My friends on The Forum I need help make an arc fault calculation

Kyell

Member
My friends on the Forum I need help making an arc fault calculation on a bus duct.
480 volt 800-amp system. This is actually important to us here because I was injured buy a bus tap that failed and caused an arc fault to ground and we are trying to put in place a better system of Education so this never happens again. Don't worry my injury was minor compared to what it could have been I was wearing proper PPE. This alone was a major factor in me coming out of this with only a minor injury can anyone out there advise me or help me I was doing the calculation the way I have been taught in the past and it doesn't seem to be right

Uncle Bob

paulengr

Senior Member
Realistically it’s not an easy thing to do by hand. Few people who work with it even know how to do this.

Arc flash calculations starts with the short circuit current. There are lots of very simple calculation methods that give you short circuit currents that are useful for checking AIC for instance, but they give unrealistically high results. With arc flash this will underpredict the danger so simplified methods can’t be used. Probably the most approachable hand calculation method is the Bussman point to point method. In fact if you have a fuse only system Cooper-Bussmann gives you a simplified method that is pretty close to the more accurate IEEE 1584-2018 method. If you have breakers you are out of luck with Bussmann. There is software but at usually around \$5,000 for a license it’s not cheap, it usually uses either the IEC or ANSI methods but it uses the full impedance (L and R) where the standards are designed back in the days of slide rules so only the inductance is used. This is the most difficult part of the entire calculation.

Next the actual arcing current is calculated using the system voltage, the short circuit current above,’and the arc gap. It’s a fairly straight forward calculation but pretty long in the current standard. Depending on the various factors the actual arcing current is about 40-85% of the short circuit current.

Next once you have this, take the arcing current and use the time-current curve of the fuse or breaker protecting the bus to determine the fault time before it trips and the arc is extinguished. This is pretty simple but finding this information especially on older or foreign equipment can be difficult.

Finally using all the above information plus the size of the bus can (LxWxH) and the distance between the arc and the prospective victim, you can calculate the heat flux in cal/cm2. Again the calculation is long and tedious but the sort of thing that you can do in a spreadsheet.

You can purchase the IEEE 1584-2018 book for around \$600. They actually produced a little over 3000 arc flashes at several testing labs and collected a ton of data in order to arrive at the calculations. Understandably they charge that much to help with paying off the cost of the research. Older versions of NFPA 70E give the older 2002 version of the equations in IEEE 1584 and is available for under \$100. The new calculations are not included partly because of space.

I know this is not the answer you are looking for but it should give you a flavor of what is involved and why realistically arc flash calculations are done in software.

Sent from my iPhone using Tapatalk

Kyell

Member
That is okay. Still working on it

That is okay. Still working on it

Still working here but NOT Convinced the answer is correct. Thank you

wbdvt

Senior Member
That is okay. Still working on it

Still working here but NOT Convinced the answer is correct. Thank you
Not sure of what you meant but Pauleng answer is correct. In order to accurately determine the incident energy value, you need to start at the utilities connection with the available fault current and X/R and model the system to your point of interest. The X/R is important to determine if any of the equipment is not properly rated for the fault current, i.e. overdutied. This has ramifications on incident energy. A coordination study is also part of a standard arc flash study and you may find lack of coordination which can also impact the incident energy level.

Overall, this is best left to experienced professionals who regularly do this type of study and have the software to perform the study.