Well, I certainly didn't intend to get you upset. I will reduce my hypothetical scenario to 30 Amps. 20 from the battery and 10 from the PMA. In your answer, you continue to use a high voltage to show that the battery could not contribute to the total current in the event of a partial short, and of course I agree. What I have described is a partial short that would reduce the voltage to something less than the nominal battery voltage so that both the battery and the PMA would then supply the excess current caused by the short. Once the voltage goes below 14.5 Volts, the PMA regulator no longer does anything so the PMA is then free to deliver whatever its maximum current capability is to serve the demands of the short. Likewise, the battery would also contribute to the excess current demand from the short once the voltage has dropped below the nominal charge of the battery, typicall 12.6 Volts, so both the PMA and the battery would feed the excessive current demands of the short. The batteries contribution would decline as the charge diminishes, but in the time that the battery is providing current to the short it could blow the 20 Amp fuse, but the PMA would be supplying its maximum output of 10 Amps so the total current being fed to the short would be 30 Amps.
OK I agree if the load is such that the maximum the PMA can output is exceeded the PMA voltage will drop to at some point below 12.8 v and the battery at that point will supply current as well.
This is a very real situation that happens all the time if for example when using heated clothing. Using GS Suzuki numbers, you would typically have at 4K RPM and above 15 amps being provided by the PMA of about 20 amps available. About 11 amps go to light, ignition, headlamp (say 3A,3A,5A respectively ) with 4 amps charging current to the battery. We then have total PMA current 3A+3A+5A+4A=15 amps total but only 11 amps through the fuse box.
If you now add a load on it's own fused circuit as in the case of heated clothing if you now pull another 5 amps, then the PMA is providing it's maximum 20 amps and the 14.5 voltage will start to drop but still well above the 12.8V needed to discharge the battery.
So what really starts to happen as you approach this 20 amps is the battery voltage drops reducing the battery charging current. By the time you drop the voltage to 12.8V you now have the full 20 amps going through the fuse box. The currents are now 3A,3A,5A and 9A for lights, ignition, headlamp and accessory. There is nothing flowing through the main fuse as the battery terminals are at 12.8V.
So what happens as you add even more load? Well if it is heater load and you pull only one more amp you blow that fuse. If it is an ignition short you only get to pull 7 more amps before you blow that fuse. The battery fuse (i.e. the main) never blows.). So the worst case with a non standard heater load of 10 amps (just enough to avoid blowing the fuse) you can short out any other circuit and you can't pull more than 27 amps without blowing a circuit breaker(fuse). But that is the design; the wire powering the fuse box needs to be handle in worst case 40 amps if there are 4 10 amps circuits. realistically you should never pull that much and you should never pull that much from the battery. On a GS if you have a standard load (11 amps with no battery charging) and 4 amps of accessory you are at the limit for a 15 amp main fuse.
This is really the worst case for a 5 fuse fusebox and 10 amps of accessory load. The main fuse only saw 7 amps. The downstream fuses are fusing the individual circuits, and the main is there to protect the battery in case it is shorted not to protect a fused load.
If it were not for the 10 amp accessory load, you would still be at most 7 amps away from blowing any circuit but the current through the fusebox would not exceed 15+7=22 amps. I guess if your XS650 PMA can only generate 10 amps then you would pull current from the battery sooner but the loads are still the same and you are still a long way from blowing the main fuse. It really takes a shorting R/R to blow the main fuse.
This really draws to question the sanity of having a 20 amps main fuse for the battery. The bike only needs 11 amps to run if it is off the battery. I would need to have a 9 amp accessory load running off of a straight battery to blow the main. The GS standard main is only 15 amps.
Yes, I did introduce the issue of high voltage from both types of alternators due to a faulty regulator and, yes , I agree that has nothing to do with fusing. I was hoping that you could include some protection from high voltage in your design as that is a more common occurrence. In the case of the stock field excited regulators, you could provide a cutoff to the current to the regulator if it was controlled from your product as I think that you have voltage sensing capability and the regulator is powered from your product. Even if the faulty regulator was uncontrollable, you could cycle it on and off to maintain a suitable average voltage to the battery and provide an alarm to the rider.
I thought I told you I incorporated ISO 7637-2:2004 testing guidelines into the design. Much of the guidelines have to do with load dump. I'm taking a wild guess by this time you do not know what load dump is nor are you familiar with the standards. Down below you say even suggest they might not be relevant as they are auto standards and not motorcycle standards.
The PMA would be a bit more challenging. Using the same algorithm as the stock alternator, you could perhaps introduce a relay that is switched on and off in response to the high voltage sensed at the battery to maintain a good average battery voltage.
Isn't that a SERIES R/R you are describing?
An answer to these over voltage issues would be very useful, so I am dismayed that you "won't entertain this comment any further."
Without handing you a schematic and a BOM, I can only refer to the standards which I have now done 3 times. More than that I basically told you that there are shunting diodes incorporated. I'll tell you more ; there are 1500 watt zener snubbers on each low impedance line coming into/out of the SSPB. That is 6 1500 watt parts protecting against +/- 100V load dumps. Here is a part number. 1SMC20AT3
Basically the SSPB will protect anything attached to it from load dump from any source.
If it goes beyond the ISO test levels it has a sacrificial fuse to disconnect everything.
Also, I believe that there is a separate set of ISO electrical standards for motorcycles and mopeds so the use of automotive ISO standards may be inappropriate.
There is nothing here related to load dump. I guess you might also presume that load dump is not relevant to motorcycles.