Need help

[saltyflicker]

All right guys, here it is;;

One of my jobs has an interesting situation. An inspector has made a call which I think is incorrect. He says that my 200-amp Cutler Hammer subpanel's 10k AIC rating must match my 1200-amp RSE Sierra switch gear's 42 AIC rating...even though I am feeding the subpanel with 200-amp type T fuses. My understanding of the code is that if type T fuses are used the AIC rating is good from 10K to 200K. Am I wrong?

Thanks,

Salty Flicker

 

[ron]

Re: Need help

The type of fuses that you are using upstream for protection of the downstream subpanel, has no bearing on the required AIC rating of the subpanel. You must calculate the required AIC for the subpanel utilizing the source and downstream impedances to the subpanel.
Sometimes you may be able to get a series rating with the upstream fuse and downstream breakers, but you must check with the manufacturer. A series rated combination still has the net effect as the high AIC rating.

 

[charlie b]

Re: Need help

Here?s a quick way to get an idea of whether a 10K AIC rating is sufficient: Take the line-to-line voltage at the 1200 amp board?s rating (presume that you have full voltage there, no voltage drop from the source to that board), and divide it by the resistance (in ohms) of the feeder to the 200 amp panel (include both the phase and neutral conductors). If the answer is less than 10,000 amps, you are done. If not, the problem gets more complicated. You can take credit for the resistance of the utility?s system and the resistance of the service cables. But you may need a software package and an engineer?s help to get an official answer.

 

[roger]

Re: Need help

Ron and Charlie b, why would this be an issue for an "inspector"? Wouldn't a "fully rated" or "series rated" system be an engineering problem that couldn't be designed and signed off on (stamped) by an EC or Electrical Inspector.

If an inspector is requiring this, isn't he putting himself in some "Engineering" liability issues?

Just asking.

Roger

 

[templdl]

Re: Need help

The type 'T' fuse must be listed in combination with the down stream panel.
However, I do see C-H laodcenters that are rated 42 and 100kaic without using the type 'T' fuse. These High interupting main breaker laod centers use CHB4 and CHH main breakers which are series rated with CH, CHT and CHP breakers.
So consider eliminating the fusing and replacing the panel with a C-H loadcenter with a CH32(or 42)H200(or 225)L(or R) that use the CHH main breaker.

 

[jim dungar]

Re: Need help

Roger,

The NEC (110.15?) says that ALL devices intended to interrupt current must be rated for the fault current at their line terminals. Enforcement of this article is part of the inspectors job.

It is a very widely held and totally inaccurate belief that AIC ratings are a concern at the service entrance OCPD only.

 

[roger]

Re: Need help

Jim, the article is 110.9 and it says " Equipment intended to interupt current at fault levels shall have an interupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment.

Equipment intended to interupt current at other than fault levels shall have an interupting rating at nominal circuit voltage sufficient for the current that must be interupted..

It is a very widely held and totally inaccurate belief that AIC ratings are a concern at the service entrance OCPD only.

I don't agree, I think the term "Series Rated", as in system, puts that to bed


Roger

[ February 25, 2004, 03:37 PM: Message edited by: roger ]

 

[engy]

Re: Need help

What is the available fault current? 10k, 40k ,etc.

How long is the feeder?

The inspector cannot make that call without a fault study (what he/she should have asked for initially)

They make current limiting devices for a reason.

Bussman JJS (type T) will have a 9,000Arms let thru on 40,000Arms symmentrical available. (200A 600V)

 

[engy]

Re: Need help

That being said, it is not as simple as just doing the math. The combo's need to conform to listing requirements.

 

[roger]

Re: Need help

Michael,

The inspector cannot make that call without a fault study (what he/she should have asked for initially)

and

it is not as simple as just doing the math.

thanks, this is what I'm getting at.

I can't see how this is an ispectors issue with out the numbers already being part of an EE's design.

