UL508A disconnect MCCB

[Hpram]

Hello,

I am new to America as an automation/electrical engineer and was hoping I could get some assistance in deciphering UL508A.


I have read through the NEC article 409, NFPA 79 and UL508A as I will be focusing on industrial control panels.
I have decided to build to UL508A.
I have got my head around most of it but have a general question around the main disconnect on the feeder side.
This is where the mains coming into the panel will be first terminated.

Does this need to be an MCCB or can it be a standard disconnect switch?

This will be feeding 4 x branch MCBs. Three will be feeding VFDs and the other a 24VDC control power supply.

Normally where I come from (New Zealand) we would just use a disconnect switch rated to the line current of all 4 devices and specify the incoming mains protection be adequate for the installation.

I have had a look over a few installations here in America and found the following:
1. The disconnect switch is a MCCB rated to the panel FLA feeding all branch circuits ( this would be my preferred way)
2. The same as 1 except the biggest load( largest VFD in the panel) does not have protection and relies on the main MCCB. ( This does not seem correct and I'm sure is from bad design and/or engineer).

Note these are just a handful from one company and do not reflect the American industry.

I'm not looking for a single answer but more what is practically found in the industry from experience.

Any answers appreciated.

 

[jim dungar]

This is a design decision.
I have seen panels with a disconnect only and panels with a breaker. The NEC required overcurrent protection will be at the source of the feeder to this panel.

 

[Hpram]

Thank you Jim

Regarding industry, what would most industrial plants prefer? Would there be a common preference?

 

[petersonra]

Thank you Jim

Regarding industry, what would most industrial plants prefer? Would there be a common preference?

Some places prefer mccbs because you don't have to find a replacement set of fuses in the middle of the night.

Some don't care.

Sometimes your choice is limited to fuses by the required sccr of the control panel.

 

[tortuga]

I have decided to build to UL508A.

Make sure you check out these links
goodbye-ul-508-a-transition-more-than-a-decade-in-the-making

https://www.ul.com/sites/g/files/qbfpbp306/files/2020-06/UL60947_external_FAQs.pdf

 

[petersonra]

Make sure you check out these links
goodbye-ul-508-a-transition-more-than-a-decade-in-the-making

https://www.ul.com/sites/g/files/qbfpbp306/files/2020-06/UL60947_external_FAQs.pdf

The transition the link refers to is about ul508 not ul508a. It's mostly done.

 

[Jraef]

Be careful about the SCCR when using MCBs. The rarely (if ever) have any series ratings.

 

[Hpram]

Thank you for all the feedback.

I have come across the UL508 to IEC transition and yes it refers to UL508 not UL508A.

We will be using Schneider ATV930 VFDs. I have referred to Annex NHA61583 for combination MCBs to meet the required SCCR ratings.
We wont be able to meet these as we also have a Schneider Lexium which is only 12kA and would require series limiting fuses to achieve anything higher. I will specify this on the specifications for when the client is purchasing the machine.
Is that a fair call?

Also, I think I will lean towards using an MCCB disconnect as opposed to a switch disconnect(non-circuit breaker). This would ensure the cable from the MCCB switch to the bus feeding the VFD and control MCBs is protected.

Please let me know any thoughts.

 

[petersonra]

The required sccr is determined by the available short circuit current. You can't design the panel unless you know that

 

[Hpram]

The required sccr is determined by the available short circuit current. You can't design the panel unless you know that

There will only be one panel design. It will be sold to customers all over the US. This would be a redesign every time.

 

[petersonra]

There will only be one panel design. It will be sold to customers all over the US. This would be a redesign every time.

If you want to sell a single design, you probably will need to pick a design that has a reasonably high sccr like 65 ka.

 

[Hpram]

Thank you Petersonra,

This isn't proving easy.

Do you have any recommendations for current limiting MCCBs we can fit on the feeder or current limiting devices for the LXM branch circuit?

 

[jim dungar]

I would think your Schneider rep should be able to help you select the their products.
Panel SCCR should be a regular issue for them.

 

[Jraef]

You can only use devices that are tested and listed TOGETHER. Look at your individual power devices, they should have data sheets that tell you what protective devices they have been tested and listed with and that the max. SCCR is for that combination.

Unfortunately, the Schneider Lexium 32 is only rated at 12kA max even WITH fuses. That makes it VERY difficult to install without someone proving that they have no more than 12kA of available fault current. That may require having a dedicated transformer ahead of it.

The Lexium 62 can have an SCCR of 50kA if you use their line choke unit. You may want to consider that as an alternative.

 

[Hpram]

Thank you Jref,

Can I use a current limiting MCCB on the feeder to raise the SCCR?
Or having a MCCB on the feeder with a SCCR of 100kA?

 

[petersonra]

Thank you Petersonra,

This isn't proving easy.

Do you have any recommendations for current limiting MCCBs we can fit on the feeder or current limiting devices for the LXM branch circuit?

You can't just add a current limiting fuse or cb and get a higher SCCR. UL has to bless that combination.

If the lexium servo drives do not work directly, it might be possible to add a transformer in the feeder circuit to that drive. You would need to look at the sccr supplement to ul508a to see what you can do.

There are other brands of servo drives that could be used that don't have this problem.

 

[Hpram]

Thank you very much everyone for your responses.
I’ve reached out to Schneider to discuss this.