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Bolted Fault Current on a DC System

Bolted Fault Current on a DC System

Bolted Fault Current on a DC System

(OP)
I am looking at an arc flash calculation for a battery bank. It will be done using Annex D of NFPA 70E. I have read that, read Chapter 12 in Jim Phillips book and reviewed the manufacturer of the battery's material.

In order to begin I need to determine the bolted fault current at the battery terminals. In order to do this, I need the internal resistance of the battery bank. Lacking any testing, how does one determine that?

Thanks in advance.

RE: Bolted Fault Current on a DC System

Battery manufacturers publish the internal resistance of their cells. Adding more cells to a battery does not increase the bolted fault current because that calculation method assumes that the shorting element has zero resistance: doubling the number of cells also doubles the resistances in series so the bolted fault remains the same. In reality the interconnecting link(s) add a bit of resistance so the theoretical short circuit value is not achieved. Whose cells are they?

RE: Bolted Fault Current on a DC System

(OP)
Thank you ScottyUK. I could not find any published internal resistance for SBS batteries.

RE: Bolted Fault Current on a DC System

(OP)
I contacted the manufacturer and they provided the values for the specific cells at time of manufacture.

RE: Bolted Fault Current on a DC System

Be aware of the terminals of a battery bank normally is upstream of the main DC breaker. In other words, there will be no interrupting device to break the arc if a bolted short occurs. Our practice is to select the battery with terminal covers, and/or plastic shielding.

RE: Bolted Fault Current on a DC System

since cell resistance increases as batt wears, my 350vdc batt bank of cells on EV has a battery managment circuit that constantly measures each cell's internal resistance & voltage and reports it via the gui. I believe they simply are watching the cell voltage drop @ current and doing ohms law r=v/i: could you not do same? just read nominal voltage, put small resistive load across the fully charged cell and measure the drop?

RE: Bolted Fault Current on a DC System

Battery manufacturers will give tables showing the short circuit current and the internal resistance, for example see:

http://www.enersysreservepower.com/documents/US-SB...

If you wanted to be especially conservative you could just use this figure for you study, but in real life you would be way over the top.

As an example I recently had to do a calculation on a telco centre battery rack which consisted of 7 strings of 4 x SBS 190F batteries. From the link above the short circuit rating of an SBS 190F is 3800 amps (3.3 x 10-3 ohm), so with 7 strings the fault current was 7 x 3800 = 26.6 kA.

The battery resistance can also be worked out from the nominal voltage (12 volts) divided by the short circuit current (3800 amps): 12/3800 = 3.16 x 10-3 ohms - very close to the given figures in the PDF.

Each individual string providing 3800 amps.

Now take into account the connecting devices.

Each battery link, 3 per string = 1.5 x 10-3 ohm.

Each cable from battery to circuit breaker, 2 per string = 3.5 x 10-3 ohm.

Circuit breaker resistance = 1.2 x 10-3 ohm.

Add that all up and you get about 2.5 x 10-2 ohms.

Now use the nominal string voltage = 48 volts divided by the worked out resistance = 48/2.5 x 10-2 = about 1900 amps.

So with 7 stings in parallel we have 7 x 1900 = 13.3 kA.

Just by adding in all the link elements, the short circuit current from this battery bank was brought down from the conservative 26.6 kA to a figure of 13.3 kA.





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