Roger

 

[charlie b]

Re: Need help

Roger: What the Inspector can do is require that evidence of a completed fault study be presented for his review. It might be beyond the Inspector?s province to call into question any technical aspect of the fault study, but he can ask to see it. The Inspector can also compare its results against the AIC rating of the various panels.

Let me add that a blanket statement to the effect that the AIC rating of a sub-panel must match that of the upstream panel is utter nonsense.

 

[Ed MacLaren]

Re: Need help

They make current limiting devices for a reason.

a blanket statement to the effect that the AIC rating of a sub-panel must match that of the upstream panel is utter nonsense.

I tend to agree with the above quotes, but I have heard people argue that current-limiting fuses/breakers do not reduce the available fault current seen by downstream equipment.

I would like to hear opinions from the EEs in the forum as to whether the installation in the sketch is acceptable from an engineering (not code) perspective.

Ed

 

[charlie b]

Re: Need help

Ed: I am an EE, but I have not worked with current-limiting fuses, and specifically I have not tried to take credit for one as the only method of ensuring that the available fault current is lower than a panel?s rating. But let me ask first, is there a typo in your drawing? Did you mean to show a 13kA let-through current from the fuse and a 13kA available current at the next panel?

The reason I ask is that I expect the fault current that is available at any given panel to be lower than that available at the upstream panel. The reason is simply that the impedance of the feeder reduces the current available downstream.

Let me ask my question another way. If you were to draw your sketch again, this time without current-limiting fuses, is it your intent that the available fault current at Switchgear A is above 30kA, and that it is only the presence of the current-limiting fuse that reduces the available fault current to a value lower than 30kA?

 

[jim dungar]

Re: Need help

Ed,

It is all but impossible to series rate two current limiting devices. Definitely, there are too many dynamics involved for a simple mathematical analysis.

For a device to be current limiting it must "open" completely. So, if only one fuse opens having the second one in the circuit is irrelevant. If both fuses begin to open then the current level may be reduced, due to "dynamic impedance", to level that affects (slows down) their opening times. If the fuses take longer to open then their current limiting ability is reduced and the resultant let-through energy will be higher.

This dynamic impedance is the same reason that the old "up over and down" method is not valid for protecting miniature molded case circuit breakers (this method was only valid for "iron frame" power circuit breakers).

 

[wilson9254]

Re: Need help



[ February 26, 2004, 06:15 PM: Message edited by: wilson9254 ]

 

[Ed MacLaren]

Re: Need help

Thanks Charlie and Jim, for the comments.

The sketch is one that I?ve been using during system protection seminars, where the principles of current-limiting fuses were discussed. I realize that I need to learn more about the subject, and just wondered if the sketch made sense from an engineering viewpoint.

is it your intent that the available fault current at Switchgear A is above 30kA, and that it is only the presence of the current-limiting fuse that reduces the available fault current to a value lower than 30kA?

That is what I have believed up to this point.

For example, let?s say a new service and transformer have increased the available fault current (AFC) at the transformer to 50 kA, and the existing Switchgear A has a withstand rating of only 30 kA. Why wouldn?t the main service current-limiting fuses (X) with a let-thru current of, say 17 kA, permit continued use of the existing Switchgear A?

If not, how can the Bussmann company make these statements?

?Modern current-limiting fuses, under short-circuit conditions, can force the current to zero and complete the interruption within a few thousandths of a second.?

?Because single-element, fast-acting fuses have a high speed of response to short-circuit currents, they are particularly suited for the protection of circuit breakers with low interrupting ratings.?

?When the short-circuit current is in the current-limiting range of a fuse, it is not possible for the full available short-circuit current to flow
through the fuse ? it?s a matter of physics.?

Ed

 

[saltyflicker]

Re: Need help

Thanks a million for everybody's input! The situation was resolved today with the inspector. The first point was that the Cutler Hammer Type BR breaker I'll be using is rated for 100K. The type T fuse is rated for 200K. Needless to say I was more than covered. Also it seems that the inspector never looked inside the main switchgear, so he did not realize it was a type T fuse. So, all's well. Thanks again.

Salty Flicker