Install Current Limiting Fuse to Reduce KAIC

Mr. Pickle

Mr. Pickle

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Rogers MN
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Electrical
Made up scenario for this discussion: SCCR of a chiller is lower than the available short-circuit current. example: AIC at chiller is calculated at 30kAIC, Chiller is rated for 10kSCCR. What to do? (don't get hung up on this question though, the real question is below)

I have been reading a lot of posts that really suggest that installing current limiting fuses in a circuit to limit KAIC downstream doesn't work. Fingers point to IEEE Buff Book 242-1986 article 2.3.6 which states that over current protection devices shouldn't be factored in to short-circuit calculations, unless a manufacturer has tested it as part of their system to reduce the KAIC to their equipment or to give them a higher SCCR value for their equipment. SKM models for short-circuit won't include over-current protection devices for this reason.

NEC says that a current-limiting device can be used in the circuit per designed and approved by a protection engineer. What software would do this, and if IEEE 242-1986 says this is not kosher, why does NEC say it is OK? A lot of posts I have read state that engineers would have to be nuts to put their license on the line to do such a thing, but is it really that complicated?

Then to add another twist, how is adding a current limiting fuse to reduce the KAIC upstream of a piece of equipment any different from putting in a fuse upstream to reduce available energy for arc-flash. Adding current-limiting fuses to reduce available current is completely OK with IEEE 1584 in determining incident energy. I just can't wrap my mind around how this would be any different at all?

I really need someone to explain to me what is OK, what isn't, why, what is different, and where should I be looking for more info?
 
First the terms are:
SCCR - the amount of short circuit current a device or equipment can tolerate until a protective device operates.
AIC - the amount of short circuit current a protective device can interrupt without self destructing.
SCA - the amount of short circuit current available at the point of the fault. This can go by other names such as AFC.

So current limiting devices reduce the amount and duration of a short circuit fault.
AIC can only be determined by the device manufacturer and testing to UL standards.
 
The NEC only allows a current limiting OCPD to to “reduce“ available for current, for existing equipment. See Section 240,86(A)
The dynamic impedance of any downstream over current device (such as in the starter) that is attempting to open when a fault occurs will mess with the ability of an upstream current limiting device to operate as the up over and down method would predict
 
Here is an earlier thread on the same discussion which might shed some light for you.................................

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
 
ron said:
The dynamic impedance of any downstream over current device (such as in the starter) that is attempting to open when a fault occurs will mess with the ability of an upstream current limiting device to operate as the up over and down method would predict
Click to expand...
I don't understand why the myth current limiting fuse are always the answer still exists. I was taught this at a Bussmann 'training university' back in 1993.
 
electrofelon said:
Ok so say I do it. Whatcha gunna cite?
Click to expand...
110.10 for the SCCR
110.3 for the behavior of the fuse.

Read the literature from the fuse manufacturers, for more than 40 years they have been saying the Up Over Down methodology only applies to conductors, like cables and busbars, not to equipment. UL listings require tested combinations of fuses and equipment except for some small control circuit fuses, <30A.
 
electrofelon said:
We are not talking about series ratings, the discussion is about SCCR.


Ok it says "reduces the current flowing
In the faulted circuit". Is that different?
Click to expand...
As mentioned by Jim, 110.10 requires stuff to be properly rated. The only way around that is series ratings in 240.86 "Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating"....
Other than series ratings, you are stuck with fully rated equipment per 110.10.
240.86 does not allow you to use the CL OCPD unless it is existing with a PE analysis (including me, I don't know of any PE's that would be willing to do this)
 
jim dungar said:
110.10 for the SCCR
110.3 for the behavior of the fuse.

Read the literature from the fuse manufacturers, for more than 40 years they have been saying the Up Over Down methodology only applies to conductors, like cables and busbars, not to equipment. UL listings require tested combinations of fuses and equipment except for some small control circuit fuses, <30A.
Click to expand...
That does not seem to be what is said in this document from bussmann. The document seems to be dated 2005, is this out of date?


For example it says,

It gives the specifier the necessary information regarding the short-
circuit current or withstand rating of electrical circuit components, such as
wire, bus, motor starters, etc. Proper protection of circuits will improve
reliability and reduce the possibility of injury. Electrical systems can be
destroyed if the overcurrent devices do not limit the short-circuit current to
within the withstand rating of the system’s components.
Click to expand...

Them further down it offers a more detailed list of "circuit components"

The following components will be analyzed by establishing the short-circuit
withstand data of each component and then selecting the proper current-
limiting fuses for protection:
• Wire and Cable
• Bus (Busway, Switchboards, Motor Control Centers and Panelboards)
• Transfer Switches
• HVAC Equipment
• Ballasts
• Circuit Breakers
Click to expand...
 
ron said:
As mentioned by Jim, 110.10 requires stuff to be properly rated. The only way around that is series ratings in 240.86 "Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating"....
Other than series ratings, you are stuck with fully rated equipment per 110.10.
240.86 does not allow you to use the CL OCPD unless it is existing with a PE analysis (including me, I don't know of any PE's that would be willing to do this)
Click to expand...
It seems the confusion is in the definition of "available fault current", which the NEC does not provide. Doesn't a properly selected CL fuse , when used ahead of a passive device, lower the AFC? Unless I am missing something, that is exactly what the NEC definition of CL fuse and the manufacturer literature says it does. Again 240.86 does not apply to a discussion on SCCR, I am not talking about two OCPD's in series.
 
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