When are CL breakers or fuses required?

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Ragingmain

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Please be gentle. First post. Not sure if I am in the right forum.
I am having issues understanding the requirements a new EE I haven't worked with before is placing on my current project.
The situation is as follows:
  • My company deploys DCFC of 60kW rated@ 480v 80A, and
  • for all of our previous installations, none of the previous EE spec'd CL fuses or breakers.
  • Our DCFC has an SCCR of 10k.
  • Not sure of the calculated fault current on this project, as the EE has yet to release the drawings, but he has indicated the AFC is higher than the DCFC rating. Which at 10K isn't hard to reach =)
  • The current switchgear does not have CL Breakers. It is currently being built, and changing breakers would require a change order the client is reluctant to pay.
  • I did provide him with time-current curves of the MCB and branch circuit breakers.
EE is asking for any of the following.
  • external disconnects with CL fuses. He states the internal disconnect with the class T CL installed fuses doesn't count.
  • new CL breakers.
I guess my questions are, and yes, I have these questions out to my EE.
  1. What section of the NEC requires CL, and when does it apply?
    1. is it for equipment protection or personnel protection when working on gear?
  2. Why doesn't the internally fused disconnect count? Isn't it there for a purpose? To protect the gear?
The reason for posting here is working with this particular EE has been quite arduous. Slow to answer, curt in his responses, etc. I am trying to educate myself as much as possible.
TIA for all your help.
 
Current limiting fuses or circuit breakers do not do you any good. If the piece of equipment is only rated at 10 k amps short circuit current rating, there is nothing externally you can do if there is more than 10,000 amps of available short circuit current at the terminals of the device.

The code does not require current limiting fuses or circuit breakers at all.

About all you can really do is add more conductor to reduce the available fault current so that it is less than the short circuit current rating of your piece of equipment. Or maybe a transformer in line. Or line reactors possibly. But current limiting anything isn't going to help.
 
110.9, 110.10, 110.24, 440.10, 285.7 all have requirements related to not exceeding the short circuit current rating of the equipment. 110.10 references use of selective coordination to limit fault current of a lower rated equipment. One means is to use CL fuse or breaker. Code is not specifying how you acheive the protection. Another means is the use of natural voltage drop of conductors length (Most times not practical). The CL device would need to be located at the higher rated source protecting the lower rated equipment.
If your source has a much higher sccr than the panel being installed, the higher AFC coming into your panel can cause catastrophic failure of the equipment that can cause injury or other issues. The use of the CL device at the source to limit the AFC downstream to you panel is inteneded to prevent catastrophic failure of the equipment. Sometimes there is a need to have a series of such devices to get down to the particular fault rating of the equipment installation.
AFA the customer not liking the added costs, they never do. But explaination of the safety concern should eleviate objection, but you have the obligation to provide the safe installation. The option to the customer should be either pay for a correct installation or not have it done.
 
petersonra said:
Current limiting fuses or circuit breakers do not do you any good. If the piece of equipment is only rated at 10 k amps short circuit current rating, there is nothing externally you can do if there is more than 10,000 amps of available short circuit current at the terminals of the device.

The code does not require current limiting fuses or circuit breakers at all.

About all you can really do is add more conductor to reduce the available fault current so that it is less than the short circuit current rating of your piece of equipment. Or maybe a transformer in line. Or line reactors possibly. But current limiting anything isn't going to help.
Click to expand...
Where does it say I cant use a CL fuse for SCCR purposes?
 
Look at the thread

HVAC Short Circuit Withstand Rating

So I have a chiller, and like almost all HVAC Equipment, it has a short circuit withstand rating of 5KA. It has a MCA of about 500 amps, so I'm obviously going to supply it with a system that has a much greater available fault current than 5KA, which is a code violation. I can add a fused...
forums.mikeholt.com
 
nhee2 said:
It says it reduces the current flowing in the faulted circuit. Are you saying it does not actually reduce the short circuit current?
Click to expand...
I'm saying while they reduce the fault current scene by most of the stuff downstream unless they have been UL tested and listed as a series combination with overcurrent protection devices downstream, it does not do you much good.
 
petersonra said:
I'm saying while they reduce the fault current scene by most of the stuff downstream unless they have been UL tested and listed as a series combination with overcurrent protection devices downstream, it does not do you much good.
Click to expand...
I see no reason to CL fuse cannot be used for downstream SCCR purposes on passive devices. If the code writers did not want that, perhaps they should just come out and say it,
 
electrofelon said:
I see no reason to CL fuse cannot be used for downstream SCCR purposes on passive devices. If the code writers did not want that, perhaps they should just come out and say it,
Click to expand...
If it could, Bussmann would be shouting it from the rooftops. But they aren't, because it doesn't work.
 
