Available fault current vs. SCCR

[overkill94]

I'm doing an inspection on a piece of equipment with a nameplate SCCR of 5 kA but the calculated available fault current at the panel is 14 kA. They contest that since they're using 150 Amp RK1 fuses as feeder protection, the let-through current is only 3 kA. Is this valid code-wise?

Thanks!

 

[ron]

FAIL

The code requires a tested assembly if that logic is used.

 

[overkill94]

FAIL

The code requires a tested assembly if that logic is used.

Thanks Ron, that's what I've been trying to explain to the client but he's adamant that the trip curves are proof enough that he's compliant.

 

[majorminotaur]

What is the fault current available at the equipment connection? It was get knocked down from 14kA.

 

[jim dungar]

FAIL

The code requires a tested assembly if that logic is used.

I agree with the fail.

However the NEC only requires listed testing for a series combination of protective device. Some listings, such as UL508A, allow for 'non-tested' combinations.
Doesn't most equipment contain some sort of protective device?

Following this logic path, the equipment has a tag that says 5kA, therefore there is no method available to field calculate a 'series rating'. However, if no SCCR rating was listed, then the AHJ could allow field methods similar to those found in UL508A based on NEC 110.10.

 

[charlie b]

I do not agree with the "fail." This is not a situation that involves a "series-rated combination." You are not passing the fault current through two devices in series, with the expectation that, on the basis of some prior testing, the combination will sucessfully trip the circuit before anything gets damaged. Instead, someone calculated that the amount of fault current available at the equipment was 14kA. However, that person should have changed that calculation in light of the specific overcurrent device (note: singular, "device") that was being installed. The presence of that device changes the amount of fault current available at the equipment. If there were a circuit breaker instead of a current-limiting fuse, the system might have be able to inflict 14KA of fault current. But with the fault limiting fuse in the circuit, the amount of fault current available is now just 3kA.

 

[overkill94]

I do not agree with the "fail." This is not a situation that involves a "series-rated combination." You are not passing the fault current through two devices in series, with the expectation that, on the basis of some prior testing, the combination will sucessfully trip the circuit before anything gets damaged. Instead, someone calculated that the amount of fault current available at the equipment was 14kA. However, that person should have changed that calculation in light of the specific overcurrent device (note: singular, "device") that was being installed. The presence of that device changes the amount of fault current available at the equipment. If there were a circuit breaker instead of a current-limiting fuse, the system might have be able to inflict 14KA of fault current. But with the fault limiting fuse in the circuit, the amount of fault current available is now just 3kA.

The rating of the panel is not in question - the nameplate says 5 kA and from what I can tell, it's due to some Allen-Bradley contactors on heater branch circuits.

I've told them that I can not certify this equipment as long as the SCCR of the panel is lower than the calculated available fault current. If the upstream fuses are indeed limiting the fault current, then that needs to be reflected on the arc-flash label. Is my reasoning correct?

 

[jim dungar]

I do not agree with the "fail." This is not a situation that involves a "series-rated combination." You are not passing the fault current through two devices in series, with the expectation that, on the basis of some prior testing, the combination will sucessfully trip the circuit before anything gets damaged. Instead, someone calculated that the amount of fault current available at the equipment was 14kA. However, that person should have changed that calculation in light of the specific overcurrent device (note: singular, "device") that was being installed. The presence of that device changes the amount of fault current available at the equipment. If there were a circuit breaker instead of a current-limiting fuse, the system might have be able to inflict 14KA of fault current. But with the fault limiting fuse in the circuit, the amount of fault current available is now just 3kA.

Are there 'circuit breakers' inside of the equipment? 240.86 pretty much prevents blanket approval 'just because the upstream device is a current limiting fuse'.

Even Bussmann talks about changing the SCCR of the equipment, not simply lowering the let-thru current 'just because the upstream device is a current limiting fuse'.
http://www1.cooperbussmann.com/2/SCCRSolutions.html

Personally, I have a problem with most fuse calculations that are not based on 'umbrella fuses'. Too many times I specify brand B fuses, to find out that brand A were installed instead (evidently purchasing agreements are more important than my design). Another problem involves arcing faults that are greater then the SCCR but less than the point where the fuse is actually current limiting.

 

[GoldDigger]

I've told them that I can not certify this equipment as long as the SCCR of the panel is lower than the calculated available fault current. If the upstream fuses are indeed limiting the fault current, then that needs to be reflected on the arc-flash label. Is my reasoning correct?

I would not go so far as to say that it needs to be reflected on the arc-flash label, but if that were done it would be a simple way out of the apparent non-compliance.
The function of an arc-flash label is not primarily to determine mandatory device ratings downstream. A simple fault current calculation (by a PE if that is required to satisfy you) would be enough, and there is AFAIK, no requirement that the arc-flash label has to be the lowest possible rating that could be assigned, just that the actual available energy be no higher than the label value.

 

[JDBrown]

I'm doing an inspection on a piece of equipment with a nameplate SCCR of 5 kA but the calculated available fault current at the panel is 14 kA. They contest that since they're using 150 Amp RK1 fuses as feeder protection, the let-through current is only 3 kA. Is this valid code-wise?

Thanks!

I see that your location is listed as Los Angeles. It is the official policy of LADBS that "fuse let-thru" is NOT an acceptable means of limiting fault current.

Link to LADBS Electrical Plan Check Checklist - see Section F, Item #6.

