AIC ratings >200kA

[thinfool]

I installed 2 - 6000amp services in NYC several years ago...the equipment was rated for 200,000. To compensate for the unlimited aspect, we provided 'cable limiters' which looked like a fuse crimped onto each se conductor. As I remember, they added a current limiting aspect to the circuit ahead of the main switch/fuses.
I think they were made by Bussman.
Check with ConEd, they should be able to point you in the right direction, as should a good PE who regularly designs systems in NYC.

check here.

 

[ron]

Thin,
Thanks for the suggestion.
Luckily or unlucky ..... The engineer that is familiar with Con in NYC is me
Usually they use (6) 2000kVA transformers to serve many of the buildings I design, but this time it will be (6) 2500kVA.
For the 2000kVA xfmr design, their standard response to the fault current question was always 200kAIC. Now that the transformers are larger, they don't have such a definite response, which it why I've calculated the values that I have.
A current limiting fuse (cable limiter) would only help reduce downstream fault ratings if I had lab tested series ratings as required by code to be >200kAIC.
The downstream network of protective devices are dynamic (faster than the current limiting fuse) and not passive (slower than the fuse), so that I cannot count on the cable limiter in every instance predictably.

 

[ramsy]

When you have Z's in parallel your statement is correct. We do not have Z, we have %Z. Same rules do not apply.

Using Z = (%Z * E) / I, for (Ohms / 6), I still get 0.96 %Z = (I * Z / E) for the Xfmr bank, on both primary & secondary buses.

Plugging 1k ft of 13.8kvac upstream-feeder impedence, into a 0.96 %Z, 15000KVA, Pri., 50kA finite, SCC manages nothing less than 750kA SCC after the 10 ft, 480vac, bus feeders.

Do they make fuses larger than 300ka AIC for 6ka running? I realize proper ANSI & international STD's require certified verification of cable metrics and ASCC calcs, but unless we prove otherwise this rough estimate suggests more impedence may not be available in the network.

 

[ron]

Roger,
The fuses may be listed for 300kA, but I need a switch that they are rated with at 300kA. All switches that I've seen take the 300kA fuse but are only listed to 200kA.

 

[ramsy]

A current limiting fuse (cable limiter) would only help reduce downstream fault ratings if I had lab tested series ratings as required by code to be >200kAIC.
The downstream network of protective devices are dynamic (faster than the current limiting fuse) and not passive (slower than the fuse), so that I cannot count on the cable limiter in every instance predictably.

Ron, that fuse mentioned above has a min 10 sec. 10% current-limiting delay, if your sure you're near 300kAIC.

 

[ramsy]

All switches that I've seen take the 300kA fuse but are only listed to 200kA.

Are those switches required to open energized? Arn't the switches designed to open after the fuse blows? If the switch AIC ratings are higher before opening, that may explain the mismatch.

 

[kingpb]

Look in ther Cutler Hammer Consulting Application Guide, it is also available on line. They have a whole section that describes the basics of Networks and equipment. The difference between spot networks, and networks depends on whom you are talking too, but either way the equipment is the same. I also checked with out distribution guys, they say that the transformers supplied, usually have a higher impedance to help limit fault current.

My take is that it seems a bunch of current limiters are used to reduce the fault current to acceptable levels.

Wish I could be of more help, but I can say this is one thing I have never done a design for.

 

[ron]

The switch must be able to withstand the full fault until the fuse clears. It gets a rating as an assembly with the fuse. It doesn't seem that anyone has tested/listed the switch with a 300kA fuse yet.

 

[ron]

King,
I hope that the utility transformers supplied will be a high impedance, but I doubt it, as it is secondary metered, and the utility would be paying for the losses.
After 1/01/07 it may have to be NEMA TP-1 energy efficient, if the utility is bound by the DOE federal rules.

 

[kingpb]

King,
I hope that the utility transformers supplied will be a high impedance, but I doubt it, as it is secondary metered, and the utility would be paying for the losses.
After 1/01/07 it may have to be NEMA TP-1 energy efficient, if the utility is bound by the DOE federal rules.

As far as I know Utilities are pretty much not required to comply with anybody. Nevertheless they will follow alot of standards but vary as needed to suit their needs. They are not even bound by OSHA.

