What am I doing wrong with this 400V DC battery short circuit and arc flash calculation ? 1

[bdn2004]

Question: Find the Arc Flash Potential at the battery terminals that has these specs:

16.8 kAmp hours: which means over 8 hours => 16800 A-hrs / 8hrs = 2100 A
Lithium ion batteries are usually 3V / cell.
400V = 400V/3V = 133 cells in series to make the 400V
Per the specs, the ohm value for the entire bank is 54 m-Ohms = .054 Ohms

When I put these values into the modeling software I get bogus results for the Arc Flash potential. But I get that the short circuit about 9kA, which is reasonable per the literature.

How am I modeling this wrong ?

 

[mparenteau]

What answer are you getting?

Mike

 

[dpc]

You have no protection to clear the fault. The fault will last to the maximum time default in the software. This looks like EasyPower, and the out of the box default is 1000 seconds. This is found in the arc-flash options under Short circuit options. If there is a fuse between the battery and BUS-1, then you need to add that to the model. If not, it is what it is. You will probably want to set a maximum arc time less than 1000 seconds, but for dc, there is no recognized guidance on what to use. "Engineering judgement"

Stay safe.

Dave

 

[bdn2004]

Yeah you are right. Duh.

This is where not actually doing the math makes me look stupid...What is the value of t? You’d be asking if you did the calc by hand.

On the AC systems that value is 2 seconds max we use : the amount of time a person will naturally back away from the explosion - on an unprotected bus - which I am assuming he’s asking. Why would it be any different for dc ?

 

[bdn2004]

There actually is a fuse in the circuit.

A pyro type fuse with the specifications "At a current of >2300A for a duration of ~ 1mS the pyro fuse disconnects the current within 3mS". But when I try to simulate this with the Easy Power model with any fuse it doesn't take the melting time of the fuse associated with the arcing current into the calculation - it still goes to the 1000s? That seems odd.

 

[mparenteau]

What's your arcing fault current? Use that to find your clear time on the fuse curve.

Mike

 

[bdn2004]

That's what I'm saying - the arcing fault current is the tic mark on the TC curve above - approximately 3700Amps or so. And the arc time would therefore be around 10 seconds on the upper part of that curve. I can put any fuse in there and it does the same thing - 1000sec.

 

[dpc]

The software is treating the fuse as a "Main" and is ignoring it - the default. In the arc flash options, in upper right corner, switch it to "Including Main" and try it again. For typical LV ac equipment the main breaker in a panelboard or switchboard is normally not considered for arc-flash purposes and the incident is calculated on the line side of the main device, not the load side. That's "Excluding Main".

 

[dpc]

Even 10 seconds will give you a pretty high incident energy.

It's probably reasonable to use 2 seconds for dc arc flash, but there is nothing that says it OK or recommended.

 

[bdn2004]

dpc,

ok! That works, you should work for Easy Power. This is a much more reasonable result.
Even with this generic fuse it's not much energy - and the real one is faster than this.

 

[bdn2004]

and here is the TCC for that....

 

[dpc]

Just be aware of the situation on the battery side of the fuse.