Bolted fault current

[charlie b]

Re: Bolted fault current

If we had perfect knowledge of what is inside a facility and how each item is to be operated, then I could calculate, with great precision and accuracy, the current that you would see at any point in time throughout the day. Lacking a complete and perfect set of input information, the best we can do is to say, "the current will never be more than XXX, so it is acceptable to use YYY type and size of conductors.

Our methods come in five steps: Obtain the input information, apply the math, do a "sanity check" of the results, hand in the report, and send the bill.

We know that we will not know everything about a project, that the input information will not be complete and perfect. But we also know how a change or difference in the input information might influence the final answer. We know what is likely to be a gray area, and whether we need to add a little "safety margin," to allow for the possibility that the gray area might prove to be more severe than is normally taken into account. We know what is gray, why it is gray, how gray it can get, and how to make the final product both useful and safe, despite the gray.

 

[templdl]

Re: Bolted fault current

I'll never forget when a maintenance man of a school came to me with a complaint that one of our breakers failed to trip (i.e. was defective) when a student kicked out an outlet and it started an arcing fault. At first I though, yes, the darned breaker was defective, what gives?
It was at that time that I took a more serious look a arcing faults as they are related to breakers. Does the arc draw enough current to trip the breaker thermally? If not, how long does an arcing fault need to continue before it trips the breaker thermally. Knowing that a breaker can't physically "see" an arc fault as it is occurring it is solely dependent upon the magnitude and length of time current passes through it current that passes through. Will the instantaneous current be high enough to trip the breaker magnetically and clear the fault? If the breaker doesn't trip magnetically the arcing fault must conduct enough current for a given amount of time to trip the breaker thermally if in fact it is ever reached, the inverse time current element.
The question is can one predict or determine what the time and current will be with every arcing fault that may occur. Once an arcing fault is initiated what happens after that? Does the peak current occur when the arc is struck and what happens as the metal evaporates and the air is ionized and it becomes a conductor.

 

[charlie b]

Re: Bolted fault current

Originally posted by templdl:The question is can one predict or determine what the time and current will be with every arcing fault that may occur. Once an arcing fault is initiated what happens after that?

One cannot predict such things. This is a good example of what I had tried to say about not having complete and perfect input information. But what we can predict is the highest voltage and current that can be achieved, should the arcing behave in the worst possible way. The worst way is when the voltage across the arc point has hit a maximum, and is just starting to drop, when the next arc hits. You can get an extremely high build up of voltage. That will cause a build up of heat, and can set nearby materials on fire. I gather this scenario is the reason given for requiring AFCI protection in dwelling units. I also gather that the reason the rule applies to bedrooms is that the authors didn't think they could sell the idea of AFCIs throughout the house, at least not for a few more code cycles.

. . . what happens as the metal evaporates and the air is ionized and it becomes a conductor?

If you are lucky, you will get a continuous, high current flow, and it will trip the breaker. If you are not lucky, you might get a fire.

 

[templdl]

Re: Bolted fault current

Yes, it's often a crap shoot. That's why ground fault protection should be considered which can help. The arcing fault in L-L or L-N it "may" extend to ground where a GF device most likely pick it up and clear it. The GF device could be anything from class A and B to class 1 and 2 depending upon the application which commonly have sensitives much less than the magnetic pick-ups of the breaker that provides protecting for that circuit.
And the AFCI has been designed not only with CL B GF protection but the manufacturers say that they have the ability to detect the electrical signature of an arcing fault.

 

[AK_Engineer]

Re: Bolted fault current

It would be ideal if all protective devices incorporate arc fault sensing, indicate the type and distance to the fault.

Until then . . .

 

[steve66]

Re: Bolted fault current

It would be ideal if all protective devices incorporate arc fault sensing, indicate the type and distance to the fault.

Until then . . .

I don't think that would be ideal from either a economic or reliability standpoint. But yes, it would be at least "very good" from a safety standpoint.

Steve