# How to choose the correct fuse in a double paralled battery supply system?

Suppose I have the following system:

simulate this circuit – Schematic created using CircuitLab

As you can see we have:

1. Two 500V battery with an estimated 500mohm average internal resistance. Each battery is fused with a fuse having a (cold) 100mohm resistance. This fuse is given by the battery supplier.
2. The batteries are in parallel and are connected with a suitable cable to a distribution box (B) fused with a 200mohm fuse. Voltage is controlled as to avoid circulation currents in batteries. Protecting each load line with a fuse is better as far as availability is concerned, but a single fuse in the distribution box has been selected as a preferred solution.
3. The distribution box feeds 3 loads. The total load current is about 200A which means that, after derating, a 300-350 A main fuse is needed.
4. The wires have a section such that their resistance is negligible compared to fuses and internal battery resistances.

For a short circuit fault:

• In A: minimum short circuit current (for each battery, worst case) is 500V/600mohm = 833 A and the battery fuse should cover that scenario, according to the battery supplier.
• In B: minimum short circuit current is 500V/800mohm = 625 A which means that, ideally, each battery will supply 312.5 A.

Problem: I need to choose the main fuse

The problem I am facing here is that if I set the main fuse to be able to withstand the rated power and surges of power (read: current) from the batteries, in case of short circuit in B it will open only after several seconds (often in the order of 100s) which will probably damage the cables' insulation.

Assume that the battery fuse is fixed SPFJ 160 A: it can be seen from the datasheet that it opens (in case of a short circuit in poit A of 800 A) after 2 s. However, if a short happens in point B, the short circuit current per battery is 312 A therefore the battery fuses open in 1000s therefore the main fuse should intervene to avoid damages. Since the main fuse is higher rated than the battery fuses, it will be inevitabily slower to burn! This means that it will will not open sooner than the battery fuses in case of short circuit in B. For example, for a 350 A fuse, at 625A short circuit current in B it will blow in 300-400s which may be unacceptable for the cables' insulation.

How should I select the correct main fuse?

All the fuse data I mentioned comes from here.

• why you need an additional fuse 200 mohm? – Marko Buršič Sep 21 at 13:36
• @MarkoBuršič in case of a fuse at point B the battery fuse (which cannot be changed) will blow too slowly (hundreds of seconds). – mickkk Sep 21 at 13:42
• First, if the fuse can't be changed then buy a battery with external fuse (changeable). It makes no sense to have a fuse that can't be changed, I am suspicious on that claim. Second, without your main fuse, everything is OK. – Marko Buršič Sep 21 at 13:46
• Or install a thermal magnetic switch or circuit breaker. – Marko Buršič Sep 21 at 13:59