what is SCCR if it doesn't prevent arc flash?

M

malachi constant

Senior Member
Location
Minneapolis
I am a PE with 25 years experience, but don't yet understand a nuance here. I understand "available fault current" and the need to brace equipment with an SCCR that is higher than than the AIC. I understand how arc flash is a function of both how large the fault current is, and how long it is allowed to continue before clearing. So there are a couple basic concepts you probably don't need to walk me through.

What I don't understand is, what exactly is the SCCR getting you? I have read that for equipment with appropriate SCCR "damage may occur, but it must not present a shock hazard, fire hazard, or expel projectiles from the equipment". So say if you calculate 45.4k available fault current, you would rate the panel at 65kAIC. But then the perfect storm of a fault happens, and an arc flash event occurs. On an intuitive level I would think the SCCR would mean the equipment is built more "heavy duty" or with bussing further apart so that when the short circuit occurs it does not turn into a ball of flame. Obviously, I know, that is not how it works - the ball of flame may still be coming. So what level of safety are you getting with the increased SCCR? Is it "when the arc flash occurs the panel blows up a little bit less", or what?

I am not articulating this perfectly, but would appreciate you taking stabs at answering what you think I am asking and maybe that will help me ask it better. Thanks in advance!
 
SCCR is about the equipment not being damaged in case of a short circuit. If the equipment cannot accept the maximum scc available before it clears, it's possible it might explode.

Arc flash has little to do with scc. It is how much incident energy is available to be dissipated in case of an event.

My personal opinion is SCCR is way overstated as a risk.
 
petersonra said:
SCCR is about the equipment not being damaged in case of a short circuit. If the equipment cannot accept the maximum scc available before it clears, it's possible it might explode.

Arc flash has little to do with scc. It is how much incident energy is available to be dissipated in case of an event.

My personal opinion is SCCR is way overstated as a risk.
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So with proper SCCR the equipment itself "does not explode" per se, but an arc flash plasma ball may burst out of the opened enclosure. Which effectively destroys the equipment, right? I assume equipment that experiences an arc flash gets replaced 99% of the time.

Does this mean that though the plasma ball bursts out, the equipment itself "holds together" - that the sides and top don't blow off, or something like that?
 
malachi constant said:
I am a PE with 25 years experience, but don't yet understand a nuance here. I understand "available fault current" and the need to brace equipment with an SCCR that is higher than than the AIC . I understand how arc flash is a function of both how large the fault current is, and how long it is allowed to continue before clearing. So there are a couple basic concepts you probably don't need to walk me through.

What I don't understand is, what exactly is the SCCR getting you? I have read that for equipment with appropriate SCCR "damage may occur, but it must not present a shock hazard, fire hazard, or expel projectiles from the equipment". So say if you calculate 45.4k available fault current, you would rate the panel at 65kAIC. But then the perfect storm of a fault happens, and an arc flash event occurs. On an intuitive level I would think the SCCR would mean the equipment is built more "heavy duty" or with bussing further apart so that when the short circuit occurs it does not turn into a ball of flame. Obviously, I know, that is not how it works - the ball of flame may still be coming. So what level of safety are you getting with the increased SCCR? Is it "when the arc flash occurs the panel blows up a little bit less", or what?

I am not articulating this perfectly, but would appreciate you taking stabs at answering what you think I am asking and maybe that will help me ask it better. Thanks in advance!
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Careful with you terms. For the first AIC in red I think you mean AFC. For the second red, i think you mean SCCR.
 
SCCR is based on equipment being able to tolerate/handle the amount of current that flows due to a bolted fault.
AIC is based on the ability of a OCPD being able to clear the current flowing due to a bolted fault

Both SCCR and AIC need to be greater than the amount of bolted fault current available. The available fault current does not have a single industry accepted acronym, although two of the most common are AFC = available fault current and SCA = short circuit available.

Incident Energy is calculated using bolted fault current however it comes from an arcing fault.

