Short circuit rating of panel.

[lquadros]

Hi,
I manufacture control panels. What I have problem with is dealing with panel inspectors, who say that if any component in the panel is rated less than the available fault current, it cannot be certified as a safe installation. The fault current of the panel will be rated according to the weakest component. Irrespective of OCPD - fuse or breaker with higher than available fault current ratings .Their solution is to install an isolation transformer or to source a component which is rated to the available fault current. I have problem to source servo controllers that are rated at 57KA. Installation of transformers is expensive and a waste due to being energized when not in use. Do you have an opinion about this? Thanks in advance for your answers.

 

[ron]

Re: Short circuit rating of panel.

Control panels need to have a short circuit rating. I would think that either the weakest component derives the overall rating (like a circuit breaker panelboard), or you can have the entire panel tested by a third party testing company, and see if the real life value is greater than the individual components.

 

[charlie b]

Re: Short circuit rating of panel.

Originally posted by lquadros: What I have problem with is dealing with panel inspectors, who say that if any component in the panel is rated less than the available fault current, it cannot be certified as a safe installation.

Sorry, but I agree with them. It does not matter if the OCPD can handle a large fault current, if another internal component cannot. The OCPD will pass the high current (until it trips, and terminates the event), without itself being destroyed by the high current. In the mean time, the other component will see the same high current, and will not be able to survive it.

I see two options. One is to test the panel, as Ron suggested. The other is to perform an analysis that takes credit for any internal resistance between the OCPD and your "weakest link" component. It is possible, however unlikely, that the fault current that is available at that component may be small enough to be within its rating. I have never performed such an analysis on the internals of a panel, and I don't know if the AHJ would accept it. The test may be your best bet.

 

[petersonra]

Re: Short circuit rating of panel.

it's not ANY component in the panel.

you need to do an analysis of the short circuit current available at the terminals of each device and see if it exceeds that devices ratings.

UL has a procedure for making this analysis.


http://www.ul.com/tca/summer04/qa.html


Q: How is the short-circuit current rating (SCCR) of an industrial control panel determined?

A: In accordance with Supplement SB of the Standard for Safety for Industrial Control Panels, UL 508A, the individual SCCR of each component provided in the power circuits between the input to the control panel and all control panel outputs for external loads must be determined. Listed components, such as switches, fuses, fuse holders, circuit breakers, contactors, overload relays and variable speed drives, are marked with a SCCR established by tests conducted on representative samples of the component. If a component is unmarked, a minimum rating is assigned, or the component may be subjected to short circuit testing. Once the component ratings are known, the lowest SCCR for any component located in the power circuit becomes the overall short-circuit current rating marked on the panel nameplate. If this rating is insufficient, UL 508A also provides a number of methods for panel builders to improve the SCCR for an industrial control panel, including:

Use current limiting fuses or circuit breakers under specific conditions outlined in UL 508A
Include power transformer to supply low-power circuits, such as for fractional horsepower motors or non-motor loads
Conduct short-circuit testing of representative components on circuit of required capacity
Select alternate Listed components with appropriate SCCR
While the UL 508A requirements for marking a SCCR are not effective until April 25, 2006, UL has permitted industrial control panels to be marked with short circuit ratings based on the requirements in Supplement SB of UL 508A to satisfy the requirements of some customers or jurisdictions.

 

[rcwilson]

Re: Short circuit rating of panel.

Codes and standards require the panel to be rated at the lowest SC rating of any internal device. Sometimes the motor starters or contactors are listed at a higher SC rating when used in combination with a certain circuit breaker or fuse, but I doubt that your servo control vendors have spent the money to do that testing.

Another approach would be to get your clients to reduce the specified SC rating feeding your panel. Many times we engineers calculate a 57 KA fault level at the MCC and then require a 57 KA rating for anything connected to the 480V MCC. But if a 100A feeder is run to your panel using 72 feet of #2 awg wire, the fault level at your panel is less than 14 kA, just due to the impedance of the wire. [113 ft of #1/0 would do the same for a 150A feeder.]

Working with your clients to reduce the rating to the real world may be a way to save you and them some money without compromising safety.
.

 

[augie47]

Re: Short circuit rating of panel.

isn't that normally referred to as a "withstand rating" rather than an interrupting rating (on equipment)

The technicalities involved are one of the reasons TN requires all equipmemnt to be third party (NRTL) certified.

 

[kevinsarnold]

Re: Short circuit rating of panel.

Not knowing the layout and type of equipment it is difficult to give a solution. An isolation transformer is one way assuming that the transformer is 10kVA or less. Another method would be to use a "current-limiting" OCPD such as a fuse that would limit the available fault current to a value less than the short circuit current rating (aka withstand rating) of the component in question. However, according to UL508A SB Bulletin, the current-limiting device would have to be in the "feeder/main circuit" of the panel. If you are willing to share some specifics, you can contact our engineering group at Bussmann for further details as we are working on solutions as we speak.

{Moderator's Note: Edited to remove personal contact information. If you wish to contact this person, please send a Private Message.}

[ September 08, 2005, 10:39 AM: Message edited by: charlie b ]

 

[templdl]

Re: Short circuit rating of panel.

It also should be noted that the kaic rating of a UL listed enclosed control is included on a label affixed to the inside of the enclosure. As was previously stated the kaic rating is not simply based upon the main OCPD because the components located down stream will be subjected to any let-through current that occurs while the OCPD is clearing the fault. I don't believe that simply doing calculations to determine what the kaic of the assembly will be can get the job. The assembly must actually be tested. It's the same with series rating OCPDs which also must be tested.

 

[lquadros]

Re: Short circuit rating of panel.

In a mass production of assembling similar panels, testing of one panel may be possible. But in case of situations where every panel is unique, then we may have a problem with testing each one of these panels.

 

[Jeff Coggins]

Re: Short circuit rating of panel.

the marking for the short circuit rating rule does not come into effect until 2006 april. Any inspector should know this.