3P bolted fault duration

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aaron892

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New to these forums, hello!

I running an arc flash study for a large industrial plant. I am using the Edsa Technical 2005 software to model the system and determine the arc flash categories and available sc / clearing times for specific feeders. When I ran the study I found that some feeds are protected very poorly and some protective devices take a very long time to trip, or do not trip at all to clear a fault. These usually are downstream of step down transformers.

My question is if the fault happens for a very long time (ie over 10 secs) is there a point where you should cut it off? For the purpose of determining the arc flash category. The problem is if the fault last for a very long time, even small arc flash energy over a very long time drives the arc flash category way up. Where do you draw the line?
 
If I was the guy blown against the wall or another peice of equipment across from it, I would hope you ran the calculation at the 1000 seconds that is usually the default. Sometimes the worker just can't get away in 2 seconds if disabled by an obstruction or pain.
 
aaron892 said:
New to these forums, hello!

I running an arc flash study for a large industrial plant. I am using the Edsa Technical 2005 software to model the system and determine the arc flash categories and available sc / clearing times for specific feeders. When I ran the study I found that some feeds are protected very poorly and some protective devices take a very long time to trip, or do not trip at all to clear a fault. These usually are downstream of step down transformers.

My question is if the fault happens for a very long time (ie over 10 secs) is there a point where you should cut it off? For the purpose of determining the arc flash category. The problem is if the fault last for a very long time, even small arc flash energy over a very long time drives the arc flash category way up. Where do you draw the line?
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You can greatly reduce your flash intensities by installing the correct type of fuses to shorten the durration These fuses are pricey but may be wrth the install to reduce the arcflash intensities.
 
quogueelectric,
Why would fuses help as opposed to circuit breakers? They are both available as current limiting devices. Depending on the fault current, many times breakers will open first if the current in not in the fuse current limiting range.
 
ron said:
If I was the guy blown against the wall or another peice of equipment across from it, I would hope you ran the calculation at the 1000 seconds that is usually the default. Sometimes the worker just can't get away in 2 seconds if disabled by an obstruction or pain.
Click to expand...

The formulas for arc flash incident energy do not properly account for long duration arcs. I have done studies that show a Limited Approach Boundary of 156,231 inches (2.5 miles) at a fault duration of 1000 secs.

These formula assume that the arc gap remains constant through out the entire duration. This is an unrealistic assumption as we know that during the arc, material will be vaporized causing an increase in arcing distance.

Some judgment needs to be applied as neither extreme is acceptable. Some people have discussed reporting the boundaries at the fault duration where the hazard risk category first exceeds 40 cal/cm? (the point where there is no longer any recognized PPE).
 
ron said:
quogueelectric,
Why would fuses help as opposed to circuit breakers? They are both available as current limiting devices. Depending on the fault current, many times breakers will open first if the current in not in the fuse current limiting range.
Click to expand...
I cant find my workbook for calculating these but I will later I have to run some errands now but I will get back with the specific info later tonight.
 
aaaron
I highly recommend you get a copy of IEEE 1584. The 2 second consideration in in Annex B, near the end of 1.2. There is some guidance on when you would not want to use it.

carl
 
The chart I will use is a 50000sym available amps and 400 amp rating . Non current limiting circuit breaker 115,000 peak amps and about 3/4 cycle; class rk5 400a fuse 50,000 peak amps about 1/3 cycle; class rk1 400a fuse 33,000 peak amps and less than 1/4 cycle; class j400a fuse 25,000 peakamps and about 1/6 cycle. If I were standing in front of this fault I would definitely pick CLASS J for this one.
 
quogueelectric,
Unfortunately, an arcing fault has lower current than a bolted fault. What is the let through differences between a fuse and breaker when the arcing fault is 10,000A or so? The current liming tables for fuses are only usable when the impedance is static. Nobody knows what the let through of the fuse will be as the arc changes the current lower ... below its current limiting range.
I'm not against current limiting fuses, but I will not agree that fuses are the solutions to all arc flash issues as Bussmann tries to advertize.
 
aaron892 said:
New to these forums, hello!

I running an arc flash study for a large industrial plant. I am using the Edsa Technical 2005 software to model the system and determine the arc flash categories and available sc / clearing times for specific feeders. When I ran the study I found that some feeds are protected very poorly and some protective devices take a very long time to trip, or do not trip at all to clear a fault. These usually are downstream of step down transformers.

My question is if the fault happens for a very long time (ie over 10 secs) is there a point where you should cut it off? For the purpose of determining the arc flash category. The problem is if the fault last for a very long time, even small arc flash energy over a very long time drives the arc flash category way up. Where do you draw the line?
Click to expand...

Are you also doing the PDC study? The trip settings often get cranked up by industrial guys (Dang breaker keeps tripping, better turn it up) that throws of the whole system. Often redoing the PDC study can lower the clearing times, or an easy fix is upgrade your breakers with modern trip units or possibly a arc flash reduction switch, like quick trip.
 
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