# Fuse design for taking in account real-world very small transients

A bit of context

I came up with this question after someone suggested that two fuses are better than a main one in the below configuration. The converters are SMPSs which regulate output voltage. simulate this circuit – Schematic created using CircuitLab

Now someone justified that F1 and F2 are a better implementation through the following example: both converters are functioning within normal limits, each drawing 1A (and F1 and F2 are both designed to trip at 2A). If Converter 1 would fail open and Converter 2 would continue working normally, then during the finite instant when the aforementioned conditions start applying, as current cannot change instantly and has only one path to follow from supply to ground, the 2A will flow through Converter 2 and potentially damage it. Since F2 is there, it would protect against the over-current condition. If converters reverse roles in the same scenario, F1 will be there to open the circuit.

The justification basically says that the fuses are there to protect against an open-circuit event...instead of a short-circuit.

That's as much context as I can give. Current can't change instantly, because that's just how the real-world works. I agree it's a weak reasoning, as the "why?" is missing, but I would like to prove it wrong or right to myself (with your help). To help answerers, I would like to know both if the parasitic inductance or anything else (other than parasitic inductance) could recreate the effects described.

Main part

Now, remove the converters and the loads and place two real-world resistors between each fuse and ground, both (resistors) drawing 1A normally. Again, the total current drawn from the power supply would be 2 A.

The questions are:

1) If one removes a resistor (ceteris paribus), will the remaining other experience the 2A through for a very short, but finite amount of time? If it does, can it be due to something else, other than parasitic inductance (and can parasitic inductance also affect things)?

2) Does 1) mean, if true, that ohm's law only applies if the time-step (or time scale) is large enough?

3) If 1) is true, is it worth taking into account this effect from an engineering perspective-i.e. during that short finite time, the 2A could damage the remaining resistor?

• What is the inductance of the power supply cable? Not very high, I bet. What is the input capacitance of a converter? Higher than the inductance, I bet. Feb 3, 2020 at 16:33