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Is there any official guidance as to what the breaking capacity should be for a mains inlet fuse? Assuming the appliance is intended to be fed from a normal domestic socket.

I understand that in an ideal world, the breaking capacity of a fuse should be greater than the prospective fault current, but in an appliance we don't know what that is. If we assumed the worst possible case (the appliances power cord plugged into a zero impedance source), we would end up with a fault current of tens of kiloamps. Fuses like that certainly exist, but I don't think I've ever seen an appliance inlet with fuseholders that could accept one.

I guess with something like a 1.5kA ceramic fuse (about the highest you can get in the 20mm form factor most mains inlet modules accept) you could argue that most of the time it's probably sufficient to break a short, and when it isn't the next fuse/breaker upstream will likely operate rapidly.

And in commercial appliances I often see glass fuses used as mains inlet fuses, the breaking capacity of those seems almost comically low, far too low to be useful in a short circuit, are they there for overload protection only?

Is there any official advice/guidance on mains inlet fuses?

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    \$\begingroup\$ I don't think you are thinking about this correctly. When a device is powered from a mains outlet, the outlet has some form of protection upstream. That protection must be rated at very high breaking capacity because the source (utility wire) has very high current carrying ability. If you get a 1500 Amp fault, your little 20 mm fuse may explode, but the next upstream fuse will blow if needed (if your little fuse explodes and arcs over or conducts through a puddle of molten metal or whatever). Also, the wire from the main circuit breaker to the outlet may limit current somewhat. \$\endgroup\$
    – user57037
    Commented Feb 21, 2022 at 20:30
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    \$\begingroup\$ 50 feet of 12 AWG wire is about 80 mOhms. 120 V / 0.080 = 1500 Amps. So in the US you would be hard pressed to get more than 1500 Amps to an appliance anyway. Similar calculations for other countries with their customary voltages and wiring sizes should yield results in the same ballpark I imagine. \$\endgroup\$
    – user57037
    Commented Feb 21, 2022 at 20:36
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    \$\begingroup\$ As I understand it, fuses in appliances are generally only there to protect the wiring from overcurrent. They're not designed to protect against any other fault class - that's what the upstream protection is for. The fuse is rated for the internal wiring of the device, and/or the permanently attached mains cable if there is one. In the case of an appliance with a removeable (e.g. IEC C13) power cable, the appliance may have a fuse to protect its internal wiring from overcurrent, and the cable may also have a fuse (e.g. in a BS1363 plug) to protect itself from overcurrent. \$\endgroup\$
    – Polynomial
    Commented Feb 21, 2022 at 21:22
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    \$\begingroup\$ The breaking capacity of those fuses then becomes irrelevant. If a 1.5kA fault somehow occurs and all of the fuses in the appliance supply path arc over, those fuses aren't responsible for handling that. As you go back up the supply path, eventually you'll hit a protection device (MCB, RCBO, contactor, fuse, etc.) with sufficient breaking capacity. \$\endgroup\$
    – Polynomial
    Commented Feb 21, 2022 at 21:25

1 Answer 1

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Is there any official guidance as to what the breaking capacity should be for a mains inlet fuse?

  • Assuming this is AC and the only requirement is to prevent a fire or wire melt-down

You choose a fuse rating for the holding capacity of the load which is not to exceed the wire. , not the short circuit safe current of the source.

Depending on design and size; there will be additional specs for ;

  • Max. Voltage Rating (Volts)
  • Interrupting Rating at Max. Voltage (Amps)
  • Operating Temperature Range
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