Way OT: UL508A SB and branch circuit protection

[JordanCClark]

Let-through helps to protects things downstream. SCCR protects things upstream.

 

[kamenges]

Originally posted by JordanCCLark:

SCCR protects things upstream.

I don't understand what you are trying to say with this. SCCR defines what the component or system being fed is capable of handling. It has no effect on what happens upstream of itself.

Keith

 

[JordanCClark]

Sorry, Keith, been ill the past few days. What I may have thought was an epiphany was more likely Nyquil induced. I'm not even sure I know what I meant by that.

It's something I almost visualize, but it's eluding me at the moment.

 

[sajjadali]

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.

 

[JeffKiper]

I would have to go back and brush up for the fine details. If you want a quick answer the lowest let through is the winner so you only get to the 5kA (that looks low) I would verify if it is a control relay or a contactor.

If you allow 200kA to pass to a device with a 5kA things tend not to be to good for the end device.

 

[jridge]

I came across a caveat in the Littlefuse whitepaper on this topic that says
“It is important to point out that step 2 modifications apply only to current-limiting devices in the feeder circuit with passive (non-overcurrent protective) devices in branch circuits. In this example, based on table sB4.2, the 60 ampere Ul Class J fuse in the feeder circuit is able to limit the ipeak to 10 ka or less if the available fault current were 100 ka. If there are overcurrent protective devices in branch circuits and the available fault current is less than 100 ka, but greater than the interrupting rating of the branch circuit overcurrent protective device, the branch circuit overcurrent device may begin to open before the feeder fuse clears the circuit, possibly causing a hazardous situation. Therefore, the SCCR of the panel cannot be greater than the lowest interrupting rating of any fuse or circuit breaker in the panel, unless it has been tested and listed by a NRTL.”

 

[sajjadali]

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.

Thanks for the prompt reply, this is what I am confused about, I understand that the peak let through current of a 200kA Class J fuse is around 30kA which is still greater than 5kA but may be I didn't explained well my question. Let me rephrase it, If a branch has a fuse as branch circuit Protection Device than under that we have a motor protection CB which is also a branch circuit protection, now just because the last protection device from the bottom down is a fuse with a Ip = 30kA with 200kA SCCR, Lets say we have a contactor rated at 35kA instead 5kA, will the fuse change the over all SCCR of the branch to 200kA provided that it is part of the branch circuit. It is not a fuse in the feeder circuit. I hope I am not creating confusion this time.

 

[sajjadali]

Thanks for the prompt reply, this is what confuses me, what if there is a fuse with current limiting capacity in the branch circuit, will it be considered a part of the branch or the feeder? and if it is a BCPD, we will just take its SCCR rating as mentioned OR will we have to consider the let through current and see what happens.