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I'm tidying up my wiring and have just realises that the short piece of 35mm2 wire from my battery bank to the isolator switch which incorporates a mega-fuse is rated at 400amp. Now surely if that ever were to blow the all of my wiring looms will be frazzled.

What confuses me is this was fitted by a professional so what am I missing, I thought all fuses were to protect the wires therefore the fuses should be rated to protect the wires themselves so to my mind 35mm2 cable should be 150ampmax. 35mm2 cable runs from the isolator switch to the pos busbar.

Hope this makes sense.😄

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Fuse could have been changed after was fitted?   A larger fuse can be installed on underrated cable, a starter motor is one example as it will blow on a dead short, but not when cranking the engine which usually pulls more amps than the cable rating.   I can’t think of other examples tho!

Edited by Robbo

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Fuses protect against cable damage in two situations: overload and short-circuit. They should be sized so that the worst case temperature rise in either situation is not damaging. The two situations are quite different because the disconnection time is quite different.

 

A short-circuit dissipates a lot of power in the cables, but the fuse fails quickly. So quickly that there's effectively no time for the cable to reject the heat via conduction, convection and radiation. The fuse therefore has to keep the instantaneous temperature rise down, and what matters if the product of fusing time and short-circuit power dissipation.  

 

Overload is different, the protection takes longer to happen, and the temperature rise in the cables also takes longer, so what matter is the equilibrium temperature. A fuse has to open once the overload is high enough that the equilibrium temperature exceeds the cable spec.

 

My guess is that your 400A fuse is short-circuit protection for the big cables, and is fine for that. It certainly isn't overload protection: that will be supplied by the individual fuses in the fuse box. As long as the sum of all of them is less than the maximum steady-state current capacity of the 35mm2 cable you have absolute protection.

 

MP.

 

  • Greenie 1

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Yes wot MP says. You have to consider the feasibility of a fault causing a current in your 35mm cable that is say between 150A and 400A. Pretty unlikely! A dead short will pass a lot more than 400A and blow the fuse rapidly, and that is by far the most likely significant fault condition.

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Thanks guys, i sort of get it, I was only checking to tightness of the connections when I spotted it anyway so I'll leave it to get on with it's job which, hopefully will be never. 

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