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How to select cable and fuse for the battery system?

mike buba

I am trying to select the cable and the fuse protection for the battery system. 

My initial approach was to use the same method I am using for the PV systems.

For example, for this PV panel Isc  is 8.80 A and Imp is 8.28 A. For this example, according to the various PV standards (IEC 60269-6, 62738 and 62548) the fuse is selected based on the short-circuit current:

PV fuses designed according to IEC 62548 standard requirements: 1.5 Isc-mod ≤ In ≤ 2.4 Isc-mod

In this example, the 15 A fuse is selected, and since the permitted current-carrying capacity of the cable must be same as or greater than the trigger current of the string fuse, the cable is 4 mm2. Please note this just a simplified example to illustrate the method. 

For batteries, are there any other standards?

How would one proceed in selecting the cable and the fuse and make sure coordination between the components is achieved?

Also, the short circuit is either not listed the datasheets (e.g., 1, 2, 3) or if it is (e.g., 4, 5), it is in kA range for ~ 5 kVA batteries. So the ration between Ibat and Isc-bat is > 100. 

If I use the fuse and the cable sizing based on the PV short circuit method (1.5 Isc-mod ≤ In ≤ 2.4 Isc-mod), this would given me the huge values for In. But then again, I need to be sure the fuse will trip on the battery short circuit and not explode due to high arc if I base my selection on the Ibat values.

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    Batteries of all rechargeable chemistries do have the potential for very high short circuit currents - unlike solar panels that depending on the solar flux can be very weedy indeed, and almost constant current collapsing voltage devices.

    So fuses for batteries need to be selected that can safely break that maximum fault current..

    The fuse rating, rather like the cable rating, needs to be decided based on the maximum credible load, (inverter load ratings will be the main contribution to  this) and the lowest fault condition or overload that you want to detect. In that sense the design process is much closer to normal mains designs, where in a similar way the supply impedance is low.

    Be aware that not all fuses sold for AC use are suitable for breaking DC, so that too needs to be confirmed.

    Mike

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  • 0

    Batteries of all rechargeable chemistries do have the potential for very high short circuit currents - unlike solar panels that depending on the solar flux can be very weedy indeed, and almost constant current collapsing voltage devices.

    So fuses for batteries need to be selected that can safely break that maximum fault current..

    The fuse rating, rather like the cable rating, needs to be decided based on the maximum credible load, (inverter load ratings will be the main contribution to  this) and the lowest fault condition or overload that you want to detect. In that sense the design process is much closer to normal mains designs, where in a similar way the supply impedance is low.

    Be aware that not all fuses sold for AC use are suitable for breaking DC, so that too needs to be confirmed.

    Mike

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