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 Post subject: DC Arcflash study and DC fuses clearing timePosted: Thu Jun 05, 2014 5:55 pm

Joined: Thu Jun 05, 2014 5:52 pm
Posts: 19
Hi there,

My co-worker provided me an available bolted fault of 16kA on a 25kW at 0.566kV DC system (Battery Storage). Somehow, he used AC module (IEEE 1584) to calculate Arc-Fault at 4.6kA and 3.9kA (100% and 85%). Then he calculated worst case Thermal Incident Energy based on the same AC model which I don't feel comfortable. I want to use NFPA 70E 2012 theoretical DC model to verify the Arcflash Ei; somehow, I had a hard time trying to determine the clearing time for a (DC) Bussman FWP - 200A fuse and (DC) SIBA 350A fuse model 2071532.350

http://cooper-bussmann.com/products/Nor ... e_187.html

According to the theoretical model I need to obtain a Arc fault @ 50% of bolted fault which is 8kA not 4kA resulting from the AC model. Somehow, base on I^2t curve the clearing of both Bussman and SIBA above will clear the fault of 8kA almost in no time or more specific less than 2 ms. I wonder if my reading is wrong?

Also, I wonder if DC Arcflash in this particular case can occur @ 115% of the let-through current which takes almost a second to clear?

Thanks

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 Post subject: Re: DC Arcflash study and DC fuses clearing timePosted: Fri Jun 06, 2014 4:18 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2173
Location: North Carolina
7604 wrote:
According to the theoretical model I need to obtain a Arc fault @ 50% of bolted fault which is 8kA not 4kA resulting from the AC model. Somehow, base on I^2t curve the clearing of both Bussman and SIBA above will clear the fault of 8kA almost in no time or more specific less than 2 ms. I wonder if my reading is wrong?

Nope. Semiconductor fuses are amazing. The only exasperating part is that they are so fast that usually multiple fuses trip simultaneously.

Quote:
Also, I wonder if DC Arcflash in this particular case can occur @ 115% of the let-through current which takes almost a second to clear?

This is where it gets a lot more tricky. Theoretically, anything is possible. However, one thing that is definitely true and known with DC systems is that we have to be concerned with trading off arc current and arc voltage, knowing that arc resistance is not a constant. It happens that maximum power transfer to the arc occurs at approximately 50% of the maximum available short circuit current...which explains what the 50% factor is all about, and that's where the arc will stay at.

There is a huge difference between the world of DC and AC, and you definitely can't just use AC models as a proxy for DC arcs. The big difference is that with an AC arc, it extinguishes at every zero crossing of current and then reignites as the voltage gets above around 150 volts or so. With DC arcs, the arc is very stable over a wide range of voltages and currents. The arc resistance quickly adjusts and stabilizes at the point of maximum arc power which happens to be at 50% of terminal voltage and thus balances 50% power transfer between the arc and the system impedance. With AC, what was found was that after a large number of tests (over 300), is that the incident energy and AC arcing curents tended to follow a long tail type distribution with a "double hump". Adding a second calculation at about 85% of bolted fault current and taking the worst case from both calculations resulted in a significantly better correlation between actual test results and incident energy. It is my understanding that the next edition of IEEE 1584 will go even further in achieving good correlation between test results and calculations.

If you get a copy of the actual papers that Annex D refers to, you will get a lot of this detail and more. DC arcs have been around for much longer than AC arcs. Some of the information I have stretches back to around the 1920's and 1930's when there was a lot of testing done to determine arcing parameters for neon lighting design. Its now very old data but still very relevant today. One of the most important equations you can get from that old data is that for DC systems, the minimum current required to initiate an arc at a given voltage is very well quantified. Below 28 V for instance, arcing is just plain impossible. It is possible but unlikely up to around 54 volts. Above around 70-80 volts though it's almost impossible not to form an arc even with extremely low currents.

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 Post subject: Re: DC Arcflash study and DC fuses clearing timePosted: Tue Jun 17, 2014 7:08 am

Joined: Mon Nov 19, 2007 5:25 am
Posts: 33
Location: Titusville, Fl.
Realizing the clearing time seems to be a known here, but shouldn't you also consider the amount of current available upon clearance as well? With that factored into your calcs should help give you an idea of the arc flash energies... Like the info about the amount of DC Voltage it takes to spark and arc - thanks....

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 Post subject: Re: DC Arcflash study and DC fuses clearing timePosted: Wed Jun 18, 2014 5:12 am
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2173
Location: North Carolina
321Liftoff wrote:
Realizing the clearing time seems to be a known here, but shouldn't you also consider the amount of current available upon clearance as well? With that factored into your calcs should help give you an idea of the arc flash energies... Like the info about the amount of DC Voltage it takes to spark and arc - thanks....

50% of bolted fault current. Di/DT plays a major role in many DC circuits, and is thus included.

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