Is my SCCR rating OK?

[wireman]

As much as I try I still have difficulty with determining SCCR ratings. So here's my question:

My customer needs my control panel with a VFD and some control components inside to comply withe an 85kA rating.
The VFD is only rated at 65kA.

Since my disconnect, main fuses, fuseholders, & distribution block and branch fuses protecting the VFD are all rated at 200kA am I meeting their 85kA spec? If not, what else should I do?

Also, am I correct here: since power supplies and control transformers are exempt all of the control components underneath them are exempt too?

Thanks in advance for any responses.

 

[petersonra]

As much as I try I still have difficulty with determining SCCR ratings. So here's my question:

My customer needs my control panel with a VFD and some control components inside to comply withe an 85kA rating.
The VFD is only rated at 65kA.

Since my disconnect, main fuses, fuseholders, & distribution block and branch fuses protecting the VFD are all rated at 200kA am I meeting their 85kA spec? If not, what else should I do?

Also, am I correct here: since power supplies and control transformers are exempt all of the control components underneath them are exempt too?

Thanks in advance for any responses.

Presumably you are using the supplement to UL508a to calculate the short circuit current rating of your cabinet. I would point out that if the UL short circuit testing shows there's no way to get past 65 kA on the vfd that is pretty much it. You might want to read carefully the section in the supplement that talks about reducing the Downstream short circuit current by applying current limiting devices in the feeder circuit. It is a confusing and convoluted section but it may offer you some hope.

I would also point out that using the supplement forces you to also comply with everything else in UL 508a. You don't get to pick and choose what parts of the specification you get to follow.

 

[Jraef]

Presumably you are using the supplement to UL508a to calculate the short circuit current rating of your cabinet. I would point out that if the UL short circuit testing shows there's no way to get past 65 kA on the vfd that is pretty much it. You might want to read carefully the section in the supplement that talks about reducing the Downstream short circuit current by applying current limiting devices in the feeder circuit. It is a confusing and convoluted section but it may offer you some hope.

I would also point out that using the supplement forces you to also comply with everything else in UL 508a. You don't get to pick and choose what parts of the specification you get to follow.

This is all correct, but I'll add that there are a FEW VFDs out there that are listed at 100kA SCCR using Class J current limiting fuses (never circuit breakers though). You may have to select your drive brand based on that however, because as Bob says you can't pick and chose, which means you can't "roll your own". They are TESTED series combinations and you must be able to prove your claim, typically by providing data sheets from the drive mfr. So if you have a favorite brand but they stopped their SCCR listing at 65kA, you have to punt in the way Bob described.

 

[templdl]

You may consider an air core reactor. As an equipment manufacturer sales and applications engineer I have supplied smaller air core current limiting reactors for MCCs OEMs to as large as those that needed a 10' x10'x10' aluminum enclosure for each of the three phases that had to be shipped on 3 individual flat bed trucks across the country.
The use of air core reactors must be considered when addressing the problem of the increased fault current availability that is becoming more common with your upgrading of the modern distribution systems.
Consult with a air core reactor manufacturer advising then of the. Size of your load and what the source ka is and what your load limitations are and they should be able to provide you with a price for it.