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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.

  • 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

  • 0

    Don't forget a DC isolator at the battery end as well.  If you are going to use an invertor that will need a AC and DC isolator one for each side of the invertor. 

  • 0

    As Mike says, the prospective short circuit current from a batter is usually reasonably high (presuming the battery is reasonably well charged that it...) - so as long as the fuse protects the conductors and has the (d.c.) breaking capacity for the fault current then you're probably good as far as short circuit protection is concerned.

    You might need to disconnect for other reasons though - e.g. if the battery is above ELV limits (120V d.c.) then you may well need to disconnect for shock protection as well - for which there are prescribed disconnection times - which might make things a bit more challenging.

    Often its easier just to go down the double/reinforced insulation route instead - which you'd likely need to do upstream of the first fuse anyway, which is often some distance away from the battery to avoid igniting flammable gases (hydrogen) that can be produced by batteries under certain conditions.

       - Andy.

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