Is there a standard definition of "minimum available fault current"?
Is there a standard definition of "minimum available fault current"?
(OP)
I’m embarrassed to ask this, but can anyone tell me if there is a definitive guide to what is meant by “minimum available fault current” on transmission systems? Traditionally, when I’ve been asked for this and I do not get more information from the requesting party, I give the fault current/system impedance based on the following conditions:
Finally, I think it is always a good idea (though I’ve not always followed the practice) to give the requesting party system impedances rather than fault currents. Of course, with one, you can derive the other. But, if they can’t make the conversion, they probably shouldn’t be applying the data.
- All generation local to the bus in question is turned off.
- The strongest line source OR strongest transformer source is out of service.
- I’ve seen some suggestions that a fault impedance should be included. However, I feel that this is not correct. The requesting party can take the system impedance data and model and add fault impence if they choose.
Finally, I think it is always a good idea (though I’ve not always followed the practice) to give the requesting party system impedances rather than fault currents. Of course, with one, you can derive the other. But, if they can’t make the conversion, they probably shouldn’t be applying the data.






RE: Is there a standard definition of "minimum available fault current"?
It's a dumb question because it does not convey what is exactly expected.
On every fault current question, I automaticaly ask what do you want it for. If they don't know, then I can't give them an answer.
The problem is people who use computer forms just ask the question for fault current without understanding what is really needed or what is being calculated.
Someone created these forms, but never took the time to write a discussion of what the program is calculating, assuming, or needing. Insert value here.
Some consultants will send a form asking for all sorts of things. And they are mostly things they don't need. I just throw the forms away, and call them to ask what are you using it for.
They will waste your time, but they don't care.
Push back and call them and ask what they need it for. Take the time to understand why they need it.
Strange how much we try to not talk to each other, despite the fact there are more phones in this country than people.
RE: Is there a standard definition of "minimum available fault current"?
So, if someone wanted to do an arc-flash study, what system condition would you give them? Incidentally, these are transmission-level impedances, so they wind up having a relatively small impact on fault current when compared to the transformer impedance. But, I like to be as precise as I can.
I really do appreciate your candor though. On a side note, you wouldn't by chance be a fan of John C. Dvorak would you?
RE: Is there a standard definition of "minimum available fault current"?
RE: Is there a standard definition of "minimum available fault current"?
If they want a fault study for Arc-Flash, then they should state that. If they want a motor start study, which usually includes a minimum credidble system, then state that.If it's for a ground grid study, so on.
I would think for an Arc-Flash study you would want to know the system maximum to set the upper limits of the study. But I have so few transmission study requests.
I think the last one was for pipeline cathotic study.
For distribution, we always say here's the transformer typical impedance for this size, it is suggested you use inifite bus for the high side. But you can't always tell how much the customer wants to shave off the fault current to justify a lower cost gear. It seems foolish to possibily undersize equipment which can hurt people. Makes personal injory lawers wealthy though.
RE: Is there a standard definition of "minimum available fault current"?
Side note: John C. Dvorak is an old-school tech journalist who started his career as a chemist. He's sort of know as a curmudgeon and he has a very dry wit. Your comments, "When I get this question, my answer is zero. It's a dumb question because it does not convey what is exactly expected.", made me think of him. He's one of the few tech-journalists that I find credible and not a complete idiot.
RE: Is there a standard definition of "minimum available fault current"?
RE: Is there a standard definition of "minimum available fault current"?
RE: Is there a standard definition of "minimum available fault current"?
RE: Is there a standard definition of "minimum available fault current"?
In most situations, the system on the secondary side of their main transformer is not going to be greatly impacted by changes in the utility fault duty. The buffering impedance of the transformer dominates.
RE: Is there a standard definition of "minimum available fault current"?
So in transmission there usually isen't much compounding and three phase events.
So where is this a concern? I know there is always a concern, but it is much reduced on the transmission system.
Or are you looking at the sub-transmission system?
RE: Is there a standard definition of "minimum available fault current"?
Thank you everyone for a great discussion. It's always good to get feedback from fellow industry professionals.
RE: Is there a standard definition of "minimum available fault current"?
“In general, two short-circuit currents, which differ in their magnitude, are to be calculated:
-the maximum short-circuit current which determines the capacity or rating of electrical equipment; and
-the minimum short-circuit current which can be a basis, for example, for the selection of fuses, for the setting of protective devices, and for checking the run-up of motors.
However, as shown in ch. 2.2 "Calculation assumptions” some of the parameters are neglected as for instance: d) Arc resistances are not taken into account.
