kA^2s

I ²t has a real physical significance - in the same way as the power dissipated in a resistor passing a given current is I² R the power times time during a fault large enough and fast enough we can ignore slow effects like cooling, is a proxy for the energy absorbed before the power is cut, and so how hot things are at the moment the ADS does its thing.

As such it allows us to say something about the ultimate temperature reached inside a given bit of equipment, if we know its resistance and the thermal mass that is being heated.  This in turn allows us to decide if something melts, or is vapourized, or indeed is redistributed to the four winds as if a small bomb  had gone off. We can then decide if we should stock a spare fuse,  a spare fuse and holder,  a replacement box, or indeed a replacement transformer and substation roof (some designs being intended to blow off gracefully for the ultimate event.)


For a thermal fuse in a well controlled thermal environment (sand filled ceramic tube anyone) this works very well.

The BUT, and it is a big BUT,  is that it is not suitable for mechanical ADS with moving contacts - inertia and saturating iron cores of solenoids conspire to limit the maximum speed, so it stops getting faster at higher current beyond a certain point.

Hence the 'death or glory' fuse in the basement of all well designed installations, though  it's true function in limiting the total energy that gets downstream, is often misunderstood.

I ²t has a real physical significance - in the same way as the power dissipated in a resistor passing a given current is I² R the power times time during a fault large enough and fast enough we can ignore slow effects like cooling, is a proxy for the energy absorbed before the power is cut, and so how hot things are at the moment the ADS does its thing.

As such it allows us to say something about the ultimate temperature reached inside a given bit of equipment, if we know its resistance and the thermal mass that is being heated.  This in turn allows us to decide if something melts, or is vapourized, or indeed is redistributed to the four winds as if a small bomb  had gone off. We can then decide if we should stock a spare fuse,  a spare fuse and holder,  a replacement box, or indeed a replacement transformer and substation roof (some designs being intended to blow off gracefully for the ultimate event.)


For a thermal fuse in a well controlled thermal environment (sand filled ceramic tube anyone) this works very well.

The BUT, and it is a big BUT,  is that it is not suitable for mechanical ADS with moving contacts - inertia and saturating iron cores of solenoids conspire to limit the maximum speed, so it stops getting faster at higher current beyond a certain point.

Hence the 'death or glory' fuse in the basement of all well designed installations, though  it's true function in limiting the total energy that gets downstream, is often misunderstood.