Location used for calculating utility available fault current

[Natfuelbilll]

In Coned service territory I have, after written requests, received available fault current data from Coned calculated at a point in THEIR system located upstream and not at the service disconnect. The fault current information came but without their feeder size and length. Coned's own documentation says the value they will provide is at the service entrance.

I questioned the Coned field engineer with, really? that much available fault current at my service?

It was then that I learned their error.

It makes me wonder.... How many services and equipment ratings have been needlessly oversized in order to meet their faulty data. (No pun intended)

 

[Ingenieur]

What is the data?

 

[Natfuelbilll]

The source is characterized by available fault current amps and impedance.

 

[mivey]

In Coned service territory I have, after written requests, received available fault current data from Coned calculated at a point in THEIR system located upstream and not at the service disconnect. The fault current information came but without their feeder size and length. Coned's own documentation says the value they will provide is at the service entrance.

I questioned the Coned field engineer with, really? that much available fault current at my service?

It was then that I learned their error.

It makes me wonder.... How many services and equipment ratings have been needlessly oversized in order to meet their faulty data. (No pun intended)

There is usually more fault current available at the service than upstream on the utility primary.

 

[Natfuelbilll]

For a system supplying many motors - yes.

 

[Carultch]

There is usually more fault current available at the service than upstream on the utility primary.

Yes. Because transformers are built for trading the combination of voltage and current, in a similar concept to the way a gear system trades speed and torque. As a result, they also amplify the fault current from their primary side to their secondary side.

 

[wbdvt]

What are you looking to do with that information?

If it is for sizing equipment to the fault current, then usually the customer service department will provide the fault current on the secondary side of the transformer with an infinite bus. This will provide the max fault current.

On the other hand, if you are looking for available fault current to perform an incident energy analysis, then it is best to get the fault current at the primary protection point. Usually this would be the riser pole fuses and then obtain the fuse data, conductor data from riser pole to padmount txf and txf data. Since this is usually utility owned, they should be able to provide that. Then since the secondary is customer owned, get that data and you can calculate the available fault current on the secondary side. The riser fuse or possibly the Bay-O-Net fuse in txf would be the clearing device for a fault on the immediate secondary distribution equipment and would give you the fault time for incident energy result.

 

[Natfuelbilll]

We need the utility information at the service disconnect for two issues: to properly specify the kaic of the customers equipment and to perform arc flash calculations.

 

[Carultch]

What are you looking to do with that information?

If it is for sizing equipment to the fault current, then usually the customer service department will provide the fault current on the secondary side of the transformer with an infinite bus. This will provide the max fault current.

On the other hand, if you are looking for available fault current to perform an incident energy analysis, then it is best to get the fault current at the primary protection point. Usually this would be the riser pole fuses and then obtain the fuse data, conductor data from riser pole to padmount txf and txf data. Since this is usually utility owned, they should be able to provide that. Then since the secondary is customer owned, get that data and you can calculate the available fault current on the secondary side. The riser fuse or possibly the Bay-O-Net fuse in txf would be the clearing device for a fault on the immediate secondary distribution equipment and would give you the fault time for incident energy result.

Any particular reason you would recommend using different data for each application?

 

[mivey]

Any particular reason you would recommend using different data for each application?

For equipment sizing you want to allow for system configuration changes including kVA upsizing, as well as lower impedance due to transformer replacement, primary wiring upgrades/feed changes, substation upgrades, transmission/generation changes, etc.

For arc flash you want the current system state.

 

[mivey]

For a system supplying many motors - yes.

Even without many motors, the secondary fault current can be higher than primary fault current and usually is.

 

[Carultch]

For equipment sizing you want to allow for system configuration changes including kVA upsizing, as well as lower impedance due to transformer replacement, primary wiring upgrades/feed changes, substation upgrades, transmission/generation changes, etc.

For arc flash you want the current system state.

Ok, that makes sense.


So for equipment KAIC selection, you'd want to be prepared for any possible fault current to be available at your transformer primary, such that no matter what the utility does in transmission/distribution upgrades, your system remains safe. The "infinite bus" calculation, which calculates the upper limit on secondary fault current from transformer KVA & impedance, assuming available primary fault current is infinite.

 

[mivey]

Ok, that makes sense.