The Code uses the term “approved method” for the issue of determining the Short Circuit Current Rating of a piece of equipment. One option offered (besides series testing to UL standards) is for a PE to engineer a system and in that, they can use CL fuse let-through currents in coordination with damage curves and withstand ratings of devices down stream. But FINDING a PE willing to stake their license and livelihood on this for something they cannot test and where they must rely on data that they cannot verify themselves is not something I have experienced. This issue has been raised several times on projects I have been involved in, not once was anyone able to find a PE willing to tackle this and stamp it.
 
Jraef said:
The Code uses the term “approved method” for the issue of determining the Short Circuit Current Rating of a piece of equipment. One option offered (besides series testing to UL standards) is for a PE to engineer a system and in that, they can use CL fuse let-through currents in coordination with damage curves and withstand ratings of devices down stream. But FINDING a PE willing to stake their license and livelihood on this for something they cannot test and where they must rely on data that they cannot verify themselves is not something I have experienced. This issue has been raised several times on projects I have been involved in, not once was anyone able to find a PE willing to tackle this and stamp it.
Click to expand...
Are you referring to 240.86? But doesn't that apply only to circuit breakers used where AFC is greater than AIC of the breaker? Then listed series ratings or PE evaluation is required. Is there a similar requirement somewhere for passive device SCCR, as electrofelon describes? [i've never really considered this nuance until reading electrofelon's reference to 240.2 definitions in the earlier post]
 
Jraef said:
......not once was anyone able to find a PE willing to tackle this and stamp it.
Click to expand...

Meet Mr @mayanees. He may be happy to assist you with your next project.

Fused vs. non-fused disconnect

I'm gonna disagree with that statement, and I use the following chart as evidence: I also refer back to 240.86(A) which allows an engineer to permit the use of fuses for existing installations where the fault current exceeds the rating of a breaker. A fuse can be used to permit such an...
forums.mikeholt.com
 
nhee2 said:
Are you referring to 240.86? But doesn't that apply only to circuit breakers used where AFC is greater than AIC of the breaker? Then listed series ratings or PE evaluation is required. Is there a similar requirement somewhere for passive device SCCR, as electrofelon describes? [i've never really considered this nuance until reading electrofelon's reference to 240.2 definitions in the earlier post]
Click to expand...
No there isn't. This comes up all the time and everybody jumps on the two ocpds in series bandwagon,, my understanding is that is not what we are talking about here. We are talking about the second device being a passive non-ocpd.
 
electrofelon said:
Meet Mr @mayanees. He may be happy to assist you with your next project.

Fused vs. non-fused disconnect

I'm gonna disagree with that statement, and I use the following chart as evidence: I also refer back to 240.86(A) which allows an engineer to permit the use of fuses for existing installations where the fault current exceeds the rating of a breaker. A fuse can be used to permit such an...
forums.mikeholt.com
Click to expand...
electrofelon 🤣
I've been faced with the situation allot because over the past 8-10 years we've done arc flash labelling for close to 200 schools in two counties in Virginia, and the fault current analysis showed that some of the schools had underrated equipment. So I/we as a company, and I think as responsible PEs, spend their money as if it's ours and go to great lengths to pass the equipment. I think we have that duty to the client, to interpret and apply technical fundamentals.
As the responsible PE I'm not gonna do anything that puts my license in jeopardy. But if there's a way to rationalize it, I'll do whatever I can to find it.
 
nhee2 said:
Are you referring to 240.86? But doesn't that apply only to circuit breakers used where AFC is greater than AIC of the breaker? Then listed series ratings or PE evaluation is required. Is there a similar requirement somewhere for passive device SCCR, as electrofelon describes? [i've never really considered this nuance until reading electrofelon's reference to 240.2 definitions in the earlier post]
Click to expand...
I am inclined to agree that there is a good chance that passive devices like switches and bus bars might well be protected by the let thru value of a current limiting device. But I am not convinced the code allows for it.


110.10 Circuit Impedance, Short-Circuit Current Ratings, and
Other Characteristics. The overcurrent protective devices, the
total impedance, the equipment short-circuit current ratings,
and other characteristics of the circuit to be protected shall be
selected and coordinated to permit the circuit protective devices
used to clear a fault to do so without extensive damage to
the electrical equipment of the circuit. This fault shall be
assumed to be either between two or more of the circuit
conductors or between any circuit conductor and the equipment
grounding conductor(s) permitted in 250.118. Listed
equipment applied in accordance with their listing shall be
considered to meet the requirements of this section.
Click to expand...
The CL device is not being protected.

110.24 Available Fault Current.
(A) Field Marking. Service equipment at other than dwelling
units shall be legibly marked in the field with the maximum
available fault current. The field marking(s) shall include the
date the fault-current calculation was performed and be of
sufficient durability to withstand the environment involved.
The calculation shall be documented and made available to
those authorized to design, install, inspect, maintain, or operate
the system.
Click to expand...
 
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