Arc flash incident energy is used to select the appropriate PPE to protect a worker from burns due to an arcing fault which usually occurs during maintenance and testing.
SCCR and AIC are used to select equipment to prevent self destruction like fragmentation, due to a bolted fault which usually occurs during installation.
 
malachi constant said:
On an intuitive level I would think the SCCR would mean the equipment is built more "heavy duty" or with bussing further apart so that when the short circuit occurs it does not turn into a ball of flame.
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Bussing further apart would help with flashover from say a transient voltage. How well the bus is braced can give a bus a higher SCCR. Magnetic forces between adjacent buses during a fault is going to try to force them apart, Higher rated SCCR busing will have better bracing so it can't move, which if it did move can lead to even more damages.
 
A question to ask is : why is there fault current and how does the equipment respond.

If someone drops a wrench on live bus bars, you can have an arc event, no matter what the ratings of the equipment.
The response time of the OCPD can impact the energy of that arc event, but that response time is entirely separate from the AIC or SCCR rating of the equipment.

If a fault occurs and the equipment does not have sufficient rating, then the equipment itself might fail or cause an arc event.
 
malachi constant said:
On an intuitive level I would think the SCCR would mean the equipment is built more "heavy duty" or with bussing further apart so that when the short circuit occurs it does not turn into a ball of flame.
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SCCR is much more than simply the spacing of the bussing. It is about preventing a small event from becoming a larger event.

Given today's technologies, you cannot prevent an arcing event from turning into a ball of plasma and flame. The best you can hope for is to prevent the arc in the first place.

Basically AIC is related to the NEC 110.9 and SCCR is 110.10. These sections have been required for almost 50 years. Arc Flash prevention and mitigation are the purview of NFPA 70E (except to recent addition of OCPD greater than 1200A).
 
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I too just started pay more attention to this as Arc Flash studies were coming in and the AIC / SCCR were always way higher than the actual available fault current from utility on down.

From my research it appears that a lot of the industrial / control equipment panels over the last 10 years are really the only pieces of equipment that might have a lower SCCR than the available fault current.

The only other time I ran into an issue with the difference of SCCR rating and available fault current was a data center project where the buss taps had a 22kAIC rated breaker.

As others have mentioned, there area lot of resources from UL, Eaton's Bussmann, S&C, etc. on this to help.

** SCCR and AIC and available fault current is a typical worst case scenario to ensure that there is no possibility of fire and damage to persons or property (for enclosed equipment). Most faults would not be your worst case and therefore most of the sizing would be overkill but that is important. It is better to have equipment fail in a "contained" or "controlled" way than have it blowing parts out like a detonation.
 
You can have the arcing portion of the fault somewhere downstream on a branch or feeder circuit, but the panelboard and overcurrent device within the panelboard need to be able to safely handle the resulting fault current without self destruction long enough for the overcurrent device to open the circuit, without blowing the OCPD up in the process.

When I was still in college we went to Square D plant in Lincoln, NE and toured the place. They made QO circuit breakers there back then, I think that operation has moved to Mexico and not sure what they do at the Lincoln facility though I think it still is owned and operated by Schneider.

Anyway, one the most remember able parts of the tour was the testing lab. They did demonstration for each group that went through. They had a vault with big heavy doors on it, inside they had what looked like the cheapest load center with very few spaces as they could find with three phase bussing, plugged a three pole breaker on it (probably 35-60 amp range) and three output leads of what was probably 6 or 8 AWG were bolted together. They turned the breaker on and closed the vault. When they energized it it was pretty loud even though it was inside that vault, supposedly was a source capable of providing 10kA of fault current. They then opened the doors, smoke was still lingering in there, someone reached in and physically reset the breaker - and it did mechanically reset. Had enough deposits on it that looked like you maybe shouldn't ever use it again but supposedly meet the 10kA standards if it would reset.
 
ron said:
Short circuit calcs are so there isn't a catastrophic failure during a fault generally when the doors are closed.
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And I think that is the Crux of the OP's question. What exactly does catastrophic mean? Do they try to design so that someone can be standing right next to the equipment and not be injured during the worst case event?
 
ron said:
Generally arc flash calculations are based on a working distance from energized conductors. So the doors are open.
Short circuit calcs are so there isn't a catastrophic failure during a fault generally when the doors are closed.
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Doors offer zero protection from an active arc fault. Their presence is ignored while calculating arc flash incident energy. Doors do prevent contact with exposed energised which might then lead to an arcing fault.

