AIC compliant

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I have a chiller rated for 5k AIC the calculation at the point of connection it is 14,700 AIC what can I do to comply with AIC rating I was thinking a fast acting fuses but it will bring the AIC to 7.3 k what also is the right way to do it
The chiller circuit it is 480v 150a 3ph
 

kwired

Electron manager
Location
NE Nebraska
5k is default for when they didn't do any testing.

Possible you can get tested (or they already have tested it but haven't marked it with the results).

Otherwise longer circuit conductors, transformers, line reactors, etc. to lower impedance of the supply circuit.
 

charlie b

Moderator
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Location
Lockport, IL
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Retired Electrical Engineer
For starters, the term "AIC" only applies to overcurrent devices. The "I" word is "Interrupting." The appropriate terms are Short Circuit Current Rating (SCCR) of the equipment and the Short Circuit Current Available (SCCA) at the fault location.

But my real question is how you came to believe that a fast acting fuse will alter the amount of short circuit current that would be flowing during a fault condition. It may reduce the amount of time before the fault current stops flowing. But it will not change the amount of current that will flow while the fault condition is present. That number depends only on the voltage source(s), the wire sizes and lengths, and any large motors that could contribute current to the fault location.

Also, do not be too quick to rely on what is called a "current limiting fuse." There are factors that can influence its ability to perform its current limiting function.
 

GoldDigger

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Location
Placerville, CA, USA
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For starters, the term "AIC" only applies to overcurrent devices. The "I" word is "Interrupting." The appropriate terms are Short Circuit Current Rating (SCCR) of the equipment and the Short Circuit Current Available (SCCA) at the fault location.

But my real question is how you came to believe that a fast acting fuse will alter the amount of short circuit current that would be flowing during a fault condition. It may reduce the amount of time before the fault current stops flowing. But it will not change the amount of current that will flow while the fault condition is present. That number depends only on the voltage source(s), the wire sizes and lengths, and any large motors that could contribute current to the fault location.

Also, do not be too quick to rely on what is called a "current limiting fuse." There are factors that can influence its ability to perform its current limiting function.

:thumbsup:

The fundamental mechanism of a current limiting fuse is that it opens quickly enough if a short occurs to prevent the current through the short (which by definition starts at zero at t=0) from rising above its limiting rating before the fuse element successfully interrupts the circuit.
That is a far easier job if the short occurs at the time of a voltage zero crossing or if there is significant inductance in the circuit. (Significant at the fault current level, even is negligible at normal load currents.)
As Charlie noted, there are a lot of things that can make it very difficult or even impossible for such a fuse to do its job.
Among other things, its action can also be influenced by the time varying impedance of a downstream OCPD which is attempting to open.
 

DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
Yeah class rk1 preferably. AIC is different than withstand rating. Look at the trip curves for rk1 fuses published by the manufacturers to figure out if it will limit your fault current to below the withstand of your chiller.

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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I have a chiller rated for 5k AIC the calculation at the point of connection it is 14,700 AIC what can I do to comply with AIC rating I was thinking a fast acting fuses but it will bring the AIC to 7.3 k what also is the right way to do it
The chiller circuit it is 480v 150a 3ph

To be candid the right way is to replace the control panel with one that is rated in excess of 14.7 kA SCCR, or as another poster mentioned insert longer conductors or something else in the feeder to reduce the available short circuit current.

It is also possible that the calculations are not correct, or are overly conservative.

Most places would just install it and not worry about it.

The owner bought something that is not appropriate and it is really his problem.
 

packersparky

Senior Member
Location
Wisconsin
Occupation
Inspector
Yeah class rk1 preferably. AIC is different than withstand rating. Look at the trip curves for rk1 fuses published by the manufacturers to figure out if it will limit your fault current to below the withstand of your chiller.

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FWIW, I have had this conversation with engineers from Cooper/Bussman and UL and was told current limiting fuses have not been evaluated as a method to lower the fault current to meet a SCCR rating.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
FWIW, I have had this conversation with engineers from Cooper/Bussman and UL and was told current limiting fuses have not been evaluated as a method to lower the fault current to meet a SCCR rating.

They have, however, been evaluated as a method to meet the AIC limit of an OCPD, yes?
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
They have, however, been evaluated as a method to meet the AIC limit of an OCPD, yes?
When TESTED and listed in a series circuit with them, yes. You cannot apply them on your own in the field and have that get you an appropriate SCCR.

