
Is there an online course somewhere that I could take that could make all of this clear and calculable?
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So far, I've been working through this document from Bussmann. It has been helpful for understanding of the concepts and language, though probably not as detailed as you would like.
 
solarmatt said:
Given these responses, it seems reasonable that I question the interpretation of the AC Surge Protection spec as equivalent to a max Available Fault Current.
I think my engineer is taking me down a path of solving a problem that doesn't need to be solved.
...
Click to expand...

Yeah, it is not standard for inverters to come with incorporated surge arrestors. Many people install surge arrestors alongside solar equipment but I daresay it isn't industry standard. The fact that this manufacturer has incorporated surge arrestors at the factory, as a nicety, does not IMO create a new limiting spec to follow. You could install a surge arrestor in an AC combiner panel and it wouldn't create an SCCR rating for the panel.

I didn't even see the 15kA spec on the spec sheet you linked to, just the description of the type of surge arrestor.
 
jaggedben said:
I didn't even see the 15kA spec on the spec sheet you linked to, just the description of the type of surge arrestor.
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I should have uploaded the document rather than a link. It appears since my post this morning, the data sheet on the website has been updated to omit the values of the SPD. Here is a snip from the data sheet I have.

CPS SPD ratings.png
 
solarmatt said:
I should have uploaded the document rather than a link. It appears since my post this morning, the data sheet on the website has been updated to omit the values of the SPD. Here is a snip from the data sheet I have.

View attachment 2576999
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That has always been an issue with SolarEdge. Their data sheets and hardware are always changing.
 
jim dungar said:
No.

There is one rating for OCPD in series. This is implied in NEC 110.9 as well as specifically addressed in the 240 for breakers.
There is an entirely different combination rating for an OCPD and non-OCPD. This is implied in 110.10 and other places.

I know of no UL tested combination rating that is not also supported by the NEC.
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Like I said, UL listings are different from NEC requirements. If equipment is UL listed then the components inside the equipment do not have to meet NEC requirements (NEC 90.7). The NEC does not support series rating of OCPD and equipment, the NEC supports UL Listing.
You are seeing a lot of "implied" in the NEC that is not there. NEC 110.9 is pretty clear:
Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the available fault current at the line terminals of the equipment.
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OCPD is not evaluated upstream of equipment to reduce available fault current, it's not "implied" anywhere in the NEC that this can be done. Engineers have been doing short circuit studies for a long time to establish equipment SCCR and AIC ratings and OCPD are specifically ignored in short circuit studies.
 
pv_n00b said:
Like I said, UL listings are different from NEC requirements. If equipment is UL listed then the components inside the equipment do not have to meet NEC requirements (NEC 90.7). The NEC does not support series rating of OCPD and equipment, the NEC supports UL Listing.
You are seeing a lot of "implied" in the NEC that is not there. NEC 110.9 is pretty clear:

OCPD is not evaluated upstream of equipment to reduce available fault current, it's not "implied" anywhere in the NEC that this can be done. Engineers have been doing short circuit studies for a long time to establish equipment SCCR and AIC ratings and OCPD are specifically ignored in short circuit studies.
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Absolutely the NEC allows you to use UL tested combinations of devices above and beyond just series rated breaker-breaker and fuse breaker combinations. 110.10 is the area to look at for allowable combinations of OCPD and non-OCPD stuff like terminal blocks motor starters even if they are in separate enclosures installed and connected under the NEC as opposed to being internal wiring using 90.7.
 
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