SCCR rating, according to UL508A

I attended a workshop on UL 508A-compliant panel design yesterday, and now I'm even more confused regarding designing for a needed SCCR than I was before.

What I understand :
As long as you use listed/recognized components in the POWER circuit (control doesn't matter), you get the "courtesy" 5kA rating.

-We can get to lower available fault currents two ways:

1) Install a transformer in front of our panel. It's impedance lowers the available fault current. Sizing tables for the transformer are part of UL508A.


2) Install current limiting fuses, or current limiting breakers where power comes into the panel. This will allow us to use components rated greater than the peak let-through of a particular type and rating of fuse / CB, with one important exception.

And this is where my understanding goes off the rails:

Overcurrent protection devices (fuses/CB's) must be rated for the available fault current, regardless of whether they're downstream of a current limiting fuse/CB. Other components, like terminal blocks, contactors, and VFD's are fine. The engineering rationale behind this wasn't well explained, and I'm looking for a better understanding of why.

The theoretical example presented yesterday:

Source w/ 50kA available fault current ---> Class-J fuse w/ peak let-through of 8kA ---> CB with 35kA rating ---> downstream components w/ at least 8kA rating.

According to the presenter, this setup could only be rated for 35kA SCCR, due to the circuit breaker.

How is this possible? The peak let-through of our hypothetical Class-J fuse is only 8kA. Why does the downstream breaker even care? If the other components are OK, why not the overcurrent protection? Short of a bunch of transformers, how does one go about actually construct a compliant panel that has a higher SCCR without ridiculously oversizing every component? I'm not going to use a 250A contactor for a 1HP motor because it has a higher SCCR.

Also, can I "daisy chain" overcurrent protection? For instance:

Source w/ 50kA available fault current ---> Class-J fuse w/ peak let-through of 8kA ---> much smaller fuse with 4kA let-through ---> downstream components w/ 5kA rating.

I'd love an explanation, because when I asked, all I got was word salad and a lot of confused looks from the presenter. His answer was "those the rules" and moved on.


Thanks,

-rpoet

Good luck with this one. Check out this thread:

http://www.plctalk.net/qanda/showthread.php?t=98516

I have never gotten a satisfactory answer to his other than the typical "because UL says so" line.

We generally use Class J and Class CC fuses in out power circuit designs wherever possible with devices that have combinations ratings with those fuse classes. Many fuses in those classes as well as their fuseholders have SCCR values of 200kA so the SCCR of downstream overcurrent protection doesn't create a problem.

All I can figure is the fuse guys have a BIG presence on the UL508A committee.

Keith

Whaaaaat? I've never heard of that. I've always just assumed, like you guys, that anything downstream just had to be rated higher than the lowered available current from the current limiting fuse.

That makes no sense to me. I suppose one workaround is to ALWAYS use J and CC fuses, even downstream of others?

kamenges said:

Good luck with this one. Check out this thread:

http://www.plctalk.net/qanda/showthread.php?t=98516

I have never gotten a satisfactory answer to his other than the typical "because UL says so" line.

We generally use Class J and Class CC fuses in out power circuit designs wherever possible with devices that have combinations ratings with those fuse classes. Many fuses in those classes as well as their fuseholders have SCCR values of 200kA so the SCCR of downstream overcurrent protection doesn't create a problem.

All I can figure is the fuse guys have a BIG presence on the UL508A committee.

Keith

Click to expand...

I just read that thread, and the engineering rationale still makes no sense to me. Either the current-limiting overcurrent protection limits the let-through current, or it doesn't. I see plenty of people, myself included, using supplementary protectors after branch circuit breakers / fuses. According to UL's apparent logic, no one should ever be able to use a supplementary protector and get a SCCR above 0.2kA.

I guess I'm just supposed to use one of these (or even larger) for the convenience receptacle in the panel then?

https://www.automationdirect.com/ad...ric)/15A-125A_(BW125_Frame)/BW125JAGU-3P015SB

Ugh.


-rpoet

I apologize to jump in but can someone please help me!! I am currently preparing for MTR exam for UL 508a Panel shop. I have a doubt about SCCR calculation.

Suppose we have a Class J fuse in Branch circuit as the first OCPD and below that fuse we have motor protection, a control relay and a motor. The sccr rating of the fuse is 200kA, Motor protection is rated at 65kA and Control relay at 5kA. In this case do we have to consider the peak let through of the Fuse and modify the SCCR of the total branch circuit or is it OK to take the lowest value of the SCCR i.e. 5kA of the control relay? Thanks a lot in advance.

sajjadali said:

I apologize to jump in but can someone please help me!! I am currently preparing for MTR exam for UL 508a Panel shop. I have a doubt about SCCR calculation.

Suppose we have a Class J fuse in Branch circuit as the first OCPD and below that fuse we have motor protection, a control relay and a motor. The sccr rating of the fuse is 200kA, Motor protection is rated at 65kA and Control relay at 5kA. In this case do we have to consider the peak let through of the Fuse and modify the SCCR of the total branch circuit or is it OK to take the lowest value of the SCCR i.e. 5kA of the control relay? Thanks a lot in advance.