Nevertheless it is mentioned further:
"Despite these assumptions being not strictly true for the power systems considered, the result of
the calculation does fulfil the objective to give results which are generally of acceptable accuracy."
In ch. 2.5 "Minimum short-circuit currents" there are some "conditions" also:
Voltage factor will be cmin, choose the system configuration and the minimum contribution from power stations, motors shall be neglected, resistances RL of lines shall be introduced at a higher temperature.
RE: Is there a standard definition of "minimum available fault current"?
If it is for an arc-hazard study, then I just give them the max available and tell them to simulate different fault impedances, from zero to a large value.
This will take them through the same range of total impedances at the point of the arc.
That way, they still have to work out the worst case I^2*t energy transfer, and my utility cannot be held liable if they use any minimum number I give them improperly.
If they are doing motor start, then I zero out any local generation and assume worst-case LIKELY scenario for lines out of service for their bus of concern.
RE: Is there a standard definition of "minimum available fault current"?
The problem is the request isen't stated clearly, so I can't be sure what is being asked. It's common to have the same issue for internal requests.
It's always better to ask and clarify what the study is being used for. Because running a study usually dosen't take that long, but running many studies because the customer isen't clear about what they want is a waste of company resources (and my time).
It's like writing a letter, get to the point, and state what you want clearly.
RE: Is there a standard definition of "minimum available fault current"?
RE: Is there a standard definition of "minimum available fault current"?
Even if they do not have the software to use the model, the Thevenin impedance of the system should be enough to allow them to use the data as they need.
RE: Is there a standard definition of "minimum available fault current"?
Screen shot from Short Circuit Analytic software below gives an example of different type fault currents including minimum, maximum, three-phase, phase-to-phase etc. that can be derived from comprehensive short circuit analysis:
The maximum available fault currents would be used for calculating total arcing current for incident energy and arc flash boundary calculations. The minimum available fault current would be used for calculating arcing current through the protection device and determining arc duration as a function of the device time-current characteristics. Screen shot from Arc Flash Analytic software below exemplifies an application of both the maximum and minimum fault currents (Available 3-ph Short Circuit Current (ASCC) and Part of the ASCC thru Protection Device respectively)
RE: Is there a standard definition of "minimum available fault current"?
If the purpose of requesting the minimum available fault current is for arc-hazard analysis (and nowadays, that is almost every request for me), our company policy is to NOT PROVIDE IT!
There is a significant block of 'consultants' that are just plugging in numbers to get 'an answer', and we fear the measures they might take as a result. The actual arcing current at the time of an incident can depend on a lot of factors. The actual fault impedance has a tremendous impact on this current.
I am aware of a case where the 'consultant' took our minimum number, simply neglected the possibility of a non-zero fault impedance at the point of analysis, and did their analysis. But the consultant did not actually find the 'worst-case' for total delivered energy (which was at a lower current and a longer event duration because of the specific time-current curve of their breaker relays).
My utility does not want to be liable for such poor analyses, nor do we want to take on any burden or responsibilities to review the work of others. So, we don't give out 'minimums'.
If you are asking for other purposes, such as voltage flicker / motor starting or Distributed Generation, that's different.
RE: Is there a standard definition of "minimum available fault current"?
RE: Is there a standard definition of "minimum available fault current"?
Thanks in advance for helping out a structural guy!
RE: Is there a standard definition of "minimum available fault current"?
It doesn't come up that much as normally there's a much stiffer source in place on the network, but if its a particularly weak source it can have unexpected consequences, thus minimum available fault current can be relevant for such applications.
RE: Is there a standard definition of "minimum available fault current"?
Boundaries? transformers are typically higher impedance devices, so to assume a small system equivlance on the high side, would allow one to calculate a not to exceed fault value on the low side, but that only works where there is a single feed. It becomes more difficult if there are two feeds, or generators involved.
Breakers don't change, or limit fault current. They interrupt it. Only current limiting fuses can limit fault current.
RE: Is there a standard definition of "minimum available fault current"?
RE: Is there a standard definition of "minimum available fault current"?
RE: Is there a standard definition of "minimum available fault current"?
Don't get me wrong, a lower impedance transformer does save on losses, but if you spend those savings on replacing equipment, that transfers future losses to the present as a cost of replacing equipment.
Reactors are nothing new, but could have been included in the transformer (so much for being too concerned about energy losses).
RE: Is there a standard definition of "minimum available fault current"?
Seems to work.