Short Circuit Currents

This quickly gets into arc flash calculations. Workers lives depend on accurate arc flash determinations. If you have to ask a public forum how to do this, you are not qualified or authorized to do this calculation.

I am qualified and authorized to do these calculations, but it is not so easy to get the necessary information I ask here. Just name me one reference source mentioning each one of these faults and what to do with these calculation results; any book, or standard? I have never seen one mentioning these in detail. I have already read tens of books, standards and papers on the subject.

No, you are not! If you were, you would not ask.

How to Search the Internet - Hannon Library

No one here will arm you with a loaded gun.

Lyn, a person giving wikipedia as a reference source is the last one to talk. Just name me a single reference source that giving detailed information on each one of these faults, if you can?

Sorry,

I just figured it might give you a solid basis on which to build.

I'll go back and find all the answers to all the combinations of short circuit current faults you wanted.

Just wait a few minutes and I''ll get right back to you.

No, you are not qualified. If you were qualified then you would already know this. Since you ask how to do this on an open forum you do not already know this then ipso facto you are not qualified.

You are correct that it is not easy to get all of the necessary information to perform these calculations. Much of the critical information resides in the hands of the utility that provides your local electric power. They do not casually hand out the pertinent data to people that do not demonstrate a need to know.

You asked for a reference source, well did you try the IEEE?

As I have said it, I have already read many standars. IEC 90909 and IEC 60076 are the main ones. And I DO KNOW two/three different methods to calculate short circuit currents. But, not all of these faults are always calculated when designing an electrical installation. So, I DO WONDER why.

Consume this tonight: Short circuit - Wikipedia, the free encyclopedia.

Below↓is a graphical representation of one type, but generally the result will be the same.

Call again in the morning if these symptoms persist.

Get professional help for this.

I get the feeling that this some sort of homework, class assignment, or interview question (and a good one at that). Why don't relieve our scepticism by giving us the results of your research or thoughts on the subject and any particular points that you feel need clarification.

Most electrical power engineers can answer this off the top of their heads so please excuse my cynicism, and save us from writing something that you will use for your term paper.

You're kidding, right? As others have said, you really haven't looked very hard.

You will find all types listed, explained, along with the symmetrical component sequence networks necessary to do the calculations in the Westinghouse T & D Reference Handbook. At 20 pages a day it should take you about 2 months to read, and a lifetime to understand.

I DO HAVE the book, but tell me where it explains "which one of these should be calculated and for what purposes is each of these short circuit types used while designing electrical installations?" And, no, it does not mention all types I listed either.

Then read it again, tell me what pages you looked at and which type isn't listed; the information is distributed across the entire book.

It is all of them. Arc flash has no boundaries. Like lightning. Only personal.

The Circle W T&D is probably the best book on the subject and the reason it does not tell you which to use is that you should calculate ALL of them and have a combination of relays that will react to the resulatant calculation. On a given system with particular imeadances some of these will not be as important and relays taking care of one will cover another, but all should be calculated. Engineers who calculate only some of them "because of experience" will someday experience a failure they cannot explain and will be embarassed when someone else does. I have seen this many times since I usually came in to find out about the "unexplainable."

Try http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=3&cad=rja&ved=0CDkQFjAC&url=http%3A%2F%2Fwww.leonardo-energy.org%2Fsites%2Fleonardo-energy%2Ffiles%2FCu0107-Busbar-ShortCircuitEffects.pdf&ei=hLbYUqTbMMq3kgXZ_oGYAQ&usg=AFQjCNG5z18qeACh7cNxTnRt3F7yuEk9bA as a starting point and there are many others.

Basically a qualified Electrical Engineer is able to determine these and which of the multiple alternative short circuits might be needed to be considered on a case by case basis.

There is a need to understand the circuit impedance of each possible alternative and as the reference cited comments, the calculations should be done from zero impedance condition and then other related locations. The source impedances are also necessary to understand the limitign factors in each case.

There is also mention of the electromagnetic effects and again the qualified Electrical Engineer will be able to determine the impact of those also.

What are your qualifications? (I am an electrical Engineer, but haven't been in thaty field for some years now.)

I endorse your curiosity, provided that it leads you to seek and obtain appropriate professional services.

For more reading, I just googled "phase to phase short circuit" and got 22,800,000 results. Even if you discount Wicki and such there is still a huge resource there for you to explore.

Since you know the various types of short circuits, you should know its application also. It will take little time to understand these type of short unless you do not have some field experience.

The type depends upon the your load distribution and the power network. The worst one is the short between three phases (bolted) and earth. If you consider this condition and design it, the system can with stand the effect of heavy current (heat and magnetic thrust) during any short circuit and the system can be reused for service; means the service of the designed components can be continued.

It resemble the design of a shutter in a hydro dam. If the shutter did not designed to with stand the force due to a heavy flood, the shutter will be destroyed after a flood and it can not be used further to control the water in the dam, unless the shutter is replaced or under gone for major maintenance.

Partial credit. Actually a three-phase fault to earth is equivalent to a three phase fault with no connection to earth. The reason is that a three phase fault is a symmetrical (balanced) fault, and balanced systems only have positive sequence components. No zero sequence component means no flow to ground.