So for equipment KAIC selection, you'd want to be prepared for any possible fault current to be available at your transformer primary, such that no matter what the utility does in transmission/distribution upgrades, your system remains safe. The "infinite bus" calculation, which calculates the upper limit on secondary fault current from transformer KVA & impedance, assuming available primary fault current is infinite.

Yes. Infinite source admittance may be extreme in some cases but not too far off in others. It is conservative but not necessarily so if other variances are ignored (which they are many times).

You also need to allow for impedance variance ranges (use low end of range for fault current, high end for flicker analysis).

You also should allow for reasonable transformer size increase if it is reasonable to expect growth (like commercial build_out, building additions, beyond temporaries, etc.).

Sometimes you see infinite bus (conservative) and ignoring service drop (conservative) and ignoring variance (liberal) with the prayer that all will be ok. I also see completely ignoring the fault calc altogether and just installing whatever is handy (moreso with small commercial and residential): sloppy work and sloppy inspections.

 

[Ingenieur]

The source is characterized by available fault current amps and impedance.

So no voltage?
no location? ie xfmr primary terminals
what was the form of the impedance? x/r or R + jX ???

what were the actual values or the sheet they sent you
service info, voltages, kva, etc
can you scan it? Or post a pic

 

[mivey]

So no voltage?
no location? ie xfmr primary terminals
what was the form of the impedance? x/r or R + jX ???

what were the actual values or the sheet they sent you
service info, voltages, kva, etc
can you scan it? Or post a pic

It would be revealing to see what the utility provided.

 

[electrofelon]

I routinely see utilities give, what I consider to be, unreasonably high figures for fault current. Last time I did a new medium sized 480 service, the utility gave me a figure of 39K, and when I calculated it from the transformer assuming infinite buss, it was a third of that. Some people say, "you have to use what the utility gives you" but now I would pretty much always go with the actual transformer data if it is available or I can get it. I can see providing for a larger transformer due to customer expansion, if that is of significant likelihood, but as far as the theory that utility may replace it with a a larger unit "they have on hand" if it fails, or with one with an impedance drastically lower (which I have never seen in 20 years), those are too far in the "what if" field for me.

 

[mivey]

I routinely see utilities give, what I consider to be, unreasonably high figures for fault current. Last time I did a new medium sized 480 service, the utility gave me a figure of 39K, and when I calculated it from the transformer assuming infinite buss, it was a third of that. Some people say, "you have to use what the utility gives you" but now I would pretty much always go with the actual transformer data if it is available or I can get it. I can see providing for a larger transformer due to customer expansion, if that is of significant likelihood, but as far as the theory that utility may replace it with a a larger unit "they have on hand" if it fails, or with one with an impedance drastically lower (which I have never seen in 20 years), those are too far in the "what if" field for me.

The strike odds are in your favor if that is how you want to do it. Not sure about the financial odds but maybe those too.

 

[ron]

If this is Con Ed in NY, then it was probably 200kA with no other supporting information. We take that number to be at the service transformer paralleled bus. If it is a large facility with their own vaulted transformers, the number would be at the buss'd service end box.

 

[NewtonLaw]

In Coned service territory I have, after written requests, received available fault current data from Coned calculated at a point in THEIR system located upstream and not at the service disconnect. The fault current information came but without their feeder size and length. Coned's own documentation says the value they will provide is at the service entrance.

I questioned the Coned field engineer with, really? that much available fault current at my service?

It was then that I learned their error.

It makes me wonder.... How many services and equipment ratings have been needlessly oversized in order to meet their faulty data. (No pun intended)

You did not supply any numbers but from you comments you seemed surprised to see it was such a high number. If the POCO is Consolidated Edison, they supply NYC area and your service could be from there underground system. Most of the service lengths are 40 feet or less from the UG system to the customers switchgear so that impedance would have to be added to their system number. It would not be unusual if the numbers they gave you were in the hundreds of thousands of amps from there grid at the location nearest to your service.

I would suggest you contact them again for clarification or, if you would, post the data and location you were given the information for and the service size supplied to your customer.

Hope this helps,

Newton Law

 

[Natfuelbilll]

After some discussions with the account Representative he went back to his engineering and they recalculated at the point of termination. It turned out to be lower when they removed their assumptions and calculated based on what is installed.