The Plane Power alternators use the field voltage, as measured at the alternator field terminal, as the sense voltage. As Vern describes, if your field voltage is to low, the alternator will be fooled into thinking your main buss voltage is also low. When this situation exists, the internal PP regulator will command the alternator to output a higher voltage to the buss. This will then trip the over-voltage crowbar, which is internal to the alternator, when the alternator output exceeds the per-determined value. The crowbar does this by SHORTING the field terminal to ground thus tripping your breaker.
One thing to note, the Vertical Power creates a tiny voltage drop through the unit due to the way it switches. You will never see the field voltage at the same level as the buss voltage when you use the VPX system. This small voltage drop at the field output terminal will cause the PP alternator to regulate . 2-.3 volts higher then if you connected the field directly to the buss. If the (Ask me how I know) This in no way is a bad thing, just something to note.