Calculating the AIC

[Hv&Lv]

I understand that the 'infinite bus' method is generally considered more conservative, because the actual short circuit current on the output of the transformer will be lower than that given by the infinite bus method.

But aren't there some situations where the higher available short circuit current can result in lower incident energy, because protective devices operate faster? Or do the calculations for incident energy take this into account?

Thanks
Jon

I don’t think so, not in the numbers you see with infinite bus or those with impedance added.

a 10 or a 22kA breaker may try to operate on a large fault but be unable to break the fault if it were, say, about 36kA ASSC.

from a relay perspective, the quickest time will be achieved if the instantaneous pickup is in the trip equation. We do that for personnel protection when someone is working on the circuit, but you never want to leave it like that.

if the curve pickup is the only overcurrent, I doubt the device will open any faster. Were only talking about 8 cycles at max fault

 

[wbdvt]

The utility can provide the available short circuit current at their primary fuses. When I do electrical studies that include incident energy (arc flash), I request from the utility the available short circuit current and X/R at the primary fuse, riser cable data and transformer data. I then model the system using that information. This provides the most accurate assessment of the entire electrical system in terms of short circuit currents and equipment duty. The available short circuit current is required by IEEE 1584 (the document that has the incident energy equations) and for services greater than 1200A by the NEC.

But aren't there some situations where the higher available short circuit current can result in lower incident energy, because protective devices operate faster? Or do the calculations for incident energy take this into account?

Not sure what you mean by higher available short circuit current as it is what is available. Higher short circuit current can result in both a lower or higher incident energy as there are other variables at play. The most accurate is to use the available short circuit current obtained from the utility. Sometimes this takes repeated requests as your first contact will be a customer service person with a chart of short circuit currents based on infinite bus. In one case a utility refused to supply the information requested even after much explaining why it is needed. I had to file a complaint with the state PUC and after educating them on it, they did meet with the utility and at the end of the meeting the utility agreed to provide the information and develop a policy for this in the future.

Yes, utility systems do change but that is why one is supposed to review the study every 5 years. One part of the review is to request the available short circuit from the utility. In study updates that I have done, I have seen the incident energy values change based on utility fault current change but never to the point of moving to another level of PPE.

 

[electrofelon]

has anyone successfully obtained primary SCC and taken that into account for smaller services? How much did that lower SCC after the transformer? Say something in the ballpark of a 500KVA 120/208 bank with 1.8 %Z. That would be 77K infinite primary. What range of values might I get taking primary impedance into account?

 

[jim dungar]

I would conjecture it is partly due to the size of the customer. I dont see a utility paying their engineers for researching and investigating primary SCC - and informing the customer down the road of any changes - on a secondary metered service where the transformer reduces the SCC to a reasonable value (perhaps not the case for some network services). I would also conjecture that taking into consideration limited primary SCC over infinite has a relatively small effect as compared to that of the transformer.

The utilities are providing current based on who owns the transformer. I can take you to a small industrial plant, on the north side of the road the production facility is given actual fault currents, but immediately next door (connected by a walkway) the office building is given 'design' level currents only. They are fed from the same POCO pole.

As an engineer it is my duty to make assumptions (also called What-If) as part of the process of determining worst case conditions. But if I have to guess at the starting point, I might as well just guess at the results.

 

[kwired]

The utility can provide the available short circuit current at their primary fuses. When I do electrical studies that include incident energy (arc flash), I request from the utility the available short circuit current and X/R at the primary fuse, riser cable data and transformer data. I then model the system using that information. This provides the most accurate assessment of the entire electrical system in terms of short circuit currents and equipment duty. The available short circuit current is required by IEEE 1584 (the document that has the incident energy equations) and for services greater than 1200A by the NEC.



Not sure what you mean by higher available short circuit current as it is what is available. Higher short circuit current can result in both a lower or higher incident energy as there are other variables at play. The most accurate is to use the available short circuit current obtained from the utility. Sometimes this takes repeated requests as your first contact will be a customer service person with a chart of short circuit currents based on infinite bus. In one case a utility refused to supply the information requested even after much explaining why it is needed. I had to file a complaint with the state PUC and after educating them on it, they did meet with the utility and at the end of the meeting the utility agreed to provide the information and develop a policy for this in the future.

