SCCR of control & starter panels

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winslowfam

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We are paying attention to the SCCR (short circuit current rating) of equipment purchased by our clients as we design the power supply to these panels....such as Air Compressors, HVAC equipment, etc. We want to ensure that the panels are rated for the available short circuit curent from the power system where the client wants it installed. When we ask the vendor or OEM for the SCCR of their equipment, we either get a "deer in the headlights" response or they give us a value like 5kA for the rating. This equipment is being fed from 480 volts, so the available short circuit curernt is always greater than 5kA...this forces us to installed current limiting fuses to limit the current these panels will see during a fault. Is this normal for the industry? Are OEM's really unaware of this problem?
 
I believe our facility runs 30kVA to 70kVA depending on where you are in the shop on 480V 3P. We only use current limiting fuses in the buss head to the machine.
 
it seems that current limiting fuses will be necessary all the time if the equipment SCCR is going to be in teh range of 5kA from the equipment manufacturer. I think this is a new code requriement since 2005 that the panels come with a short circuit current rating. I wonder, too, if the vendors are just picking 5kA as their rating, since we keep running into that rating? I don't know why 5kA...it is supposed to be the lowest rated device in that particular panel.
 
NFPA 70E does require local labeling on certain electrical equipment. At minimum I believe this includes: voltages 240V and greater and only 240V that is supplied by 112.5 KVA XFMR or greater. I don?t think 208V is required per say by my be desired?
 
5kA is the typical base value a panel can have unless all of its components have been verified to be higher. The most commonly followed procedure appears to be UL508A.

Simply adding current limiting fuse upstream is not a solution.

For example: say you have 20kA of short circuit amps available (SCA). Your equipment has a main breaker rated for 18kAIC. You now have two protective series in series, so you they need to have a series-combination ratings in order to meet the NEC.

Another example: say you have 6.283kA available (300kVA, 480V, 5.75%Z), and your equipment is being fed from a 400A Bussmann LPS-RK1 fuse, At this fault level the fuse does not enter its current limiting range so it offers no protection to your 5kA SCCR equipment.
 
winslowfam, I also find it very frustrating. A high percentage of the equipment I inspect fails to have the required data per 409.110 and the "deer in the headlight" look is very prevalent until you work your way up the ladder to the guy that says "oh h*#*, are you one of THOSE inspectors".
There are a couple of local EEs that have been busy providing engineering documentation to solve the problem which often is accomplished with current limiting fuses accompanied by their engineering documentation.
The slow implementation of NFPA70E seems to be helping somewhat as
the same data is needed for proper PPE.
 
I can't see how anyone can calculate the fault current they need without getting into calculus; nor am I aware of any way to measure impedence in the field.

That's what it comes down to: the inpedence of the circuit, which is greatly influenced by the length of the circuit. Whether the load is next to the panel or across the room makes a huge difference. I don't see where you can assume any particular 'Z' value.
 
renosteinke said:
I can't see how anyone can calculate the fault current they need without getting into calculus; nor am I aware of any way to measure impedence in the field.

That's what it comes down to: the inpedence of the circuit, which is greatly influenced by the length of the circuit. Whether the load is next to the panel or across the room makes a huge difference. I don't see where you can assume any particular 'Z' value.
Click to expand...

My apologies, if this is in relation to my post. I should have clarified it as being a 300kVA transformer, with a 480V secondary, and a %Z=5.75 will have a maximum fault current of 6,283A. My point was to illustrate, how it is possible to have fuses never enter their current limiting range.
 
tryinghard said:
NFPA 70E does require local labeling on certain electrical equipment. At minimum I believe this includes: voltages 240V and greater and only 240V that is supplied by 112.5 KVA XFMR or greater. I don?t think 208V is required per say by my be desired?
Click to expand...
The labeling for NFPA70E deals with arcing fault incident energy for selecting PPE. Labeling is required whenever PPE needs to be determined. The existing 240V exception deals with the calculations.

NEC 409 requires control panels to be labeled with their SCCR (short circuit current rating).
NEC 110 requires equipment to be applied within its ratings.
 
Manufacturers are challenged with educating customers to what the customer needs. If we tell a customer he needs our panel with a 100ka rating. The csutomer goes to the competition who say nothing about the SCCR and offers a 10ka panel at a significant discount. We loose the business and are eventually out of business. Thus our Salesmen are reluctant to educate the customer. This closes the never ending circle of customer dissatisfaction.
 
jim dungar said:
My apologies, if this is in relation to my post. I should have clarified it as being a 300kVA transformer, with a 480V secondary, and a %Z=5.75 will have a maximum fault current of 6,283A. My point was to illustrate, how it is possible to have fuses never enter their current limiting range.
Click to expand...

