Arc Flash Analysis

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I am doing an arc flash calculation using PTW32 software. I have a 25 KVA transformer from 480V to 120V and the program is telling me that the available fault is 900A, but the arc-flash protection required is a level 3. This seems to make no sense to me as to why the arc flash level would be so high. It shows the time delay for the breaker to trip of two seconds. Since the bus arcing fault is 900A on a 100A breaker, I would expect the breaker to trip in a much shorter period of time than 2 seconds. If you could enlighten me as to what I may be doing wrong, I would appreciate it.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
I am doing an arc flash calculation using PTW32 software. I have a 25 KVA transformer from 480V to 120V and the program is telling me that the available fault is 900A, but the arc-flash protection required is a level 3. This seems to make no sense to me as to why the arc flash level would be so high. It shows the time delay for the breaker to trip of two seconds. Since the bus arcing fault is 900A on a 100A breaker, I would expect the breaker to trip in a much shorter period of time than 2 seconds. If you could enlighten me as to what I may be doing wrong, I would appreciate it.

Right-click on the bus you're analyzing for the arc-flash incident energy level and Go To Arc Flash + TCC drawing. The software will plot the upstream protective device and will overlay the arcing-fault current, from which you can see what's happening.
And if your maintenance fee is paid, call their tech support line and they'll walk you through it. My experience is that their tech support is excellent.
 

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
Since it is tripping at 2 sec which seems like you have set it to the IEEE 1584 max time value, the arcing current is in the long time region of the breaker.

From the transformer size and secondary voltage (120 V), it appears to be a single phase circuit which is exempt from analysis. Situations like that I default to <1.2 cal/cm2.

If not single phase but 3 phase with secondary voltage of 120/208 V, then I would use the IEEE 1584 exemption for transformer less than 125kVA and voltage below 240 and default it to <1.2 cal/cm2
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Available fault current and Arcing Fault current are two different items.

It is not unusual to find arcing fault currents to be fractions, like 15%, of available (bolted) fault currents.
Low arcing fault currents are partly why current limiting fuses are not a one-size fits all answer.
 
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