Low Voltage Protective Device Arc Flash Analysis - Protective Device Bolted&Arcing Fault Current

[djr3203]

I am using SKM to perform a power system analysis (short circuit,coordination, and arc flash). When performing he arc flash calculations through the software per IEEE 1584, the table summary gives me the bus bolted fault current, bus arcing fault current, protective device bolted fault current, and protective device arcing fault current.

1. Why would the bolted fault current at the bus be different than the bolted fault current at the protective device?
2. Why is the arcing fault current at the bus different than the bus arcing fault current?

Based on my studies, IEEE 1584 calculates the bus arcing fault current based on several factors (including gap distance between conductors, whether arc is in a box or open air, etc.). Where can I find information that discusses how this derived value is further reduced to come up with the protective device arcing fault current?

As always, I appreciate you taking the time to read and comment.

DJR

 

[dpc]

1. The fault current through the upstream device that is tripping may not be the same as the total fault current due to motor contributions, additional sources, etc.

2. Arcs have resistance, so the arcing current will always be less than the calculated bolted fault current (zero fault impedance). IEEE 1584 has an equation to convert bolted fault current to arcing fault current. The difference between bolted fault current and arcing fault current becomes greater as the voltage is reduced.

 

[djr3203]

I didn't realize the fault current through the upstream device that is tripping would be different, but now that I think about it, it makes sense. I had read the equations of IEEE 1582 on how to calculate the arc fault current, but I didn't realize it also had equations for determining the protective device arcing fault current, which is based on the calculated arcing fault current which is in turn based on the bolted fault current (sorry for the run on sentence). I found the equations describing the arcing fault current at the protective device in the help library sent over by SKM support.

Thanks for the input DPC!

DJR

 

[waross]

ARC FLASH takes clearing time into account. The longer the clearing time, the greater the arc flash energy. Changing the trip settings does not lessen the arc intensity, it lessens the clearing time of arcing.

whether arc is in a box or open air, etc.).

This does not increase the fault current, it increases the energy that may be directed towards a worker.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[djr3203]

Waross,

Agreed changing the trip time does not lessen the arc intensity but lessens the clearing time of the arc which impacts the energy at the fault.

I wrote the statement too fast. I agree the location of the arc has no affect on the magnitude of the arcing fault current, as this depends on the impedence and not location. It is the incident energy at the worker that is impacted by the location of the fault (box or open air). Actually that whole statement was incorrect, as the factors I was describing was for incident energy calculations and not the calculations for the bus arcing fault current.

Thanks for the corrections as the statements were definitely incorrect. I should reread my posts better before submitting.

DJR