AIR/AIC rating when using fuses in combination with breakers

[SunFish]

Example Scenario:

Supply side connection with available fault current from transformer at 58 kA. Main service breaker has an interrupt rating of 65 kAIC. PV system installation will use a fused disconnect and fuses with a 200 A kAIC rating for the interconnection. An AC combiner panel with a main breaker is used to combine multiple inverter outputs.

Is there any reason to use a main breaker in the AC combiner panelboard that also has a 65 kAIC rating or can this breaker have an AIC/AIR rating that is lower than the available fault current? I've been told that fuses have a much quicker fault clearing time and therefore the kAIC rating of the main breaker in the AC combiner panelboard can be disregarded so one could use a main breaker than has, as an example, a 25 kAIC rating because the fuses will clear a fault before the main breaker even has a chance to trip.

I'm curious to get other's opinions. Should one use a 65 kAIC rated main breaker in this scenario or is it acceptable to use a lower rated breaker/panelboard to save $?

 

[Carultch]

It is only the case when the fuse class and breaker model are listed as a series-rated combination, that you can do what you are proposing. Which I will add, is strategic and is something I would recommend, provided that you have the opportunity to review product selection with attention to detail. Communicate the requirement clearly on the drawings for which devices you are specifying to have a series-rating, and have the contractor submit the datasheets and series rating tables, for your review.

The breaker manufacturers have tables in their product documentation that identify the series-ratings among breakers they build, and specific classes of fusing. The products need to be tested together, and listed as a series combination, in order to be able to disregard the KAIC rating of a breaker that has upstream fuses. This also allows downstream breakers to have a lower KAIC than upstream breakers, provided that the combination carries this rating.

Otherwise, the breaker has to have a KAIC rating that meets or exceeds the fault current available at its location in the circuit, as it would be if there were no other OCPD's upstream of it. It is possible for fault current to diminish with circuit length due to the impedance of the feeder wire, but fuses do not reduce it in a way that is brand-agnostic for all breakers downstream.

 

[electrofelon]

What sizes are the 3 OCPD's? ( The fused disco, MB, and branch/inverter breakers).

Also, what is the purpose of the combiner main breaker?

 

[SunFish]

What sizes are the 3 OCPD's? ( The fused disco, MB, and branch/inverter breakers).

Also, what is the purpose of the combiner main breaker?

Well, this is kind of just a made up scenario, so lets say 200 A fuses, 200 A main breaker, and four 50 A inverter breakers with the AC combiner panel and Inverters to be installed at a not readily accessible location so the main breaker at the AC combiner acts as a disconnect grouped with the inverters to shut all four off at once.