rpoet
Member
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
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