The most common cause of bolted fault currents is wiring errors during installation. Bolted faults are typically noticed when turning the power on.
Arcing faults are typically caused by inadvertent touching of exposed energized components typically during maintenance and troubleshooting.
 
kwired said:
You can have the arcing portion of the fault somewhere downstream on a branch or feeder circuit, but the panelboard and overcurrent device within the panelboard need to be able to safely handle the resulting fault current without self destruction long enough for the overcurrent device to open the circuit, without blowing the OCPD up in the process.

When I was still in college we went to Square D plant in Lincoln, NE and toured the place. They made QO circuit breakers there back then, I think that operation has moved to Mexico and not sure what they do at the Lincoln facility though I think it still is owned and operated by Schneider.

Anyway, one the most remember able parts of the tour was the testing lab. They did demonstration for each group that went through. They had a vault with big heavy doors on it, inside they had what looked like the cheapest load center with very few spaces as they could find with three phase bussing, plugged a three pole breaker on it (probably 35-60 amp range) and three output leads of what was probably 6 or 8 AWG were bolted together. They turned the breaker on and closed the vault. When they energized it it was pretty loud even though it was inside that vault, supposedly was a source capable of providing 10kA of fault current. They then opened the doors, smoke was still lingering in there, someone reached in and physically reset the breaker - and it did mechanically reset. Had enough deposits on it that looked like you maybe shouldn't ever use it again but supposedly meet the 10kA standards if it would reset.
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I have always been fascinated with these Labs that test the stuff. I heard once that square D had a very nice lab and actually did testing for other manufacturers too? Not sure if that's true or not. Also, I've always wondered, what does the power company think of them causing all these big short circuits?
 
electrofelon said:
I have always been fascinated with these Labs that test the stuff. I heard once that square D had a very nice lab and actually did testing for other manufacturers too? Not sure if that's true or not. Also, I've always wondered, what does the power company think of them causing all these big short circuits?
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I was still young and somewhat ignorant at that time. This would have been in probably either 1988 or 1989 when we went there and I was only 19 or 20 years old with little to no in the field experience yet. Seeing the same thing now would be even more informational to me, plus I'd have better idea of what questions to ask them.
 
electrofelon said:
I have always been fascinated with these Labs that test the stuff. I heard once that square D had a very nice lab and actually did testing for other manufacturers too? Not sure if that's true or not. Also, I've always wondered, what does the power company think of them causing all these big short circuits?
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Square D had a couple of labs like this. Their biggest was in Cedar Rapids IA. They did rent it out to others, it was expensive so they didn't like it to sit idle. Yes the utility Inter connection was designed with the test lab in mind.
 
Hi all, really appreciate the insight. Bolted fault (typ during installation) vs arc fault (typ during maintenance and testing) was very illustrative.
Also the description of the Square D testing lab imagery. Really, just about every response here helped push my understanding forward.

I'm preparing some material to teach younger staff the basics of short circuit studies. Trying to teach something sure makes you realize how little you know, right? This will surely not be my final line of questions on the topic :)
 
malachi constant said:
Bolted fault (typ during installation) vs arc fault (typ during maintenance and testing) was very illustrative.
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This is an important concept.
NFPA 70E considers testing part of 'normal' activities while installation of and changes to wiring is 'not normal' and therefore typically requires more precautions and often higher rated PPE.
 
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