Click to expand...

If the control relay is not in the "power" circuit, then you do not need to consider it at all.

If the relay is being used in the power circuit, then it must be included. The entire concept of using current limiting fuses is for those who are doing SERIES LISTING of devices that are going to be submitted to UL for TESTING in series, you cannot simply apply them at-will, unless you follow that up with testing.

But MOST components used in power circuits HAVE ALREADY been series listed by the component manufacturers, you just need to access their data sheets on what OCPD it is listed with and at what level. If your component has NOT been series listed, then you are better off finding one like it that is. You COULD in theory submit your own, but it is VERY expensive as a one-off task so it is generally not practical to do so just to save the small amount of cost of changing components.

So if your "control relay" is actually being used in the power circuit, you likely will not find any higher SCCR for it even in series with fuses, because as a control relay, the manufacturer would not likely have gone to the expense. But if you can substitute it with a CONTACTOR, that likely IS going to have a series SCCR listing with fuses or with the motor protector.

You cannot use fuses to apply rating to downstream circuit breaking devices. Simple as that. You can use the peak fuse let through current on non-circuit breaking devices...contactors, terminal blocks...whatever. The problem is with downstream devices such as breakers or fuses opening up on their own due to current when the high current is needed to open the preceding device. Not really hard to understand.

Fuses NEVER EVER have an SCCR. They have interrupt ratings. That simple fact must be understood before even considering talking about SCCR.

LegacyLee said:

You cannot use fuses to apply rating to downstream circuit breaking devices. Simple as that. You can use the peak fuse let through current on non-circuit breaking devices...contactors, terminal blocks...whatever. The problem is with downstream devices such as breakers or fuses opening up on their own due to current when the high current is needed to open the preceding device. Not really hard to understand.

Click to expand...

I don't understand why it would be a problem if an over-current device in a branch circuit opens before a fuse located upstream in a feeder circuit. Can you explain why this would be a problem?

My understanding...which is most certainly incomplete.



If the fault current is higher than the downstream device's interrupting capacity, AND the downstream device opens before the upstream one, you have a dangerous condition where the downstream device is trying to interrupt a higher fault current than it's rated for.

And that makes sense. You don't want a breaker trying to open a current that is higher than its interruption rating.



But if the upstream device has a peak let-through current that is lower than the downstream device's interrupt rating, I don't see how the downstream device could ever be in a situation where it is trying to interrupt a current higher than what its rated for.



To me it seems like over-current protection devices should be protected by upstream current-limiting fuses the same as passive devices such as power distribution blocks, contactors, etc. But for some reason SB4 of UL508a does not allow this and I don't understand why.

Logically...I agree. But..if downstream devices can have faster trip/blow curves than the upstream ones they could open first. Or, under fault conditions, it may not be 100% predictable/guarantee-able that the upstream would open first.

But why would it even matter if the downstream device opens first if the upstream device limits the fault current to a value that is less than the downstream device's interrupt rating? I'm struggling to understand how that could be a hazardous condition.

We're talking millisecond (or less) timing between the devices. If upstream takes just 1 ms longer to open than downstream, that could be long enough to cause a dangerous failure.
BTW, I'm kind of speculating here, just trying to reason out a possible explanation. I would think that the upstream's peak let-thru should protect the downstream too. I'm trying to come up with an scenario where it wouldn't. Trip times is what I came up with, but there could be another reason. I don't have access to UL508 so I don't know if the standard goes into detail about the reasoning.

Originally posted by joseph_e2:

I don't have access to UL508 so I don't know if the standard goes into detail about the reasoning.

Click to expand...

Unfortunately it does not. It simply states what it states without any supporting rationale. Granted, that is pretty typical of standards.

The funny thing is, from a facility supply standpoint, this argument doesn't apply, at least as far as I know. Enclosures are given an SCCR so this number can be matched against the facility supply available fault current.
Lets assume an "enclosure in question" has a properly calculated SCCR of 20kA. The plant available fault current at the plant panelboard will be MUCH higher than this. Let's further assume an available fault current at the connection point of the enclosure in question of 15kA. Many things go into the available fault current calculation. So let's assume in Case #1 that the plant supply OCPD has a relatively high peak let-through (say 40kA) and this 15kA available fault current is largely the result of the torturous path the supply conductors take through the facility before they get to the enclosure. Then lets assume for Case #2 that the enclosure in question is mounted within 15ft of the plant supply entrance and that the 15kA is the result of a current limiting circuit breaker. As far as I know, for the purposes of enclosure connection, these two sources are treated the same from an available fault current standpoint.

If I'm not reading the standard right on this please let me know. I am on the OEM side so I am more familiar with SCCR than with AFC. But the first thing I hear plant guys talk about when AFC becomes an issue is changing the plant side OCPD.

Keith