AIC fuse...now I'm confused

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DanZ

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Living in: Orient, OH ; Working in Central Ohio
After the last few jobs that required Arc Fault Current Calc's from the PoCo, my boss wants to know if there is some way we can fuse the wiring before or after the meter can that will take care of any arc fault issues, so we won't have to go back to the PoCo for more calcs.

I didn't bother trying to explain arc fault current to him, since he couldn't grasp why running wires sized for 500A to a wireway, then splicing (2) sets of 200A and a set of 100A wires with wire nuts wouldn't work.

So, aside from going back to the PoCo for more calcs, or severly up sizing the AIC rating of the panels, is there anything we can put in to take care of AIC issues, or should we go back to the PoCo. I'm thinking the PoCo would be the easy and cheap way, but not the fastest.
 
Huh? Now I am confused, what are you asking? The Poco gives you available fault current, not arc current, they should also give you an X/R ration for your fault current calculations, but arc fault current is not the same thing.
 
But if your basic question is an easy way to obtain the AIC for proper purchase of distribution equipment, you either need to get the info from the utility or install the highest AIC equipment available. In most cases a phone call will be cheaper.
 
Well thats easy, your utility should have someone assigned to this for commercial and Industrial projects, you should get to know that guy.

After you get your values there are ways to reduce the fault current if you have high values, for a dentist office you should not have any problems.
 
Well, there's two different people. One doesn't know anything about it, but they're in charge of "customer requests" and the other is the engineer. Either way, it takes about a week to get the numbers. Just another thing to plan on.

How can you reduce your fault current? The last few jobs had AIC ratings over 21,000A so we had to up size from the "standard" 10,000A AIC rated panels to 35,000A. I'm not sure what the cost difference is.
 
DanZ said:
Well, there's two different people. One doesn't know anything about it, but they're in charge of "customer requests" and the other is the engineer. Either way, it takes about a week to get the numbers. Just another thing to plan on.

How can you reduce your fault current? The last few jobs had AIC ratings over 21,000A so we had to up size from the "standard" 10,000A AIC rated panels to 35,000A. I'm not sure what the cost difference is.
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You do not ask for AIC information from your utility.

The utility provides Short Circuit Amps (SCA) or "fault current". You then use this fault current to determine which SCCR (short circuit current rating) you need for your panels and what AIC (amps interrupting capacity) of your protective devices need to be.

Most utilities provide a design (worst case) level of current so they simply look it up in their application book. In the vast majority of situations I get this number in one phone call (however getting anything other than the published value does take awhile).
 
jim dungar said:
You do not ask for AIC information from your utility.

The utility provides Short Circuit Amps (SCA) or "fault current". You then use this fault current to determine which SCCR (short circuit current rating) you need for your panels and what AIC (amps interrupting capacity) of your protective devices need to be.

Most utilities provide a design (worst case) level of current so they simply look it up in their application book. In the vast majority of situations I get this number in one phone call (however getting anything other than the published value does take awhile).
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I requested fault current info. It was what the inspector asked for. It took a week to get this.

Available Fault MVA (Symmetrical) L-G= 17. 8 MVA (2337A)

X/R Ratio = 3.71
X0/R0= 2.23

In letter form, for the inspector. Now that I have a direct line to a project engineer, it may not take as long to make it thru the chain of command, but I wouldn't want to plan it that way.
 
Sizing for equipment or PPE?

Sizing for equipment or PPE?

Be aware that most electric utilities will have published data for the available fault current based on the transformer size. It is normally given as the maximum based on the lowest impedance transformer they have in stock and assuming a bolted fault and infinite bus. You will not normally have the lowest impedance transformer, a bolted fault, or an infinite bus.

With this in mind, you can not pick out PPE from this data but you can select electrical equipment. :)
  • Bolted Fault. A fault with no impedance such as arcing.
  • Infinite Bus. A fault on the secondary side of a transformer will not cause the primary voltage to sag.
 
