UL 508A short circuit calculation

[ZeroSeq]

Hello,

I am looking to provide a rating for an industrial control panel with solid state relays (using UL 508A Supplement SB).

I've been unable to find a reference to which standard the "available short circuit current" must be calculated.

When rating gear and breakers, it is obvious - ANSI equipment uses the ANSI C37 standards, IEC equipment uses IEC60909.

However, for UL listed equipment, I have not come across the reference for which method is required.

Does anyone happen to know this?




Thank you.

 

[jraef]

If the component has a tested and listed SCCR value, you use that. The information should be available from the manufacturer of the SSR. If not, you are relegated to what is colloquially called the "courtesy" rating of 5kA for untested components. Then of course if you have one component that has a 5kA SCCR rating, your entire panel is limited to 5kA. The best option is to shop around for an SSR that has a higher SCCR, or one that has a tested and listed "series rating" with a protective device already done by the manufacturer.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ZeroSeq]

Hi Jeff, thanks for the quick reply.

For the SCCR of the SSR, I've simply applied table SB4.1 to achieve the 5kA rating.

However, what I am looking for is if there is a standard required as per UL 508A for the available fault calculation?

For example, my ANSI rated circuit breaker requires that I use the ANSI C37 calculation methodology to acquire the available fault currents to compare against the breaker rating.

Is there a similar standard UL 508A points to? Or, is my simple lumped impedance calculation method acceptable?



Thanks

 

[wbd]

I think you have to use the ANSI method. I would suspect that if you model the entire system from the utility primary connection with the available fault current (not infinite bus), you may find 5kA will be exceeded.

 

[wroggent]

ANSI seems like it would be a good choice for applications using UL listed or recognized devices.

 

[LionelHutz]

UL508A is only concerned with developing the correct fault rating for control panels.

You would use a method that is generally well accepted in North America to calculate the actual available fault current.

Can you share the complete reference to the complete standard number and section in the C37 standards that says a certain method of calculating the fault current must be used? Can you also give the complete standard number that contains the calculation method.

Overall, I'm finding it very hard to believe that any of the ANSI/IEEE standards would says that a certain method of calculation must be used. I could believe they recommend a method lacking any other calculations, but not that they state a certain method must be used.

 

[ZeroSeq]

You bring up a good point, LionelHutz. After reading ANSI C37.010 and C37.13, I couldn't find a stipulation explicitly stating the calculation methodology as described herein MUST be utilized in acquiring the maximum available short circuit currents for the purposes of confirming acceptable equipment ratings.

I had always assumed that it was self evident that to adequately rate a circuit breaker built to ANSI standards, the ANSI calculation methodology must be used. This would ensure that the characteristics of the prospective fault current (DC time constants, AC decrement multipliers, induction load contribution, etc.) match that of the test conditions.

Good thing to think about.

 

[controlsdude]

I would look at all components SCCR ratings. Usually they come in a set variety as said above 5k, 30k, 60k, and 100k. Look at all your components and maybe visit your panel shop and have this discussion. That way you have a handle on what parts can be used together to get the same rating. Also, look at your PPE equipment minimums that your company looks at versus your customers. PPE and SCCR ratings work together and drive sometimes to same goals.