600,000 A seemed a bit high to me.
 
petersonra said:
600,000 A seemed a bit high to me.
Click to expand...
it would to me also.
My first post used the US notation of "." (dot) for signifying decimal, my second used the US notation of "," (comma) for thousands. I hope i did not confuse all of the European readers that reverse these two notations.:grin:
 
jim dungar said:
The labeling for NFPA70E deals with arcing fault incident energy for selecting PPE. Labeling is required whenever PPE needs to be determined. The existing 240V exception deals with the calculations.

NEC 409 requires control panels to be labeled with their SCCR (short circuit current rating).
NEC 110 requires equipment to be applied within its ratings.
Click to expand...

Maybe they should get this done too, it's got alot of the same math don't it? :)
 
winslowfam said:
... When we ask the vendor or OEM for the SCCR of their equipment, we either get a "deer in the headlights" response or they give us a value like 5kA for the rating. ... Is this normal for the industry? Are OEM's really unaware of this problem?
Click to expand...
UL and seveal other authorities have made a big push to get the word out there. All of the major mfrs have white papers, webinars and do training of their sales force to get the word out. I have offered 2 classes in the past year alone, no sign-ups. Yet in several cases where I have discussed this with people I KNOW were invited, in person by me and my cohorts, they claim they had no idea of what SCCR ratings are.

My belief is that most OEMs know, but the "deer in the headlights" look is a dodge. They think if they feign ignorance they can get people to let them off the hook "just this one last time". They just don't want to trouble themselves with adding on another layer of engineering responsibility
 
I work for an air compressor OEM and we purchase our control panels from a few different suppliers. Most of our panels have a SCCR of 5kA or 10kA based on the lowest rated panel component per UL508A. It would be very difficult, and costly, to build standard control panels with a rating that would meet most fault conditions. Regardless of the panel rating, different supply systems have widely varying available fault currents which must be addressed on a case by case basis to ensure proper protection.

Regards,

Mark
 
mbeatty said:
I work for an air compressor OEM and we purchase our control panels from a few different suppliers. Most of our panels have a SCCR of 5kA or 10kA based on the lowest rated panel component per UL508A. It would be very difficult, and costly, to build standard control panels with a rating that would meet most fault conditions. Regardless of the panel rating, different supply systems have widely varying available fault currents which must be addressed on a case by case basis to ensure proper protection.

Regards,

Mark
Click to expand...

Amen. Just getting the end user to tell us what what short circuit current is available is all but impossible most of the time.

They often get the deer in the headlight look when that question is asked. We don't really care. We just make what the customer pays us to make. Often that is whatever is cheapest.
 
As a builder of UL listed panels, heres the scoop: Both UL and NEC requires the panels to have a SCCR. Neither requires the panel manufacuter to coordinate this rating with the available fault current. (How can they know where it is going to be installed?) That is the responsiblity of the installing contractor or the owner. 5k is the minimum rating of most power components so they, the panel manufactures, simply list the panels at 5K.

If you need a higher rating, you need to specify it when you order it. you will most probably have to pay more because it will take more time, effort and maybe, more expensive components to achieve the requested rating.

Current limiting fuses ahead of a panel is a simple way to get it coordinated and may be significantly cheaper then paying for the upsized components (especially breakers)

Looking at a utility transformer book, a 1000KVA, pad mounted transformer only has an available fault current of 21K. throw some wire in there and do the calculations and you find the available fault current at your panel will be significantly less. If you are on a factory floor with a buss-duct system, be prepared for some fusing.
 
Cmdr_Suds said:
Current limiting fuses ahead of a panel is a simple way to get it coordinated and may be significantly cheaper then paying for the upsized components (especially breakers)
Click to expand...

I do not mind repeating myself; 'current limiting fuses' are not a one-size fits all solution to a low SCCR.

According to the copy of UL508A-2001 table SB4.2, on my desk, even a 30A Class RK1 fuse will only limit the maximum let-through current (Ip) to a minimum of 10kA, which does not help with 5kA SCCR labeled equipment.
 
jim dungar said:
I do not mind repeating myself; 'current limiting fuses' are not a one-size fits all solution to a low SCCR.

According to the copy of UL508A-2001 table SB4.2, on my desk, even a 30A Class RK1 fuse will only limit the maximum let-through current (Ip) to a minimum of 10kA, which does not help with 5kA SCCR labeled equipment.
Click to expand...

Bussman's table shows LPS-RK_SPI 100A under 5kA. Interestingly enough the graph that goes with it shows about 11kA.

Our 480V buss runs range from 30kA to 70kA with the buss head drop to the machine using only Low Peak Current Limiting fuses. The fuses are 300kA interrupting capacity. Frequently we use 30A that show 5kA or less on the attached graph.
 
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