Which in many cases should/could result in distribution equipment with a higher AIC than may be necessary? Which other than from a cost stand point is not a bad thing?
 
charlie said:
With this in mind, you can not pick out PPE from this data but you can select electrical equipment. :)
  • Bolted Fault. A fault with no impedance such as arcing.
  • Infinite Bus. A fault on the secondary side of a transformer will not cause the primary voltage to sag.
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You can if you are using the tables. or rather you can determine IF you can use the tables with this value and knowing the max clearing time.

Most use the tables without having any clue if the system they are working on falls within the paramaters of the tables.
 
brian john said:
Which in many cases should/could result in distribution equipment with a higher AIC than may be necessary? Which other than from a cost stand point is not a bad thing?
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It could make coordination difficult. If you put in an overcurrent device that will open 100kA and there is only 18kA and there is a fault, how many cycles will it take for it to open and how is the downstream equipment going to react? :-?
 
zog said:
You can if you are using the tables. or rather you can determine IF you can use the tables with this value and knowing the max clearing time.
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The problem is that you can not know the maximum clearing time since most transformers are fused to protect the primary line and not the transformer itself. As an example, we fuse a 15 kVA transformer with a 15T fuse. A 15T fuse will carry 150% for an indefinite period of time, now do the calculations on how much kVA it will take to blow the primary fuse with a 7.62kV primary and a 15 kVA, 1? transformer. :)
 
charlie said:
It could make coordination difficult. If you put in an overcurrent device that will open 100kA and there is only 18kA and there is a fault, how many cycles will it take for it to open and how is the downstream equipment going to react? :-?
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Charlie, Could you give me the short story for that question.
 
Is it possible that Dan's boss was thinking of current limiting fuses, as they used to be (incorrectly) understood.

My understanding was for many years, 'current limiting fuses' were treated as reducing fault current downstream of the fuse, in _all_ circumstances, including for protection of downstream OCPD. This is no longer accepted practise, and 'series rating' would be required for this sort of use...but this sure sounds like what Dan's boss was describing.

-Jon
 
charlie said:
The problem is that you can not know the maximum clearing time since most transformers are fused to protect the primary line and not the transformer itself. As an example, we fuse a 15 kVA transformer with a 15T fuse. A 15T fuse will carry 150% for an indefinite period of time, now do the calculations on how much kVA it will take to blow the primary fuse with a 7.62kV primary and a 15 kVA, 1? transformer. :)
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Right, you do get that problem across a transformer, good point, you need to have the protective device and the know fault current on the same side of the transformer. That should be a note to the notes on the tables, it is common knowledege to some but maybe not so obvious to others.
 
charlie said:
The problem is that you can not know the maximum clearing time since most transformers are fused to protect the primary line and not the transformer itself. As an example, we fuse a 15 kVA transformer with a 15T fuse. A 15T fuse will carry 150% for an indefinite period of time, now do the calculations on how much kVA it will take to blow the primary fuse with a 7.62kV primary and a 15 kVA, 1? transformer. :)
Click to expand...
15T? Do you have a 1.5T maybe? We have 1,2,3 T etc but no 1.5. We do have a 1.5X but we would use a 2.5X or a 3 amp Kearney type K or T. The idea being to open in 5 minutes at 3 times full load.

The FLC of the 15 kVA at 7.62 kV is 2 amps. On my T curves, this has a 5 min minimum melt at 6 amps and 5 min total clearing at 7 amps.
 
winnie said:
Is it possible that Dan's boss was thinking of current limiting fuses, as they used to be (incorrectly) understood.

My understanding was for many years, 'current limiting fuses' were treated as reducing fault current downstream of the fuse, in _all_ circumstances, including for protection of downstream OCPD. This is no longer accepted practise, and 'series rating' would be required for this sort of use...but this sure sounds like what Dan's boss was describing.

-Jon
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I'm pretty sure that was what he was trying to describe to me. His theory is that that would be safer than getting panels with a higher AIC rating, since the PoCo can (and admits they) change their Fault Current ratings without notification.

On another note, when I handed the electrical inspector/plans examiner the letter from the PoCo, he did the same thing I did the first time I looked at it. He read the 2337A and took that to be the AIC rating.

Please bear with me on this. I'm not an EE, or an electrician. I'm just the drafter/project manager that needs to figure this stuff out so I can get a permit, while being safe and code compliant. That's why I'm here asking.
 
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