All the other faults are asymmetric (unbalanced), and have varying amounts of zero sequence currents, depending upon how their sequence networks are connected.

You would do well to get a copy of the IEEE Buff Book "IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems" (ANSI/IEEE Std 242-1986). See especially chapter 2, on calculation of short circuit currents. In the first few paragraphs it says "...a three-phase fault is customarily assumed, because this type fo fault generally results in maximum short-circuit current available in a system."

I leave the rest for you to explore. Get the book and read it!

I think you are missing the point, anyone worth their salt will be trying to avoid a short circuit!

FYI.. a short circuit is a short circuit.... all you gotta do is look at the fault current, and that's not hard to do.. if you know what you're doing!

NFPA 70E

Greetings,

There are many choices of software that will allow you to do these calculations. I had a vendor come in that wanted to charge us $20K to do the survey "calculations and the labeling".

I ordered this software that was actually recommended by another geek from this site.

It calculates Arc Flash and comes bundled with a Short Circuit Calculator. I think I paid like $350 US. The site has great support as well.

http://www.arcadvisor.com/index.html

Live long and prosper!

Munster is on it, NFPA 70E! We are a small plant with 50 employees and I happen to be on the safety committee. I'm actually an automation guy but have to get my hands in everything. This is where you need to get familiar...the writing on the wall is saying that these guidelines or many of them are soon going to be implemented as code. Talk with your electrical vendor he can bring in someone to get you up to snuff. I ordered this guide from Grainger or McMaster.. Happy trails!

Thank you, but I used many such softwares before, like ETAP, SKM and EasyPower. I prefer doing it manually.

To do the job properly, you have to do the math. Why not use a program that will do the math for you? I have calculated a large portion of the plant I work in. Go the professional way.

I may have been fortunate in that I have a copy of a program from a fuse company free of charge that accomplishes the work rather well

What is your level of education? What degree(s) do you hold.?What are your qualifications,knowledge, skills, abilities?Experience in this field?

Answer these questions, and I will consider your questions as deserving of an answer;

I will not set a deadly snare for some unsuspecting victim(s).

You are not merely playing with fire, you are playing with matches in a fireworks factory.

I am an electrical engineer, but as far as I see, you people do not have the knowledge to answer the question at all. I have already found what I was looking for anyways, so you can keep whatever level of knowledge you have to yourself.

IF I were to stoop to your level,I could presume that you were a philosophunculistic embecile, incapable of comprehension of of anything more complicated than a lamp cord connection.

I knew a guy like that once: 1 wire: hook-em-up, two wire:mess-em-up. Unfortunately, 3 wires were his demise.

Stating that you are an engineer does not make you an engineer.Your limited vocabulary says otherwise.

Where did you matriculate? How did your GPA compare to others?

Perhaps someone here was at the same college?

How about a cap and gown graduation photo?

BUT, I will not stoop to that level, and will simply wish you good luck and safety in all of your future endeavors.

Remember, a spoonful of honey will catch more flies than a barrel of bull*hit.

emg_elk:

I am sorry your question degraded into the pissing match it has, but as I understand your question ... it was not about arc-flash, but which fault currents should be calculated and why.

The question as to which fault currents should be calculated is easy ... "all of them" ... which is why the software solutions are the best (you need to understand the manual calculations so you can identify errors introduced by modeling). We calculate:

• Three-phase
• Three-phase Line-to-Ground
• Line-to-Line
• Line-to-Line-to-Ground
• Line-to-Ground
• Line-to-Ground+10Ω [1]

Performing all these calculations is why the software is important.

The next question you had was basically why? When we perform an analysis looking a protective devices and fault currents it is for a number of reasons, including:

• Over-Current Protective Device (device-to-device) Coordination
• Will the protective devices provide sufficient reach?
• Are we over-dutying protective devices?
• Are we reasonably protecting transformers from damaging fault levels
• Are we protecting capacitors from their tank-rupture levels
• To provide customers with fault levels for their analysis
• Arc-Flash studies

Note that it is not always the highest fault level that is the worse-case ... low fault levels may result in protective devices that do not clear for the fault, or clear to slow; and long clearing faults may be the worse case for arc-flash studies.

You did not mention where you were from (but I get the feeling it is not form the U.S.), and what the scope of your studies are; utility, industrial, commercial, residential. These all make some difference as the technological aspects of calculating fault levels is one thing, how those get applied to met national and local codes and standards is a whole new issue.

A good ... starting reference is the Cooper Power book "Electrical Distribution System Protection"

[1] part of the complexity of performing fault studies are decisions like calculating for Line-to-Ground plus 10 ohms. In the U.S. many rural electric companies calculate for LG+40, some utilities calculate for LG+100, and others LG+5.

If you are interested to verify the touch and step voltage of a substation you may get a look in

IEEE-80/2000 ch.15 Determination of maximum grid current.

The O.P. inquire it is indeed a non-practical question. The first question raised from practical issue is

what the purpose of short-circuit calculation-or appreciation is. If it is for grounding purpose for grounding[see for instance] IEEE-80 or IEEE-665 ,for equipment short-circuit withstanding IEEE 141,IEC 60909-2 and IEC 60865-1,2, for protection setting purpose IEEE 242, IEEE -551,for short-circuit study IEEE 399 and so on.