Let's say all the other breakers in the panel are standard (i.e. not ACFI). Would it help to have the main breaker AFCI or CAFCI?
Conversely, let's say the other breakers are all CAFCI. Can I use a standard main breaker?
My concern is to have the best security+convenience mix. In special, I don't want a water flood in the basement bathroom to trip the main and loose my refrigerated food or become unable to open the garage.
What does the code says about that?
Your idea is not weird. The European practice is to have a whole house master GFCI (their term is RCD), but there are a few key differences.
AFCI needs to be per branch circuit. It depends on detecting the "sound" of arcing on the lines, and if it's protecting too many circuits, the sound is too dampened to hear.
Arcing literally is a sound; in fact it's the same "crinkle crunch" sound you hear when hooking up speakers or sticking headphones in a plug. If you hooked plain, dumb speakers wound for 120/230V mains power, you'd be blasted out by the normal 50/60Hz "hum". If you added a notch filter to block that, you'd literally hear line noise and arcing. This is easily analyzed by a 1990s era DSP.
One cost-saver is to have the GFCI serve (or be the main breaker of) a small subpanel. However you don't want that serving too many circuits, or you'll have the European problem of minuscule ground faults stacking to cause nuisance trips.
You would not be able to do this strategy with AFCI breakers, because 2-pole AFCI breakers of large amperage are not made.
An alternative to AFCI protection is to run the entire circuit in metal conduit. The metal conduit conducts heat very well, so it distributes the heat from series arc faulting, preventing it from concentrating in one space enough to start a fire. As for parallel (shorting) arc faults, the metal conduit carries large amounts of current back to the breaker, assuring a trip. Since such a failure is also a ground fault, it will trip a GFCI quite a bit sooner.
I can't see any reason why you would want the main breaker to be anything except pure overcurrent protection. You need to have overcurrent protection in case something really bad happens. But short of that, you don't ever want to have the main breaker trip because then you have no power. That isn't just because of refrigerators, but you also would not want any bad wiring (backstabs, nail through a wire, switch gone bad, whatever...) to make the whole house go dark. The protection provided by AFCI, GFCI, etc. can cover everything at the circuit level and, effectively, provide whole-house protection since the types of problems that they protect (arcing wires in walls, ground faults in appliances, etc.) are not likely to happen inside the panel.
Always follow local code. National code does not require the main to be afci. 2017 NEC update states all residential living space 15a & 20A branch circuits to be afci protected either by breaker, or receptacle. Exception is kitch, bath, laundry, garage where you need gfci. You must update to current code when replacing, or repairing an existing circuit.
-Ted D (Boston)
