Incorrect. For existing installations under engineering supervision it doesn't have to be a tested combination. The calculation approach is to provide upstream protection that functions in concert with the existing protective devices to safely open the circuit under fault conditions. This is generally a field-engineered solution employed to avoid having to replace all equipment when an increase in AFC causes existing equipment to be underrated.
Compatibility with series rated systems will in all likelihood be limited to circuit breakers that (1) remain closed during the interruption period of the fully rated OCPD installed on their line side and (2) have an interrupting rating that is not less than the let-through current of an upstream protective device (such as a current-limiting fuse).
Not true either. At one time there was a calculation (up, over, down) but it turns out to be faulty and no longer accepted. The theory is correct but in practical reality because of dynamic impedance the only practical way to predict it is by actually testing a combination.
But let’s be clear this is when the TCCs overlap. If they don’t overlap, have at it. Of course the only region anyone is interested in is the short circuit/instantaneous region.
CL fuses are a fine way to provide breaker protection at higher AIC. There are tons of breakers out there with backing fuses when you get into 65 kA and above ratings. But once again, we can’t predict timing so again must be a tested combination.
And of course there is the old approach. We document the fact that a miscoordination does or may exist and move on.
As far as the SCCR vs AIC argument this is missing the point. Say I place the downstream equipment in a black box. You can’t look inside but I can label the box for you with an SCCR. So whatever you do the available fault current has to be less. A CL device allows you to do exactly that, at least with mild decreases. Now when I open the box and let you look inside, there’s a breaker in there. Now let’s just go ahead and put your external device in the box. Now we raised SCCR. It’s not AIC but has the same effect.
The problem is coordination may or may not work though. And that may or may not be important. If there are no branches in the circuits…true “series” rating, I’d argue that coordination is no critical. It’s a much different story though if we have say a fused disconnect feeding a panel board. We don’t need fuses popping and killing all the loads every time something like a motor burns up. That’s the point of series ratings along with the SCCR/AIC argument,
But I’ve tried to solve this for the 5-10 kA equipment (cheap IEC stuff) before with fused disconnects. The answer every time turns into adding a small isolation transformer where eventually magnetic saturation limits current. Going from 40 kA to 35 kA is reasonable and doable. Going from 40 kA to 10 kA with current limiting, series rated or not, isn’t going to work.
