Arc Flash calculations

[OCPD]

Hello All,

I am performing arc flash analysis on small MV system, 12.47kV is the service voltage with some 12.47 equipment and goes down to 208V. I have couple of questions on the calculations:

1. For arc flash, IEEE 1584 equations are used which are typically for 3-phase faults. Why is line-ground fault not considered for arc flash? Is the reason that line-ground fault quickly turns into a three phase fault in 15kV or small systems?

2. Now the code requires to install a arc flash mitigation system for systems rated for 1200A or more. These arc flash mitigation is typically designed for a 3-phase arcing fault. For example a maintenance setting is provided for a 3-pole 2000A breaker. The maintenance setting reduces the instantaneous to 5000A to lower the arc flash energy, but the ground fault is set for a instantaneous of 2000A, which is still lower than the maintenance setting. Why is still the L-G not considered for arc flash?

 

[wbdvt]

Basically the premise in IEEE 1584-2002 is that a fault that would cause an arc flash is a 3 phase one and all the testing done to develop the equations were 3 phase. They consider that an arcing fault is started by a L-G fault would rapidly progress to a 3 phase fault. There are no equations to do a single phase arc fault so there are two ways to handle that: calculate it as a 3 phase knowing that the result will be conservative or state that it is exempt from analysis and give it a default incident energy level.

Is this your first time doing an arc flash analysis? If so are working under the guidance of an engineer experienced in arc flash studies?

 

[publicgood]

Basically the premise in IEEE 1584-2002 is that a fault that would cause an arc flash is a 3 phase one and all the testing done to develop the equations were 3 phase. They consider that an arcing fault is started by a L-G fault would rapidly progress to a 3 phase fault. There are no equations to do a single phase arc fault so there are two ways to handle that: calculate it as a 3 phase knowing that the result will be conservative or state that it is exempt from analysis and give it a default incident energy level.

Concur.

 

[OCPD]

Basically the premise in IEEE 1584-2002 is that a fault that would cause an arc flash is a 3 phase one and all the testing done to develop the equations were 3 phase. They consider that an arcing fault is started by a L-G fault would rapidly progress to a 3 phase fault. There are no equations to do a single phase arc fault so there are two ways to handle that: calculate it as a 3 phase knowing that the result will be conservative or state that it is exempt from analysis and give it a default incident energy level.

Is this your first time doing an arc flash analysis? If so are working under the guidance of an engineer experienced in arc flash studies?

Thank you.

This is not my first one and yes it is performed under the guidance of experienced PE. I got this question since on open switchyard typically line - ground fault is fused for arc flash calculation. i assume the reason being the distance between buses larger in open switchyard compared to lower voltage systems. This is only reason I was prompted to ask the question if my understanding of not performing the single-line to ground arc flash on less than 15kV systems, was not correct.

 

[wbdvt]

If it is a switchyard or substation you may be in realm of OSHA 1910.269 for utility or utility like equipment where arc flash is specifically mentioned. In Appendix E there is a table of recommended methods to calculate incident energy. One of them for single phase <15kV is using Arc Pro and IEEE 1584 for 3 phase. Above 15kV the table has Arc Pro

 

[Brad45]

Phase to ground Fault

Phase to ground Fault

FYI, stats show that most (70%-80%) may begin as phase to ground faults but then, due to the expansive nature of conductive metals and electricity, very quickly progress to 3 phase faults.

Im with wbdvt on this one.