Main switch short circuit capacity.

Actually, the nice thing about fuses is that as the fault current rises they get faster, more or less as a constant energy device - the energy to evaporate so many grams of fuse wire is almost fixed, once it is all so fast that there is no time for it to cool down -  this drives the I 2t is a constant approximation - I squared R is wattage, and I squared  R times time is energy .

Given that resistance of the fuse wire is always the same for all fuses of a given construction and rating, so the energy to blow that fuse is also fixed. Then it follows that I squared times time is also pretty much constant  - and indeed if you look at fuse curves, in the fast blow side of the chart, you see this is pretty much true.

In extreme (well beyond the ratings faults), the fuse does not interrupt safely, you may get a running arc inside the fuse between the opening ends, and all the broken bits of ceramic have to be swept up, and the welded and discoloured end caps may need to be levered out of the holder, but even then fuses do not generally fail to open, pretty much ever. (The sand fill supresses this arcing tendancy)


Circuit breakers do not have this nice (almost) constant energy feature - yes initially the contacts move faster with more fault current, up to a point, but there is a limit to how fast contacts can be separated. For example to open a 3mm gap in 10msecs is 300mm/sec, which sounds quite slow. However, given the inertia of the contacts, and the fact the magnetic core of the sense coil saturates at high current, very few normal circuit breakers ever open in less than a few milliseconds.

Then there is a potential problem, as the let through energy rises at higher fault currents, - for a high enough fault current, a breaker may absorb enough energy and weld shut or blow up before the contacts manage to open.

So we always want at least one fuse in the system for the day it all goes wrong. It won't blow in half a century of normal use, but it will be ready when needed. Unlike a mechanical thing that may stick.

The REC4 will not be opening or closing onto a fault, so there is no arcing expected, just thermal heating of the contacts. I do not expect a problem.

Let us know what they say

Actually, the nice thing about fuses is that as the fault current rises they get faster, more or less as a constant energy device - the energy to evaporate so many grams of fuse wire is almost fixed, once it is all so fast that there is no time for it to cool down -  this drives the I 2t is a constant approximation - I squared R is wattage, and I squared  R times time is energy .

Given that resistance of the fuse wire is always the same for all fuses of a given construction and rating, so the energy to blow that fuse is also fixed. Then it follows that I squared times time is also pretty much constant  - and indeed if you look at fuse curves, in the fast blow side of the chart, you see this is pretty much true.

In extreme (well beyond the ratings faults), the fuse does not interrupt safely, you may get a running arc inside the fuse between the opening ends, and all the broken bits of ceramic have to be swept up, and the welded and discoloured end caps may need to be levered out of the holder, but even then fuses do not generally fail to open, pretty much ever. (The sand fill supresses this arcing tendancy)


Circuit breakers do not have this nice (almost) constant energy feature - yes initially the contacts move faster with more fault current, up to a point, but there is a limit to how fast contacts can be separated. For example to open a 3mm gap in 10msecs is 300mm/sec, which sounds quite slow. However, given the inertia of the contacts, and the fact the magnetic core of the sense coil saturates at high current, very few normal circuit breakers ever open in less than a few milliseconds.

Then there is a potential problem, as the let through energy rises at higher fault currents, - for a high enough fault current, a breaker may absorb enough energy and weld shut or blow up before the contacts manage to open.

So we always want at least one fuse in the system for the day it all goes wrong. It won't blow in half a century of normal use, but it will be ready when needed. Unlike a mechanical thing that may stick.

The REC4 will not be opening or closing onto a fault, so there is no arcing expected, just thermal heating of the contacts. I do not expect a problem.

Let us know what they say