Hello

I hope you can help me. I study the following education : Academy Profession Degree in Service Engineering (Electrical) in Denmark.
In this context, I have chosen to base my exam assignment about Arc Flash Hazard. Because I find it very interresting, because it does not really exist in Denmark yet.
I have during my practical stay, worked a little with SKM PT, and I now want to do a manual calculation of the results as SKM PT is reached.

I borrowed IEEE 1584 and I am in the process of making the calculations. BUT there is one thing I simple do not understand.

What is meant by "gab between conductors" and where do I find the information?

I hope you can help, and I apologize for my not-so-good English.

If it is easier to help me through Skype or Google Talk, then it is a possibility.

/ / Mads Andersson, Denmark

Hello Mads Andersson,

Arc flash is definitely an interesting subject, good luck!

With gap they mean the gap between the arcing conductors. See figure 5 on page 25. Check table 4 on page 12 for typical gaps between conductors.

In SKM PTW you can see the gap when you run the arc flash study, see image. You can change the gap by selecting other equipment types.

Hope this helps, if it isn’t clear please let me know.

Hey Luc

Page 25, is help on understanding what it means.

I don't have access to SKM any more, but i have a Demo of EasyPower.

I have looked at the table 4 on page 12, but if i'm doing a manual calculation i need to know the exact gap between the conductors.
I have calculated backwards, and found out that a Siemens 3VL4 - H, has a gap between the conductors at 46,5 mm.
But i have to find some documentation for my assignment.

Because of you explanation and the reference to the IEEE 1584, i now know what to ask for, when i contact Siemens.

Thanks.

Hello Mads

Welcome to forum.

Within Denmark there is a company called "SELCO", in the community of Roskilde. Selco is now owned by a USA company, Littelfuse. Selco offers a variety of arc-flash mitigation products, based on optical (and optional current) sensing. My colleagues in Australia have used the Selco products for years. Only recently have the Selco products received approvals for North America.

If you search for Selco Denmark, you will find information about the company, the products and arc flash.

Hello Mr. Andersson
The IEEE guide offers the suggestion of using gaps of 13, 25, 32 and 40mm for low voltage calculations. The gaps suggested come from estimated gaps in different types of equipment built to North American Standards. However, even in the equipment that the guide describes the real gaps can vary very widely and the suggested gaps probably reflect a minimum gap better than they reflect an average or maximum. Unluckily the complexity does not end there. Associated with each gap is a "distance exponent" shown in table 4, page 12. The distance exponent is used in the incident energy calculation and attempts to reflect the effect of the enclosure around the arc and how it may, or may not focus energy towards the exposed worker or plane. Interstingly in North American equipment the same gap and factor may be used for the rear and the front of the equipmen though the geometry of the enclosure around possible arcing points may be quite different. Furthermore if you use gaps other than the ones in the table, I am not sure what distance exponent you should use. If the equipment you have has an actual measured gap in excess of that shown in the tables I am not sure the guide tells you how to address that. Under the assumption that the guide is "not" suitable for that range of gaps what is then the conservative way to analyze the situation? I could offer some options but I cannot guarantee any of them are correct:
1) Use the gap you have to calculate arcing current (Ia)... ensure your protection is at least sufficiently sensitive for that theoretical minimum current. I always advise to go for the most sensitivity possible as you never know what else is wrong in the model that could produce lower than expected arcing current. To me this represents a conservative estimate for require protection sensing.
2) Then perform the calculation again using a gap in the table that may reflect how your equipment is built such as 32mm or 25mm and the type of equipment it is such as individually mounted switchboard or a group mounted type construction for the front. Use that arcing current & exponent to calculate Incident Energy based on your overcurrent device response time. At the 85% Ia of this gap, or the real gap, whichever yields a higher value of energy.
3) Always keep in mind that the calculations are an "estimate" and many factors can render the estimate not conservative enough.

@ AB P.E. - Thanks for your answer.

@ Marcelo - If the take 85% of the gap size i have calculated i will get apx. 40mm, and then it will fit in to the tabel.
I will now read some more of IEEE 1584, and after that i will read you reply again.

I am grateful that you will spend your time helping me.
Have a nice day.

Gap length is fairly consistent in some instances such as spacing in North American 480/600 V lugs on breakers (25 mm). Indeed it does vary all over the place outside of this fairly narrow set of circumstances. The conductor gap causes an increase in incident energy and higher arcing currents due to the longer arc length. The distance exponent has more to do with the geometry of the equipment. Do not correlate the two as there is no intentional correlation.

Maximum "allowable" gap length varies with voltage. Once the gap length reaches a certain point, IEEE 1584 equations are in no way representative of what really happens and at this point, you are on your own. EPRI (Electrical Power Research Institute) which is a North American research institute dedicated to working on utility interests has been sponsoring a project to study the effect of large open air arc gaps that are common in outdoor utility systems but there is no "equation" in a book yet. The same organization has done several arc flash studies on more common equipment-specific utility gear and recommends much lower values than IEEE 1584 equations for a lot of conditions for which IEEE 1584 is inappropriate such as with open air 400-600 V power cables, some dead front pad mounted transformers, and CT cabinets. In the latter two in particular, the arc gaps are well beyond the values where the IEEE 1584 empirical equation is accurate.

That's not what he said. He said take 85% of the Ia at the real or tabulated gap. Big difference.

Thanks for the help every body.