Yes, utility systems do change but that is why one is supposed to review the study every 5 years. One part of the review is to request the available short circuit from the utility. In study updates that I have done, I have seen the incident energy values change based on utility fault current change but never to the point of moving to another level of PPE.

Incident energy involves amount of current and the time it takes place. High current but fast response by OCPD can have less incident energy than low current with slow response.

 

[wbdvt]

Incident energy involves amount of current and the time it takes place. High current but fast response by OCPD can have less incident energy than low current with slow response.

It also involves other factors such as open air vs in a box, enclosure size, electrode configuration, conductor gap, system voltage and working distance.

Yes, high current will produce a faster response time by the OCPD but what if that high current is above the OCPD's AIC rating? Typically in that case, the OCPD is assumed not to operate to clear the fault and the next upstream device is used. Since this is usually a larger device and with coordination, would be slower to operate. Therefore, the incident energy would not be less.

 

[kwired]

It also involves other factors such as open air vs in a box, enclosure size, electrode configuration, conductor gap, system voltage and working distance.

Yes, high current will produce a faster response time by the OCPD but what if that high current is above the OCPD's AIC rating? Typically in that case, the OCPD is assumed not to operate to clear the fault and the next upstream device is used. Since this is usually a larger device and with coordination, would be slower to operate. Therefore, the incident energy would not be less.

And is only good for whatever point in the system you done all that evaluation for. NEC's requirement to post available fault current at service equipment is somewhat pointless for this reason and what they do require is available fault current which still doesn't tell you incident energy all by itself.

 

[wbdvt]

And is only good for whatever point in the system you done all that evaluation for. NEC's requirement to post available fault current at service equipment is somewhat pointless for this reason and what they do require is available fault current which still doesn't tell you incident energy all by itself.

That is why a system study should be done

 

[Open Neutral]

Well, Hv&Lv was astute to start with the PSE references.

I used the Bussman calculation per Cow, but my question was how much, if any, the secondary cabling lowered the possible fault current. And Bussman never includes that.

My concern was if it exceeded the rating of the existing downstream breakers. The EC is now talking about a main disconnect cabinet. I learned earlier in this saga that a higher-rated disconnect can protect downstream panels if the paperwork deities are mollified.

 

[Hv&Lv]

but my question was how much, if any, the secondary cabling lowered the possible fault current. And Bussman never includes that.

In all fairness I really believed the conductor part I posted in post #2 would have answered the question...here it is again..

Conductors: %Z wire = ((reactance of wire in table 9) x kVA / voltage squared) x ((length of conductor / number per phase) x 100)

 

[Open Neutral]

Did not mean to imply it didn't address it, but I'm not familiar with table 9.

reactance * 50/240^2 * 80/2 * 100

 

[Hv&Lv]

Did not mean to imply it didn't address it, but I'm not familiar with table 9.

reactance * 50/240^2 * 80/2 * 100

I’m sorry, didn’t know. It is table 9 in the back of the NEC

 

[electrofelon]

Well, Hv&Lv was astute to start with the PSE references.

I used the Bussman calculation per Cow, but my question was how much, if any, the secondary cabling lowered the possible fault current. And Bussman never includes that.

My concern was if it exceeded the rating of the existing downstream breakers. The EC is now talking about a main disconnect cabinet. I learned earlier in this saga that a higher-rated disconnect can protect downstream panels if the paperwork deities are mollified.

I am not familiar with the bussman calculator. I would be surprised if it didn't have the capability to figure in conductor impedance. The Mike Holt one does. I looked at your screenshot and it wasn't clear to me why it was giving you no values. It worked for me as I mentioned in a previous post I think the conductors dropped it down to about 17k IIRC

 

[sgtsparky]

In Oregon You have to do the calculation for the signing supervisors test (master) and assume infinite input.

 

[don_resqcapt19]

Well, Hv&Lv was astute to start with the PSE references.

I used the Bussman calculation per Cow, but my question was how much, if any, the secondary cabling lowered the possible fault current. And Bussman never includes that.

My concern was if it exceeded the rating of the existing downstream breakers. The EC is now talking about a main disconnect cabinet. I learned earlier in this saga that a higher-rated disconnect can protect downstream panels if the paperwork deities are mollified.

Bussmann has a free app that does the transformer calculations as well as the cable calculations. Check your app store for Bussmann FC2 (Fault Current Calculator)