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

One of the government agencies I worked for decades ago was one of the few in the State that addressed SCCR ratings. It was commonplace for equipment suppliers (and sometimes manufacturers) to have the "deer in a headlight" look when rejections were issued. Normally we had to move a few steps up the chain before we heard "Oh! You are one of THOSE" and then it would be addressed.
(The same scenario often applied to our requiring NRTL listings)
 
don_resqcapt19 said:
However, you will have to find some way to limit the fault current on the line side of the 20 amp breaker that feeds these loads. Unlikely you will find one suitable for use with that high AIC.
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To add a little to this, if you have 100kA available how often do you have (or desire to have) 15 and 20 amp circuits connected to the equipment that has that much available? Often that is main distribution type equipment and only supplies feeders or if there is branch circuits they are some large capacity load. Usually your general use branch circuits are on some some feeder or even have a transformer in the path and the fault current at the origin of those branch circuits is likely below 10kA or at least has series rated components involved. At very least a 22-25 kA main breaker in that branch panel that is series rated for use with the 10kA branch breakers, which that combination will be covered even by most commonly used "load centers" you find in residential installs.
 
malachi constant said:
So all motors [technically motor controllers, but I'm going to be loose with my language and say "motors"], all motors everywhere, always, should have their available fault current calculated and compared against the controller's SCCR?
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Loose with the language is not serving you here. It is NOT about the motor, it is about the EQUIPMENT ahead of it. All electrical devices that control the flow of POWER (as opposed to control circuits) must be capable of surviving a short circuit event without entering into a “RUDE” condition (Rapid Unplanned Disassembly Event). That applies to breakers, disconnect switches, fuses and fuse holders, terminals, contactors, overload relays, solid state power devices, EVERYTHING in the power circuit, EXCEPT the motor or load device (heater) itself. Fuses and circuit breakers already have that baked into their “interrupt ratings), commonly shown as “AIC”, and fuse holders generally are rated for the fuses they can hold. But the rest of it must be tested and listed with the breakers or fuses based on the “let-through” energy they allow before clearing the fault.

SCCR is not something that we can do in the field, the equipment must STATE it now. If there is nothing done, you can assume an untested “courtesy” level of 5kA. In most older RESIDENTIAL applications breakers are rated 10kAIC, because it’s hard for a residential service transformer to allow that much fault current to go though it. So 5kA SCCR might be OK too by the time you factor in the wire impedance out to where the equipment pad is. But it’s not guaranteed, and an AHJ might demand to see calculations showing that something with a 5kA SCCR label has that little AFC at the terminals. But if you get a 5kA SCCR label on a piece of commercial or industrial equipment, it can be next to impossible to connect it.
 
Deltaforce said:
A current limiting fuse or breaker prevent or lessen it
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There will always be the arcing and always will be energy released by the arcing. Overcurrent devices can vary in how they react to the incident which will change how much energy is released in the arcing incident. Lower current but for longer time can have more incident energy than higher current for shorter time.
 
kwired said:
There will always be the arcing and always will be energy released by the arcing. Overcurrent devices can vary in how they react to the incident which will change how much energy is released in the arcing incident. Lower current but for longer time can have more incident energy than higher current for shorter time.
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The question, always injury or fire by arcing
of short-circuit cause
The intent SCCR prevent or lessen it
Failure it mean design failure
 
Deltaforce said:
The question, always injury or fire by arcing
of short-circuit cause
The intent SCCR prevent or lessen it
Failure it mean design failure
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SCCR is about equipment and not directly about people protection. If the current in an incident doesn't exceed the SCCR of some equipment this mostly means that equipment should not be throwing shrapnel at you because it couldn't withstand the forces involved.

Then there is AIC rating, more applies to items that are intended to interrupt fault currents. A 10kA rated breaker should be able to interrupt a fault of 10kA or less without damaging the breaker. There still may be some exposure element involved if you are in close enough proximity and no additional shielding is in place.
 
Side note: at the large hospital that I retired from they would have a highly qualified testing company clean ,PM & test the maybe 50 rack out 400 to 1600 amp 480 volt circuit breakers located in a basement and a tenth floor switchgear room. They had us take cleaned & PM'ed spare breakers to the other location and use them for some mechanical loads that were in VFD'S that could be powered down during the week .When they came in on a llate Saturday night to clean,PM & test breakers in larger of the two switch gear rooms sharp eyed tech noticed that we had at least a dozen think 40 KIAC rated rack.out breakers in the gear that and with all 65 KAIC . Tech told us even if you only have one lower rated KIAC breaker racked in the entire gear is only rated for the lowest KIAC installed breaker.
 
Op question and winnie remark 'If someone drops a wrench on live bus bars, you can have an arc event, no matter what the ratings of the equipment' related
NEC 240.67 provide answer.
Suitable current limiting fuse or breaker reduce arcflash category to 1 or 0 mitigate arcflash hazard in addition sccr of equipment
 
Deltaforce said:
arcflash category to 1 or 0 mitigate
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These terms are for locations which have not performed a complete arc flash study. They are commonly used as slang in our industry. For the past decade, the preferred NFPA 70E ratings would be expressed in APTV or calories/centimeter(squared).
 
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jim dungar said:
These terms are for locations which have not performed a complete arc flash study. They are commonly used as slang in our industry. For the past decade, the preferred NFPA 70E ratings would be expressed in APTV or calories/centimeter(squared).
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Hope op and winnie agree #52
 
MTW said:
Here's you one that wasn't a bolted fault or short circuit, but it burned long enough to take out a 480V 750KVA bank on a pole.

The cause was accumulated grinding dust with condensing humidity in the middle of a wet winter.
View attachment 2576786
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We had a copper smelter with conductive dust everywhere. Problems all the time. Even the articulated loaders and off road dumps they used for hauling out to the slag pile would drain the batteries overnight from the dust on top. That place was rough on everything
 
Deltaforce said:
Not concern PE?
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Assuming you mean Professional Engineer, there are far more installations designed and installed by contractors and electricians than ones designed by PE. And even there the mechanical and electrical PEs often don't talk to each other so equipment with SCCR far lower than the available fault current is specified by the mechanical engineer.
 
jim dungar said:
If not someone needs updated training.
NFPA 70E dropped 'Category 0' some 10 years ago.
Slang is okay in most circumstances but it should not be misleading.
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A correspond between cal/sq.cm and arcflash cat
Cat0 less than 1.2cal/sq.cm ( even though removed by nfpa 70e)
Cat i minimum 4 cal/sq.cm etc
So no issue
 
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