Modifying branch circuit SCCR

Tip DS

Tip DS

I'm here.
Location
The Great Meme State
Occupation
Electrical Engineer
I am aware a branch device can have its SCCR 'modified' or raised through the use of class listed OCPD (generally a current limiting fuse) up-stream of the device, assuming the let-through current does not exceed the device's withstand current rating.

Here's the heart of my question: Can the same thing be done for the entire branch circuit? For example, let's say a branch circuit has a circuit breaker that protects a contactor, overload relay and motor. Imagine all the components down-stream of the circuit breaker have a SCCR of 100kA, or have been modified to meet 100kA SCCR. NOW, the circuit breaker that's up-stream from all the components only has a SCCR of 14kA. Can I put a 200kA interrupt rated fuse up-stream of the circuit breaker with a let-through current of less that 14kA, and have the entire branch rated at the 100kA of the rest of the modified branch?

In other words, can I have a "modified BRANCH SCCR" in the same way I make a modified component SCCR - with an up-stream fuse (not in the feeder circuit?)
 
Tip DS said:
I am aware a branch device can have its SCCR 'modified' or raised through the use of class listed OCPD (generally a current limiting fuse) up-stream of the device, assuming the let-through current does not exceed the device's withstand current rating.
Click to expand...
That is not correct.
 
Tip DS said:
There is no doubt that I could be mistaken. It seems like UL508a SB4.3.3 allows for this, as it relates to components protected by current limiting fuses.
Click to expand...
This only applies to UL 508a listed control panels, and only to current limiting devices installed in feeder circuits. It does not apply in general and basically never applies to ocpd unless they have been tested.
 
petersonra said:
This only applies to UL 508a listed control panels, and only to current limiting devices installed in feeder circuits. It does not apply in general and basically never applies to ocpd unless they have been tested.
Click to expand...
I probably should have stated initially that my question was in regards to control panels. I'm great at assuming the whole world can hear the voices in my head. LOL
 
electrofelon said:
This comes up a lot and I always point out that this is not prohibited by the NEC and the fuse manufacturers say you can do exactly that. However many of the engineers on this forum so you cannot do it. I wish the industry would come up with a straight answer on this.
Click to expand...
The fuse manufacturers do not say this. They are very careful not to.
 
My understanding is that the 'calculated let through' approach (often called 'up, over, down') was _previously_ thought to be an acceptable way to calculate the short circuit interruption rating of series OCPD.

However it was learned that this doesn't always work for systems with 'dynamic impedance' (for example a breaker opening and thus changing resistance from very low to very high). Thus 'series rated' OCPD systems are required to be tested combinations.

The up, over, down method IMHO is still applicable to systems that don't have dynamic impedance, for example short circuit ratings of bus bars. It is also a useful approach for selecting which combinations of fuse and circuit breakers should be tried out with actual testing.

Jon
 
petersonra said:
The fuse manufacturers do not say this. They are very careful not to.
Click to expand...

Please see the following document:

It states:
For currents equal to
or greater than its threshold
current, the fuse will limit the
current let-through to the
system.......Curves like this are
found in Eaton’s current-limiting
fuse application data and make
it easy to check the fuse let-
through against the withstand of
the equipment it is protecting.
Click to expand...
 
petersonra said:
But it does not state anywhere that you can use the let thru value of the fuse as a substitute for an OCPD of the proper rating.

You could potentially convince an AHJ that your example of, for instance, only protecting a bus bar, maybe they would accept it. The problem is that would be a highly unusual situation.
Click to expand...
I do don't see that we are using the CL fuse as a substitute for anything, and I am not proposing trying to increase the AIC of a downstream OCPD (as that is a dynamic device and not only does it not work, but is prohibited by 240.86). Every fuse manufacturer says a CL fuse limits the current to a certain value shown on their charts when properly selected. The NEC also provides a similar definition. So why can't I use it just like I would a transformer? No one specifically says I can do that either, but for some reason nobody questions that method.
 
electrofelon said:
I do don't see that we are using the CL fuse as a substitute for anything, and I am not proposing trying to increase the AIC of a downstream OCPD (as that is a dynamic device and not only does it not work, but is prohibited by 240.86). Every fuse manufacturer says a CL fuse limits the current to a certain value shown on their charts when properly selected. The NEC also provides a similar definition. So why can't I use it just like I would a transformer? No one specifically says I can do that either, but for some reason nobody questions that method.
Click to expand...
Suppose you have a CL fuse protecting a bus bar in the example cited. Just making some numbers up here. Suppose you have 50 kA AFC. Suppose the CL fuse has a let thru of 20 kA and the bus bar is rated for 10 kA. So looks fine. Suppose further downstream someone puts in a transformer with 22 kAIC CB protecting it. What protects the CB from the 50 kA AFC if the CL fuse cannot protect the CB? What happens to the bus bar if there is a bolted fault downstream of the CB until the CB opens because the CB and fuse are not series tested?
 
petersonra said:
Suppose you have a CL fuse protecting a bus bar in the example cited. Just making some numbers up here. Suppose you have 50 kA AFC. Suppose the CL fuse has a let thru of 20 kA and the bus bar is rated for 10 kA. So looks fine.
Click to expand...

Doesn't look fine to me. Did you mix up the 20 and the 10?

Suppose further downstream someone puts in a transformer with 22 kAIC CB protecting it. What protects the CB from the 50 kA AFC if the CL fuse cannot protect the CB?
Click to expand...
The 22KAIC CB is under dutied. We would need at least a 50KAIc CB there (assuming negligible drop from The wire impedance). Not sure why ocpds are being thrown in, we are not discussing downstream OCPDs as that is not acceptable both practically and by 240.86.

What happens to the bus bar if there is a bolted fault downstream of the CB until the CB opens because the CB and fuse are not series tested?
Click to expand...

See previous response, I'm not talking about ocpds in series.
 
I'm assuming you meant the fuse let through to be below the bus bar rating.

IMHO the downstream OCPD would not be suitable, and might fail catastrophically in the situation you describe. The opening OCPD might be subject to stresses related to the original available fault current.

IMHO the bus bar would only see stresses limited by the let through current of the fuse. The bus bar would be fine.

Though I admit that my understanding of this situation is weak. If the current never actually exceeds the let-throgh value, how can the apparent stress on the downstream OCPD exceed what that current can cause? My assumption is that this has something to do with the rate of change of current in a fault, but that is a just a guess.

Jon
 
I like how few power system engineers or equipment manufacturers participate in this line of discussion. This topic is usually brought up by electricians and control panel builders.

What does the UL standards for bus bars state about current limitation.
It appears UL 508A, for control panels, pretty limits the blanket "fuses are the best thing since sliced bread" to 30A and smaller.

I never found a situation where I was willing to put my stamp on a design not explicitly sanctioned by the subject equipment manufacturer.
 
jim dungar said:
I like how few power system engineers or equipment manufacturers participate in this line of discussion. This topic is usually brought up by electricians and control panel builders.

What does the UL standards for bus bars state about current limitation.
It appears UL 508A, for control panels, pretty limits the blanket "fuses are the best thing since sliced bread" to 30A and smaller.

I never found a situation where I was willing to put my stamp on a design not explicitly sanctioned by the subject equipment manufacturer.
Click to expand...
I'd stamp only things that I have scrutinized to be safe myself. That includes having approvals from nationally recognized organizations that test and monitor how products are made. Call me choosy but I'm just covering my butt!
 
You must log in or register to reply here.
Top