Modifying branch circuit SCCR

[petersonra]

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.

I have not found a case where anything over a 30 A CL fuse does me any good in a UL508a panel. The let thru is high enough it is not helpful.

Never quite understood why the CL fuse has to be in the feeder circuit either. This has led me to put two cl fuses in series so one is in the feeder circuit.

 

[ptonsparky]

What I know about class CC fuses is using them in a control circuit limits the damage to my equipment. A failed coil results in replacing a coil and the fuse vs coil, holding, stop, & accessories.

 

[Fred B]

Basically the need is to provide for the "weakest link" in the system, while accounting for the max demand. Motor loads are an example of high surge demands, and potentially modification of upstream devices that limit the current below the motor surge demands can negatively impact desired operation.
I don't think what OP is suggesting can be universally allowed or denied. Everything would have to be listed or studied to coordinate together. MFG may have criteria that limit or restricts use of CL, or may even indicate specified means of the CL.

A reduction to the extent as indicated by OP would seem impracticable at best. Why not just use the correctly sized breaker?

 

[Jraef]

The information provided by the fuse mfrs is intended to be used by ENGINEERS who are designing series rated combinations so that they can ensure ahead of time (with reasonable expectations) that it will pass the ever-so-expensive required testing for UL to allow for a Series Listing. Without that Series Listing, you have nothing useful.

In theory a PE can sign off on a Series Rating if they feel comfortable in accepting the liability risk if they are wrong, but personally, I have never met one who would in the few times I have tried (where I was trying to help someone out of a jam after a blunder).

 

[electrofelon]

The information provided by the fuse mfrs is intended to be used by ENGINEERS who are designing series rated combinations so that they can ensure ahead of time (with reasonable expectations) that it will pass the ever-so-expensive required testing for UL to allow for a Series Listing. Without that Series Listing, you have nothing useful.

In theory a PE can sign off on a Series Rating if they feel comfortable in accepting the liability risk if they are wrong, but personally, I have never met one who would in the few times I have tried (where I was trying to help someone out of a jam after a blunder).

Note we are not talking about series OCPDS, we are talking about a passive device that has a sccr or withstand rating. I see no requirement for a tested series combination in those cases.

 

[topgone]

Note we are not talking about series OCPDS, we are talking about a passive device that has a sccr or withstand rating. I see no requirement for a tested series combination in those cases.

However, the SCCR of all equipment served by a protective device must be higher than what the said protective devices lets through. It takes two to tango, they say!