When your the big guy on the block generating all the megawatts, I guess you can make your own rules.

I have set up a lunch and learn for my dept. with the guy that runs the local utility network downtown. It should be an interesting discussion.

 

[ramsy]

Does AIR 200kA (Estimated 300kA) RMS Symmetrical, according to Bussman, qualify as a 300kaic switch?

 

[don_resqcapt19]

King,

They are not even bound by OSHA.

What about 1910.269?
Don

 

[kingpb]

King,

What about 1910.269?
Don

My understanding is that it applies to independent power producers, e.g. like the former ENRON who owned power plants. But it would not apply to publically held utilities, suh as Con Ed.

With that said, I don't think they don't all try to comply, but it's just that an OSHA inspector won't be walking around looking for violations.

Here's an intersting scenario, say a power plant is being built, and the Contractor building it certainly has to comply with OSHA. The inspector is there walking around, and see's "violations" that are part of the existing facility. What does he do? I'm sure this has happended, somewhere.

But that is getting us off topic................

 

[ron]

Roger,
Your link was to an IEC rated fuse. This install is bound by USA rules, even worse, NYC rules.
There are fuses out there that are rated at 300kA, but the problem is finding a switch that is rated at 300kA with that fuse.

 

[kingpb]

Roger,
Your link was to an IEC rated fuse. This install is bound by USA rules, even worse, NYC rules.
There are fuses out there that are rated at 300kA, but the problem is finding a switch that is rated at 300kA with that fuse.

Where is the switch going that it needs to be rated for 300kA?

Can you sketch a very simple one-line for us visual folks to look at. I'm still not seeing where you are going to have over 200kA if designed properly, in accordance with Network principles.

 

[wirenut1980]

Thin,
Thanks for the suggestion.
Luckily or unlucky ..... The engineer that is familiar with Con in NYC is me
Usually they use (6) 2000kVA transformers to serve many of the buildings I design, but this time it will be (6) 2500kVA.
For the 2000kVA xfmr design, their standard response to the fault current question was always 200kAIC. Now that the transformers are larger, they don't have such a definite response, which it why I've calculated the values that I have.
A current limiting fuse (cable limiter) would only help reduce downstream fault ratings if I had lab tested series ratings as required by code to be >200kAIC.
The downstream network of protective devices are dynamic (faster than the current limiting fuse) and not passive (slower than the fuse), so that I cannot count on the cable limiter in every instance predictably.

By downstream network protective devices, do you mean fuses that are in your control? Can these be changed out with some that would coordinate with a current limiting fuse? My apologies, I am also having trouble visuolising (sp?) your setup.

 

[ron]

King,
I'm not sure the best format to attempt to convert my autocad onelines for posting, and even then it would be difficult to read as it is formated for a large drawing.
I'll let you use you minds eye:
13.8KV distribution terminated on a "primary bus". The "primary buss" has (6) takeoffs that serve a fused utility protectors/switch for each. Each of the switches then serve as the primary side power for each of the (6) 13.8kV-480/277V transformers. Each of the (6) 480/277V secondaries then go to a fused utility protectors/switch. The output of the secondary side fused utility protectors/switch then goes to a common network 480/277V bus.
From this common network bus, there are (6) takeoffs, finally.... one for each of my (6) 4000A fused service switches.

 

[ron]

David,
The network protectors are out of my control. They are upstream of the common networked 480V bus. My control/design begins at (6) takeoffs on the downstream side of the common networked 480V bus.

 

[ron]

To simplify things you can think of the condition as a primary feeder serving a "theoretical" 15,000kVA transformer and I get the only (6) taps from the secondary bus (after the utility protector).

 

[kingpb]

Based on what you described, this is what it should look like:

View attachment 196

With this configuration the xfmr impedance needs to be at least 8%, and it gets you down to approx. 190kA through the ties (which is worst location). According to Cutler Hammer, the standard impedance for 2500KVA network xfmr is greater then 7%.

Once you add cable you should be able to make this work. You described one common secondary bus, but that doesn't seem right because that would give you no capability to isolate buses in case of a fault. You would lose the entire load, which is opposite of what a Network is supposed to do.

Hope that helps.