Arc Flash Calculations--Contribution From Utility 1

[timm33333]

I have a question about arc flash calculations. We calculate incident energy at the maximum and minimum values of fault currents provided by the utility. But in our case the utility only has the maximum value of fault current (minimum value of fault current is not available). So I will have to run the arc flash calculations for a range of utility fault current.

What should be the range of values for which the system be modeled to get the worst case incident energy? Should we assume the minimum fault current to be 5 times less than the maximum fault current; for example, if the given maximum utility fault current is 5kA, then should we assume the minimum utility fault current to be 1kA? Thanks.

 

[FacEngrPE]

I suggest this thread addresses part of your question.
thread238-423117

 

[dpc]

If you are calculating based on IEEE 1584, all equations are based on three-phase faults. So minimum fault currents that utilities worry about for their feeder protection are not a factor for your calculation.

The need to worry about a minimum fault current depends on how the utility has provided their fault data. If you are taking service at 15 kV distribution voltage, it is unlikely that the available fault current will vary significantly. You may want to calculate +/- 10% service voltage. If you are taking service at 230 kV, then there will likely be more variability, but the answer has to come from the utility. If taking service at transmission level, I'd ask for maximum fault current and fault current with largest source or line out of service.

But for a typical distribution system, the utility fault current at 15 kV is going to be heavily dependent on the sized and impedance of the transformer at their substation and the length of the feeder to your location. Unless they two transformers at the substation with substantially different sizes, there isn't much that will change the answer at your location.

 

[davidbeach]

The fault current that's probably going to cause the most concern is the current just below the instantaneous pickup of the interrupting device closest to the location for which you're doing the calcs.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[bacon4life]

The fault current on 12 kV feeders can change by an order of magnitude when the utility has equipment abnormally configured. In my area is it common for a customer near a substation to have the fault level drop from 7 kA to 1.5 kA when the closest substation is offline for maintenance. I do not know what portion of customers actually consider abnormal utility side conditions when printing arc flash labels.

In a few cases, I have provided customers fault level information under normal conditions as well as the most likely alternate configuration. Since the utility grid can be configured many different ways, it is impossible for utilities to accurately determine the minimum fault current possible after all possible storm damage or equipment failures.

One alternate approach would be to ask the utility for their minimum pickup value, and assume that the minimum fault current is at least 10% higher than the minimum pickup level.

 

[cuky2000]

The minimum available utility short circuit may impact the clearing time of te protective devices and occasional provides the worst case scenario reulting in the largest incident energy asociated with arc flash calc.