Deltaforce
Senior Member
- Location
- India
- Occupation
- Electrical Engineer
But inspector of the install?
They also miss?
what is SCCR if it doesn't prevent arc flash?
- Thread starter malachi constant
- Start date
They also miss?
- Location
- Illinois
- Occupation
- retired electrician
almost all the time
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
SCCR does not apply to loads like motors at all. Only to the upstream control equipment and to the equipment containing protective devices such as circuit breakers, but not the actual protective equipment.
And to respond to another post, many times devices have limitations on wire sizes that restrict the SCCR. Usually larger wires have higher SCCR in these cases, while smaller wires may have just the default rating. You have to look it up and not rely on blurbs that say 100kA SCCR and #12-4/0 wire. Its quite possible the SCCR with #12 is 10 kA.
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kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Some do, some do not.
If there is reason to assume rather high available fault current and you end up with a rather short length feeder or branch circuit the knowledgeable ones will be asking questions at the very least. The ones that aren't so knowledgeable on this topic may have no clue what the possibilities might be in some short circuit or arcing incident.
With enough experience, one just kind of naturally knows most the time when there is little concern vs you at least need to be thinking about the SCCR.
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
malachi, arcing fault current on low-voltage systems (<1000V) is about 55% of the bolted fault current, whereas on MV-HV systems it's closer to 95%. That's good info to present to your younger staff.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
Deltaforce
Senior Member
- Location
- India
- Occupation
- Electrical Engineer
But
Incident energy the key
How does one get Well informed on this? Do you have any documents that someone could upload? There’s so much information online, but I don’t quite know where to start for this
A while ago, someone posted documentation on Boulder motors, and it was very informative. Read it all understood it printed it off and now it’s all out of my memory so I need something to keep referencing
I do mostly residential service work occasionally. I’ll use this stuff on particular jobs around here and it’d be nice to be able to whip it out of my back pocket. Thank you for anyone that posted anything about that
A while ago, someone posted documentation on Boulder motors, and it was very informative. Read it all understood it printed it off and now it’s all out of my memory so I need something to keep referencing
I do mostly residential service work occasionally. I’ll use this stuff on particular jobs around here and it’d be nice to be able to whip it out of my back pocket. Thank you for anyone that posted anything about that
DBill64
Member
- Location
- Washington, USA
- Occupation
- I&E Technician
For determining SCCR for industrial control cabinets (in accordance with UL 508A) I referred to this to wrap my head around the subject: https://literature.rockwellautomation.com/idc/groups/literature/documents/at/sccr-at002_-en-p.pdf
I have sadly never delved into incident energy calculations for an entire building though, which I imagine is this same methodology but expanded in scope.
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
It's all in NFPA 70E. They make recommendations for software in Annex D to perform the IE calcs, then the software manufacturer's use those calculations. Currently in use is IEEE 1584-2018. If the range exceeds that of IEEE 1584, because it's only good for 208-600V, 500-106,000 A, and 601- 15000V, 200-65,000A, then other methods are used. The calculations are so involved that it's best left to the Power Systems analysis packages.
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
I would actually think the second should have been a breaker rated at 65kAIC. The enclosure and the bussing would have the SCCR rating.
brycenesbitt
Senior Member
- Location
- United States
But there's two levels, right? There's the equipment still being operational, and the equipment not "ejecting parts".
But what about the enclosure? Could we have a SCCR rating at which the explosive decomposition of the breaker or fuse is contained within the rated enclosure?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
SCCR doesn't apply to enclosures. It applies to say panelboard bus bracing and what kind of magnetic forces during such high current incidents they can withstand without compromising that bracing. The breakers have an AIC - the current it can safely interrupt without risk of destruction.
I don't know there really is any such rating for enclosures, though I think many older switchboard or panelboard cabinets will have heavy enough metal to withstand more force from inside than many newer ones will. But then there is some with ventilation openings and they will let some blast force out through those regardless how thick the enclosure walls are.
brycenesbitt
Senior Member
- Location
- United States
Well, bigger picture that would be a strategy of defense in depth, and staged "series rated" like protection. Thicker steel is pretty cheap
despite the evidence in wimpier and plastic boxes these days. Containing a big surge current in a small molded case breaker seems a much harder job, reflected in the prices for those breakers.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Then there is the situation where you are turning the breaker on, cover is on the panel but you have door open so part of the breaker is exposed. Should that breaker blow up because you closed it into a short circuit, the exposed plastic front of the breaker is still a bigger potential hazard than the steel cover surrounding it. Yet at same time correct AIC selection/cooridnation of the breaker should greatly minimize that risk.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
NFPA 70E does not consider enclosure doors or covers when evaluating ArC Flash. At this time there is no industry consensus standard for calculating Arc Blast or it's effects.
There are many, but not all, UL standards that require enclosure doors remain closed when testing the actual SCCR and AIC ratings, however the doors are sometimes allowed to be deformed and not reusable.
brycenesbitt
Senior Member
- Location
- United States
Certainly.
But that's a tiny exposure surface compared to 24/7/365.
Is there a good reference I could read on arc flash incidents by amperage. Where is this is a big problem, little problem, not a problem?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
You need to look at the history of NFPA 70E and IEEE 1584.
Most of the data involves 3 phase incidents with fault currents above 2kA.
We've been doing Arc Flash for some 20 years but still don't have good methods for calculating incident energy on single phase systems.