For what it's worth, I looked up all of the Code sections listed on the form with this requirement and none of them specifically mention fuse let-thru. However, that is the AHJ's official interpretation if you're in the City of Los Angeles.

 

[overkill94]

Thanks everyone.

Although I'm located in LA, the inspection is in Kansas City and there isn't actually a jurisdiction. I would be okay with a PE signing off on it since that would deflect liability from my company, but I'm not sure that's an option for them.

There are no circuit breakers in the panel, just the upstream fuses, a main disconnect switch, and then branch circuit fuses within the panel. I checked Table SB4.2 from UL508A to see if the 60 Amp class J fuses on the contactor's branch circuit would bump it up, but the lowest let-through is 8 kA which is still higher than the contactor's 5 kA.

They've also asked for some replacement contactor options (rated higher than 14 kA) but I've never had to buy components so I'm having a hard time tracking one down. Can anyone recommend a model they can use that either has a higher stand-alone SCCR or can be series rated higher?

 

[overkill94]

Okay, now I'm even more confused. Here's an excerpt from an article on this very site - http://www.mikeholt.com/technical.p...&type=u&title=Available Short-Circuit Current

"If the available short-circuit current exceeds the equipment/conductor short-circuit current rating, then the thermal and magnetic forces can cause the equipment to explode and/or the circuit conductors as well as grounding conductors to vaporize. The only solution to the problem of excessive available fault current is to (1) Install equipment that has a higher short-circuit rating
(2) Protect the components of the circuit by a current-limiting protection device such as a fast-clearing fuse, which can reduce the let-thru energy."

 

[jim dungar]

Can anyone recommend a model they can use that either has a higher stand-alone SCCR or can be series rated higher?

Look for something called a 'self-protected' starter. This is fairly common in IEC style equipment.

 

[topgone]

Okay, now I'm even more confused. Here's an excerpt from an article on this very site - http://www.mikeholt.com/technical.p...&type=u&title=Available Short-Circuit Current

"If the available short-circuit current exceeds the equipment/conductor short-circuit current rating, then the thermal and magnetic forces can cause the equipment to explode and/or the circuit conductors as well as grounding conductors to vaporize. The only solution to the problem of excessive available fault current is to (1) Install equipment that has a higher short-circuit rating
(2) Protect the components of the circuit by a current-limiting protection device such as a fast-clearing fuse, which can reduce the let-thru energy."

It looks like a blast from the past for me!

I've experienced this same situation where the vendor wanted to pass a low SSCR contactor even when the calculated/ specified available short circuit current was higher. The solution was to lengthen the line feeding the MCC to be able to bring down the available short-circuit current. Can you re-calculate your available short circuit current available with the secondary line longer?

 

[dkidd]

Here is a good article

http://ecmweb.com/design/sccr-liability-trap

 

[petersonra]

A lot depends on just why the panel is labeled the way it is. Many parts in control panels can be labeled as listed with a higher SCCR if protected by a CL fuse. The CL fuse is not required to be inside the panel.

Conceivably the CL fuses on the BC could serve to protect whatever devices in the panel have a low SCCR. Whoever designed the panel would need to look at it and come up with a label that defines what upstream BC protection would be acceptable.

It is also entirely possible that the panel could be modified to have a higher SCCR without all that much trouble. It might be as simple as installing a set of CL fuses and changing the nameplate on the panel.

But just installing CL fuses upstream does not cut it IMO.

ETA:

Just to be clear.

What might be an acceptable solution is that the control panel manufacturer adds a nameplate that says something like this:

SCCR of 65kA when panel is protected by class J fuses maximum 60A.

Someone than installs 60A class J fuses upstream.

 

[winnie]

A _key_ distinction that needs to be made here:

The 'maximum let through' of a fuse can not be considered as suitably limiting short circuit current when protecting downstream overcurrent protective devices, or other devices with _dynamic_ impedance. My understanding is that devices with dynamic impedance can 'see' and respond to the early part of the fault before the upstream device has actually limited the current. Thus when talking about protecting things such as circuit breaker panels, you must have a proper 'series rating'.

My understanding is that 'maximum let through', or 'up over down' can be used if the device being protected has static impedance. For example, if the SCCR rating of a device is set by the mechanical bracing of bus bars, then I believe that you can use the maximum let through of a fuse to determine if you have suitable protection.

This is not an area that I am deeply familiar with, so I am very open to corrections or education on the topic.

-Jon

 

[overkill94]

Both the client and I tried to find some documentation for a series rating on the contactor, but all we could find were IEC series ratings (not UL). If anyone feels like solving a puzzle, it's an Allen-Bradley model 100-C37*00.

Setting aside the panel's SCCR, does anyone know of a loophole or code exception that would allow a value from a let-thru graph to replace the calculated available fault current at the panel's input?

 

[winnie]

Take a look at http://www.ab.com/en/epub/catalogs/...04-D-100S-C-104S-C-100S-D-Specifications.html

Look about 3/4 of the way down, under "Short-Circuit Coordination Data"

There they describe the maximum fuse or breaker permitted given various available short circuit conditions, including under UL requirements. They list a value at 5kA available, and then have blanks for higher ratings (10kA, 18kA, 30kA....) but have ratings for 100kA. There is even a rating for 25kA available in series with an inverse time breaker....

Like I said, I am out of my element here, so take the above as my research due to my curiosity, not my expertise!

-Jon

 

[petersonra]

this contactor appears to have a sccr of 100kA if used with a max 50A class J fuse.