The entire charging circuit is unfused on the 550, except the black wire feeding the V reg. That is switched form the Key switch, and the key switch power is from the main fuse. Of course, the key switch supplies power to the entire bike, sans starter motor, too. The rectifier is not on any fused branch.
If you are instantly blowing a 30 amp fuse, you pretty much have a direct short to power to ground somewhere.
The stock bike draws about 10 amps through the fuse. This means the bikes resistance is about 1.26 ohms. 30 amps would be .42 ohm. If you had a milliohmeter, you could actually measure the resistance, note that it is low, and then keep disconnecting things until the resistance comes up above 1.26 ohms. Whenever you connect something that puts the ohms below 1.26, you found the circuit branch fault, These are "kind of" advanced techniques/skills for finding circuit faults. But, it can be done this way without blowing any fuses.
You can also stick a 1 ohm resistor in series with the fuse. It will limit current draw while you poke around with a voltmeter looking for serious voltage loss about the supply distribution. The biggest loss is where the problem lies.
A good battery will not deplete instantly, nor will it charge instantly. Takes time for the voltage to change and this is related to the discharge rat and the charge rate. For example, the starter motor draws 120 amps. The charging system only supplies about 1-2 amps for battery charging when the RPM is high enough. (The rest of the power in used by bike components.) The battery will deplete far faster than it will charge. So, look for voltage change trends instead of absolute voltages, particularly when troubleshooting with an undercharged battery.
A depleted battery with no load measures about 11.9 V and is in a full charge state with 12.6-12.8V. Under load, these voltages are lower.
As for rectifier resistance, makes sure your meter supplies enough voltage to fully activate the diodes, when measuring it's conduction resistance.
Cheers,
