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.

  • \$\begingroup\$ why you need an additional fuse 200 mohm? \$\endgroup\$ Sep 21, 2019 at 13:36
  • \$\begingroup\$ @MarkoBuršič in case of a fuse at point B the battery fuse (which cannot be changed) will blow too slowly (hundreds of seconds). \$\endgroup\$
    – mickkk
    Sep 21, 2019 at 13:42
  • \$\begingroup\$ 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. \$\endgroup\$ Sep 21, 2019 at 13:46
  • \$\begingroup\$ Or install a thermal magnetic switch or circuit breaker. \$\endgroup\$ Sep 21, 2019 at 13:59

2 Answers 2


Use a circuit breaker link:

As you see, it can trigger in less than 10s when beyond nominal current.

As you said, 200A nominal current. When short circuit you get 1.6kA. 833A per battery, the 200A breaker resistance is 1.2 mohm. It will trip in 50ms (800% overload), the fuses would survive since it would take 2s to blow them. enter image description here

  • \$\begingroup\$ A CB is not an option, I am constrained to use a fuse due to mechanical issues (vibrations) \$\endgroup\$
    – mickkk
    Mar 26, 2021 at 9:25

There's something odd about your design there. Fuses are selected by current rating, not resistance. If you put 200A through a 200m ohm fuse, it will be dissipating 8kW, which makes no sense.

Fuses are available in various speeds from "quick blow" to motor rated. If you need discrimination, you need to ensure that the fuse you want to blow first is faster. But in the event of a dead short, don't rely on only one fuse blowing.


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