Arc Flash

[Mike01]

After doing some research on this topic (have some spare time on my hands and trying to better understand ArcFlash) is it correct to say that equipment ratings are designed around a ?short circuit? study, where as arch flash energy is calculated around the arcing current (a function of the bolted fault current), just doing some poking around if I have a main fuse around 2000-2500A with a bolted fault current around 33Ka the arcing fault current @ 100% is approx. 18Ka while the 85% is approx. 16Ka it would be the 16ka that is used to calculate the arch flash (sound correct?) and when comparing that to a circuit breaker you arc flash category is actually less using a circuit breaker over a fuse? Sounds funny to me as I always hear ?just put in a fuse? but that might not necessarily solve your problem(with a main fuse)? Just wanted to get everyone?s 2 cents. Although anywhere else in the system a current limiting fuse would help downstream of the main because of the current limiting aspects of the fuse? Does this make any sense? A 2000-2500A main fuse(protecting a switchboard) with a Ibf of 33Kaic at 480v has a category 4 wow, I would have never guessed that.

 

[zog]

After doing some research on this topic (have some spare time on my hands and trying to better understand ArcFlash) is it correct to say that equipment ratings are designed around a ?short circuit? study, where as arch flash energy is calculated around the arcing current (a function of the bolted fault current)

In a nutshell, yes.

just doing some poking around if I have a main fuse around 2000-2500A with a bolted fault current around 33Ka the arcing fault current @ 100% is approx. 18Ka while the 85% is approx. 16Ka it would be the 16ka that is used to calculate the arch flash (sound correct?)

Actually arc flash software will look at many different arcing values, factor in the clearing time, and pick the worst case. On 480V systems you should at least look at the Ei from a 100% Isc (BF) and the minimum level that an arc can be self sustaining, generally recognized to be 38% of the Ibf, in your case 6840A.

and when comparing that to a circuit breaker you arc flash category is actually less using a circuit breaker over a fuse?

Sometime yes, sometimes no. Depends on the fuse type and ratings and the breaker settings.

Sounds funny to me as I always hear ?just put in a fuse? but that might not necessarily solve your problem(with a main fuse)? Just wanted to get everyone?s 2 cents.

You seem to have a good understanding.

Although anywhere else in the system a current limiting fuse would help downstream of the main because of the current limiting aspects of the fuse?

Current limiting fuses are a good solution for mitigation only when they operate in thier current limiting range.

Does this make any sense? A 2000-2500A main fuse(protecting a switchboard) with a Ibf of 33Kaic at 480v has a category 4 wow, I would have never guessed that.

Thats the common response

 

[Mike01]

Actually arc flash software will look at many different arcing values, factor in the clearing time, and pick the worst case. On 480V systems you should at least look at the Ei from a 100% Isc (BF) and the minimum level that an arc can be self sustaining, generally recognized to be 38% of the Ibf, in your case 6840A.

Current limiting fuses are a good solution for mitigation only when they operate in thier current limiting range.

Thanks for your response I was just curious wher the 38% comes from? If i am reading this correctly I would need a minimum of 6840A to sustain the arc. is there a minimum A in general to sustatin the are or is it dependant on the avaliable fault current. I only Ask because I have read that at transformers less than 125Kva and secondary voltages of 240 or less that it is a category 1 due to the an ?arc? cannot be sustained is this correct, is it dependant on voltage also? and this may sound like a stupid question but how would you know if the current limiting fuse is acting in it?s current-limiting range? Thanks again.:grin:

 

[zog]

Thanks for your response I was just curious wher the 38% comes from?

Research done by IEEE if I recall correctly, done by someone anyhow.

If i am reading this correctly I would need a minimum of 6840A to sustain the arc.

On 480V systems that is an accepted assumption.

is there a minimum A in general to sustatin the are or is it dependant on the avaliable fault current.

That value depends on the system voltage, and a few other factors.

I only Ask because I have read that at transformers less than 125Kva and secondary voltages of 240 or less that it is a category 1 due to the an ?arc? cannot be sustained is this correct, is it dependant on voltage also?

That is a expection that come from IEEE 1584, some smart people I know dont agree with it and some lab tests have shown that to be questionable, but until further research is done that is the point most OE's use as a cutoff. (You have to stop somewhere)

and this may sound like a stupid question but how would you know if the current limiting fuse is acting in it?s current-limiting range? Thanks again.:grin:

A very good question IMO, the answer is you dont know, the fuse charteristics have to be properly selected by the person doing the PDC study.

 

[Mike01]

Tcc

Tcc

On a fuse TCC where would the current limiting range be?

 

[jim dungar]

On a fuse TCC where would the current limiting range be?

Typically it is not shown on a TCC. You could assume it would be at least the point where the fuse curve crosses the .01sec line.