Really, it comes down to what petersenra said, especially the last part:
To be candid the right way is to replace the control panel with one that is rated in excess of 14.7 kA SCCR, or as another poster mentioned insert longer conductors or something else in the feeder to reduce the available short circuit current.

It is also possible that the calculations are not correct, or are overly conservative.

Most places would just install it and not worry about it.

The owner bought something that is not appropriate and it is really his problem.

If you are the electrician, there is nothing you can do NOW to rectify this after the fact, short of the suggestions on adding impedance to the circuit, making sure the calcs are correct, and suggesting he start over with his supplier.

Often times the REASON it was such a bargain is because it is coming from a supplier who doesn't know (or care) about getting the proper SCCR ratings. I'd also hazard a guess that the control panel for this chiller isn't UL listed, because the process of UL listing it would have all but FORCED them to get a proper SCCR or else actively ignore UL's emphasis on this issue now, which opens them up to some measure of culpability and a recourse path for the buyer.
 

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
I would look at the report that is saying that you have 14.7kA at that point in the circuit and see if the starting fault current is infinite bus maximum fault current or if actual available fault current from the utility was used. If the former was used you may find that the available fault current may yield lesser values.
 

ron

Senior Member
5k is default for when they didn't do any testing.

Possible you can get tested (or they already have tested it but haven't marked it with the results).

Otherwise longer circuit conductors, transformers, line reactors, etc. to lower impedance of the supply circuit.

This is the best answer.

In the 15kA range, a fast acting fuse is likely not in its current limiting range and will not be so fast acting. You can't count on a fuse to lower fault current or assist another device to withstand unless it has been tested as an assembly to do so at that new SCCR.
 

DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
FWIW, I have had this conversation with engineers from Cooper/Bussman and UL and was told current limiting fuses have not been evaluated as a method to lower the fault current to meet a SCCR rating.
Really? I have always use current limiting fuses for coordination

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DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
They can't be used to increase the SCCR of equipment unless tested. The old up-over-and-down method will not work.
Are you saying that it has to be and engineered series combination?

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jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
Really? I have always use current limiting fuses for coordination

Among other issues; for an upstream fuse to limit current it must open, while for coordination it must stay closed.

When reviewing trip curves, it is important to remember that you cannot infer anything that is not shown on the curve. Once a device crosses the lower axis (typically 0.01sec) the curve is no longer applicable. So when the available fault current falls to the right side of all devices, actual testing of the devices is required to determine coordination.
 

DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
I've been doing some further reading on the subject. The problem looks to be that of dynamic impedance in older circuit breakers. The breakers have a tendency to insert a temporary impedance in the circuit the spooling the Upstream current limiting fuse into not tripping.

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DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
Among other issues; for an upstream fuse to limit current it must open, while for coordination it must stay closed.

When reviewing trip curves, it is important to remember that you cannot infer anything that is not shown on the curve. Once a device crosses the lower axis (typically 0.01sec) the curve is no longer applicable. So when the available fault current falls to the right side of all devices, actual testing of the devices is required to determine coordination.
I understand. I have been reading UL 489

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DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
I've been doing some further reading on the subject. The problem looks to be that of dynamic impedance in older circuit breakers. The breakers have a tendency to insert a temporary impedance in the circuit the spooling the Upstream current limiting fuse into not tripping.

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*fooling

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mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
240.86 A or B

240.86 A or B

This is the best answer.

In the 15kA range, a fast acting fuse is likely not in its current limiting range and will not be so fast acting. You can't count on a fuse to lower fault current or assist another device to withstand unless it has been tested as an assembly to do so at that new SCCR.

... actually kwired meant to say that the addition of cable increases the impedance of the supply circuit, thereby lowering the available fault current.

There are only two ways this could be installed, per 240.86. One is the PE-approved 240.86(A), but that's only good if it's an existing device that has the upstream upgraded and is now overdutied. If that was the case I would look at a Cooper Bussmann CLF fuse LPS-RK_SP 150A which shows a published let-through current of 3500 amps (interpolated) for a supply current of 15kA. Yes the up-over-down method is no longer recognized, but the published let-through curves are. Then in order for me as a PE to approve the application, I'd have to overlay the new ocpd with the opening opening curve of the clf fuse and make sure the device remained passive during the opening period.
But this isn't an existing device so you need to use 240.86(B) and find an upstream fuse that is tested/listed with the ocpd in the panel.
 
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